Sulphonation of phloroglucinol succeeds at higher temperatures only, the sulphonic acid being a solid which is scarcely soluble in water, the latter then assuming a wine-red colour. The condensation product—prepared as described for resorcinol, but requiring higher temperature—is a brick-red powder, insoluble in water.

The same end-product also seems to be obtained by simply heating the sulphonic acid at a higher temperature; this also induces condensation with the formation of a reddish-brown mass insoluble in water. It is, of course, impossible to attempt any tanning experiments with this product in aqueous solution; attempts at dissolving the condensation product in alcohol proved barren of result, since only traces of impurities accompanying the substance dissolved, imparting a light reddish-brown colour to the solution. In highly concentrated alcohol, however, the condensation product is somewhat soluble, yielding a reddish-brown solution. A piece of pelt introduced into the alcoholic solution was surface tanned only after forty-eight hours, leaving the remainder of the pelt pickled; extending the experiment over a further four days produced no change in the pelt. The latter was therefore rinsed with water, lightly fat-liquored and dried, when a soft but empty leather of grey colour and good tensile strength was obtained. It appears, therefore, to be a case of pseudo-tannage, where an infinitesimal amount of synthetic tannin produces a tanning effect without, however, a true tannage being effected.

The Elberfelder Farbenfabriken have protected the use of the condensation products of di- and polyhydroxybenzenes by Ger. Pat., 282,313; owing to the high cost of the latter substances, however, it is doubtful whether synthetic tannins prepared from these materials would not be too expensive for any other than pharmaceutical purposes.

Before leaving the phenols, mention must be made of the quinones, the use of which for tanning purposes was first protected by Ger. Pat., 206,957 (30th April 1907). According to this patent, only 400 gm. of quinone are required for the conversion into leather of 400 kilos pelt, drum tannage being preferable. During the process the leather first assumes a reddish colour, changing through violet to brown; its resistance to water, acids, and alkalies is said to be considerably greater than that exhibited by all other kinds of leather.

The chemistry of the quinone tannage has been investigated, and an explanation given by Thuau [Footnote: Collegium, 1909, 363, 211.] assumes a reaction between the quinone and the amino groups of the hide protein with the formation of hydroquinone—

+-O OH 2R.NH2 + 2C8H4 = C6H4 + C6H4(O.NH.R)2 +-O OH (Pelt.) (Leather.)

Fahrion has shown that, during the tanning process, the quinone loses its active oxygen, and this can only be brought about by the amino group of the hide protein, the amino group only being capable of effecting reduction of the quinone. An analogy is here offered by dianilinoquinone. A spent quinone liquor contains considerable amounts of hydroquinone. The tannage may also be effected by exposing pelt saturated with hydroquinone to oxidation by the air. The pelt, which is unaltered by the hydroquinone bath, on being removed from the latter, and in the presence of alkali, assumes a red colour at first, which changes into violet, blue, and finally brown, the pelt being thereby converted into a quinone-tanned leather.

It may be noted that quinone only effects pseudo-tannage; quinone mixed with water deposits, in time, a black amorphous substance practically insoluble in water. This substance is easily adsorbed by hide powder, but is not capable of converting the latter into that insoluble form into which it is converted by the natural tannins.

Amongst polyhydric alcohols, the behaviour of the methyl ester of catechol, guaiacol was investigated. The sulphonic acid was prepared by heating guaiacol with concentrated sulphuric acid, the resulting water-soluble product possessing a light, brownish-green colour. On condensing the sulphonic acid with formaldehyde, the same precautions were observed as in the case of resorcinol, but complete fixation of the formaldehyde could only be obtained by finally heating the product for a short time over a free flame, at about 105 C. Condensation was indicated by the brownish appearance of the liquid. No insoluble products were formed. The condensation product easily dissolves in water, the solution assuming a rich brown colour and exhibiting the following reactions: gelatine is completely precipitated, aniline hydrochloride produces opalescence, and ferric chloride a deep brown coloration.

Tannage, with the partly neutralised product, was rapid, the pelt being nearly tanned through in twenty-four hours, excepting a small white streak in the middle; after a further twenty-four hours this streak had vanished, and the completely tanned, dark grey-coloured leather, after washing, fat-liquoring, and drying, was soft, full, and of good tensile strength, very similar to the leather yielded by the catechol-condensation product.

Of the nitro-compounds, trinitrophenol, C6H2(NO2)3OH (picric acid), was investigated. If a concentrated solution of picric acid is brought into contact with pelt it will penetrate the latter completely in a few days; it is, however, difficult to fat-liquor the resultant leather, since the fat is absorbed only with difficulty. If a pelt treated in this way be dried, a soft but rather flat leather results, the colour of which easily rubs off, the leather also tasting intensely bitter. These disagreeable qualities prevent a general use of this material for tanning purposes; in spite of them, however, picric acid, in admixture with boracic acid, salicylic acid, and glycerol, is used in the production of the so-called transparent leather. The latter is very flexible and possesses great tensile strength, but loses the latter quality when exposed to heat, and, when stored, also loses its flexibility. By simply washing with water, the leather is reconverted into pelt.

When picric acid is treated with hot sulphuric acid and formaldehyde gradually added, a dark coloured water-soluble condensation product is formed which strongly precipitates gelatine. Exposed to the action of bromine, the condensation product yields a mass which is insoluble in water.

Experience has taught that the amino bodies—the basic N-derivatives of the phenols—do not yield substances possessing tannoid properties on condensation. On account of their importance, however, a few have been included in this series of investigations.

Aminobenzene, C_6H_5NH_2, aniline, treated with sulphuric acid, yields the water-soluble aniline sulphate, which, on cautious addition of formaldehyde, yields a reddish-coloured gel, insoluble in water, in addition to a small volume of a reddish-yellow liquid. The latter precipitates gelatine, but is not capable of converting pelt into leather. The insoluble gel is likewise insoluble in alcohol, so that tanning experiments with this substance are excluded.

Dimethylaniline, C6H5N(CH3)2, when treated with sulphuric acid, yields a product soluble in water which neither reacts with nor fixes formaldehyde. Hence the substance does not precipitate gelatine.

If, on the other hand, nitrosodimethylaniline,

NO C6H4 (CH3)2

is sulphonated, and the water-soluble sulphonation product heated with formaldehyde for some time, the product remains soluble in water and precipitates gelatine. No tanning effect could, however, be detected.

Arylsulphaminoarylsulphonic acids and arylsulphoxyarylsulphonic acids precipitate gelatine but are devoid of tannoid character. The latter is acquired by compounds belonging to this class containing two or more sulphamino groups, or when they, in addition to one sulphamino group, contain a sulphoxy group and another sulphonic group. According to Ger. Pat., 297,187 (Society oc Chemical Industry, Basle), such compounds are obtained when, for instance, sodium sulphanilide in alkaline solution acts upon nitrotoluenesulphochloride, and the resulting nitrotoluenesulphamino compound is subsequently reduced with acetic acid and iron. The resulting aminotoluenesulphaminobenzenesulphonic acid is finally treated with p-toluenesulphonic chloride till the latter disappears. A compound of the composition

-NH -SO_2 - ^ ^ ^ -NH - V V V SO_2Na CH_2

is thereby obtained, which, when acidified, is readily capable of being used for tanning purposes.

The intermediary product of the aminotoluenesulphaminobenzenesulphonic acid obtained by this process may again be employed for the purpose of reacting with one-half molecule soda and 1 molecule nitrotoluenesulphonic chloride. The following compound is obtained—

-NH -SO_2 - -NH -SO_2 - ^ ^ ^ CH_3 ^ v v -NH -SO_2 - v v SO_3Na CH_3 CH_3

If p-toluenesulphaminobenzenesulphonic chloride is condensed with sodium sulphanilide, a compound,

-SO2 -NH - NaSO3 ^ ^ ^ v v -SO2 -NH - v SO3Na

is obtained which, when acidified, exhibits tannoid properties.

On condensing sodium phenolsulphonate with nitrotoluenesulphonic chloride, reducing the condensation product and condensing the latter with p-toluenesulphonic chloride, a compound similar to the above is obtained—

-O -SO_2 - ^ ^ ^ CH_3 v v -NH -SO_2 - v NaSO_3 CH_3

Again, a similar product is obtained when p-toluenesulphaminobenzenesulphonic chloride or its homologues or isomers are condensed with sodium-o-cresylsulphonate—

-SO_2 -NH - SO_3Na ^ ^ ^ CH_3 v v -SO_2 -O - v CH_3

The chloride of this compound may again be condensed, for instance, with sodium aminotoluenesulphaminobenzene-sulphonate, and yields the compound—

-NH -SO_2 - ^ ^ ^ -NH -SO_2 - ^ v v -NH -SO_2 - v v CH NaSO_3 CH_3

The three latter compounds, when dissolved in water and the solution acidified, exert tanning action.

It is also possible to employ mixtures of arylsulphaminobenzylsulphonic acids in acidified aqueous solution for tanning purposes. According to Ger. Pat., 297,188, such mixtures are obtained by nitrating benzylchloride and heating with an equimolecular amount of sodium sulphite; the sodium nitrobenzylsulphonate thus obtained is reduced to aminobenzylsulphonic acid with iron and acetic acid, and finally condensed with the calculated amount of p-toluenesulphonic chloride. A mixture o- and p-toluenesulphaminobenzylsulphonic acid [Footnote 1: Cf. also Ger. Pat, 319,713 and 320,613.] thus results.

Amongst aromatic alcohols the dihydric alcohols show characteristic behaviour; the latter combine with sulphonic acids with the elimination of water, condensation taking place without formaldehyde, and the resulting products being soluble in water and possessing tannoid properties. [Footnote 2: Ger. Pat., 300,567, of 20th September 1917.] In addition to phenolic mono- and disulphonic acids (and higher sulphonation compounds), the homologues, cresols, xylenols, and naphthols enter into reaction. The two components condense with great ease, liberating heat; dilute solutions (of the components) are heated to about 100 C., the process being complete in a few minutes. The products obtained are exceedingly pure and are easily crystallisable. Employing 1, respectively 2, molecules of sulphonic acid, the reactions take place according to:—

OH CH_2.OH OH OH }C_6H_4 + HO.C_6H_3{ = H_2O + }C_6H_3-CH_2-C_6H_3{ HSO_3 CH_2.OH HSO_3 CH_2.OH

OH OH CH2.OH CH2.C6H3{ }C6H4 + HO.C6H3{ = 2(H2O) + HO.C6H3{ HSO3 HSO3 CH2.OH OH CH2.C6H3{ HSO3

OH CH_2.OH OH OH }C_6H_3.CH_3 + HO.C_6H_3{ = H_2O + }C_6H_2.CH_3.CH_2.C_6H_3{ HSO_3 CH_2.OH HSO_3 CH_2.OH

OH OH CH_2.OH CH_2.C_6H_2.CH_3{ }C_6H_3.CH_3 + HO.C_6H_3{ =2(H_2O) + HO.C_6H_3{ HSO_3 HSO_3 CH_2.OH OH CH_2.C_6H_2.CH_3{ HSO_3

OH CH_2.OH OH OH }(C_10)H_6 + HO.C_6H_3{ = H_2O + }(C_10)H_5.CH_2.C_6H_3{ HSO_3 CH_2.OH HSO_3 CH_2.OH

OH OH CH2.OH CH2.(C10)H5{ }(C10)H6 + HO.C6H3{ =2(H2O) + HO.C6H3{ HSO3 HSO3 CH2.OH OH CH2.(C10)H5{ HSO3

The condensation products above enumerated were tested with regard to their tanning power, both non-neutralised and partly neutralised (1:10, 1:20, and 1:30 c.c. N/10 NaOH) samples being examined. In all cases rapid tannage was observed yielding firm and soft leathers of light brown colour and varying degrees of swollenness.

Relatively to their reactions, all the products strongly precipitate gelatine, whereas only the condensation products of phenol, cresol, and xylenol derivatives give a characteristic coloration with iron salts.

The tannin contents of the non-neutralised condensation products lie between 72-80 per cent.—figures which clearly indicate the purity and efficiency of these substances.

Notable amongst aromatic acids is salicylic acid, C6H4.OH.COOH, which at higher temperatures is easily sulphonated with concentrated sulphuric acid; the sulphonation product represents a white solid, which easily dissolves in water forming a clear liquid. The sulphonic acid, when mixed with about one-third of its weight of water and heated to about 120 C., is easily condensed with formaldehyde. Towards the end of the reaction, considerable frothing sets in, but in spite of the high temperature required by this reaction no insoluble bakelites are formed. A reddish-brown fluid is obtained easily soluble in water, to which it imparts a brown colour. An aqueous solution of the product completely precipitates gelatine, gives a strong opalescence with aniline hydrochloride and a deep violet coloration with ferric chloride. Neutralised as usual, the product, in a 3 B solution, converts pelt within three days into a white, full leather of good tensile strength.

This process has been patented by the Deutsch-Koloniale Gerb und Farbstoff Gesellschaft (German-Colonial Tanning and Colour Extracts Ltd.) in Karlsruhe, the letters patent also including the ring homologues of salicylic acid. Similar results are obtained when cresotinic acid (hydroxy-toluic acid), OH.C_6H_3.CH_3.COOH, is employed as base.

If the phenyl ester of salicylic acid, Salol,

HO.C6H4.CO.O.C6H5

is sulphonated, a product is obtained which is easily soluble in water, but which is identified as a mixture of the sulphonation products of salicylic acid and phenol, the salol being dissociated on sulphonation. The temperature must not exceed 80 C. by condensation with formaldehyde, or insoluble bakelite will be formed from the phenol; the aldehyde must also be added gradually. An aqueous solution of the partly neutralised condensation product has a pronounced tanning effect on pelt, and converts the latter into leather in one to two days; the leather being very similar to that produced by the salicylic acid condensation product. The qualitative reactions of the product in aqueous solution are the same as those given by the salicylic acid condensation product.

Salicylic acid may, however, also be condensed with formaldehyde without first being sulphonated; in this case, a little hydrochloric acid should be present. A product slightly soluble in water is obtained, which may be looked upon as being methylenedisalicylic acid. In alkaline solution it is easily soluble,

[Footnote 1: Its solubility in alcohol and alkalies renders this product an effective and cheap substitute for shellac.—Transl.]

the liquid possessing an intensely bitter taste. The sodium salt gives a deep violet coloration with ferric chloride, a slight precipitate with gelatine, and slight opalescence with aniline hydrochloride. In contact with pelt, however, it exhibits no tanning effect, but when dissolved in alcohol, a pickling effect may be observed.

[Footnote 2: A similar reaction is observable in the case of the sodium salts of METHYLENEDISALICYLIC acid brommated or iodised, which form a clear solution varying from red to reddish-brown.]

The attempt at preparing a condensation product from sodium-m-hydroxybenzoate by means of formaldehyde and bisulphite is worthy of attention. A dark brown, viscous liquid is obtained which is perfectly soluble in water, and the aqueous solution of which gives opalescence with gelatine, a precipitate with aniline hydrochloride, and a bluish-black coloration with ferric chloride. Its behaviour towards pelt is very similar to that of phenolsulphonic acid, and it yields a similar leather.

A very similar condensation product was obtained by condensing sodium-p-hydroxybenzoate with formaldehyde and subsequent sulphonation with sulphuric acid. From a practical standpoint, however, these substances cannot be employed, since their tanning action is only effective in acid solutions of such concentration of acid as would gelatinise the pelt.3

If, on the other hand, non-condensed methane derivatives of phenol, e.g., hydroxyphenylmethanesulphonic acid, are partly neutralised and a solution of the product thus obtained used for tanning experiments, no tanning action is observable. The acidified solution does not precipitate gelatine, and gives a dark brown coloration only with ferric chloride.

GALLIC ACID, C_6H_2(OH)_3COOH, when heated with sulphuric acid, is easily converted into the insoluble rufigallic acid, which is also insoluble in alcohol. If, however, gallic acid is heated with an excess of sulphuric acid, the product cooled and treated with formaldehyde, a deep brown condensation product is obtained which is soluble in alcohol, and in this state is capable of converting pelt into a substance similar to leather which, though rather hard, possesses good tensile strength. This water-insoluble condensation product is also soluble in alkalies, the solution exhibiting properties similar to that described above. Gallic acid, therefore, is not a suitable base for the production of synthetic tannins soluble in water.

Phthalic acid also is difficult to sulphonate: the sulphonated compound treated with formaldehyde gives only water-insoluble condensation products.

3. Condensation Of Naphthalene Derivatives

The simplest method of condensing [Greek: b]-naphthalene-sulphonic acid is to heat it at 135 C. at a pressure of 20 mm. for several hours.[Footnote: Austr. Pat., 61,061, of 10th September 1913.] The resulting product is a cheesy mass which reacts strongly acid. By reducing the acidity of the substance to 1 gm. = 10 c.c. N/1O NaOH, a grey, cheesy mass results, which easily dissolves in water, the solution being coloured a light yellow-brown and precipitating gelatine aniline hydrochloride; no coloration, however, appears on adding ferric chloride.

The condensation of [Greek: b]-naphthalenesulphonic acid, however, proceeds with much greater energy in the presence of formaldehyde. In practice, for instance, 10 kilos of naphthalene is heated with the same weight of concentrated sulphuric acid (66 B), when the mixture is converted into [Greek: b]-naphthalenesulphonic acid by heating for several hours at 150-160 C; the sulphonation completed, the sulphonic acid is cooled to about 85 C., and 4 kilos of formaldehyde (30 per cent, by weight) slowly added; finally, the product is stirred at the temperature mentioned till all formaldehyde has combined.[Footnote: Austr. Pat., 69,194, of 25th June 1915; Ger. Pat, 290,965.]

Tanning experiments with this product yielded, in a short time, a nearly white coloured leather (see later).

In addition to formaldehyde, there are other substances which induce condensation of naphthalenesulphonic acid; if, for instance, sulphur chloride is allowed to act upon [Greek: b]-naphthalenesulphonic acid, a light brown solid of pronounced acidic character is obtained; if the latter is partly neutralised with caustic soda, a greyish-brown solid results, which dissolves in water with a light brown colour, the solution precipitating gelatine and aniline hydrochloride, but giving no coloration with ferric chloride.[Footnote: Austr. Pat., 96,194.]

Tanning experiments with this product in aqueous solution gave a light brown, rather soft leather, and this, in addition to the qualitative reactions of the substance, prove that this method of condensation hardly alters the character of the product from a tanning point of view. The brown coloration imparted to the leather tanned with this condensation product owes its existence to coloured intermediary products.

Attempts at condensing chloronaphthalenesulphonic acid and nitronaphthalenesulphonic acid resulted in soluble condensation products which gave some of the reactions given by the tannins (precipitation of gelatine and aniline hydrochloride), but which were incapable of tanning pelt, a light tannage being effected on the surface only.

[Greek: a]-Naphthol dissolved in hot concentrated sulphuric acid and heated for some time on the water bath, yields the light brown, water-soluble [Greek: a]-naphtholsulphonic acid. A dilute solution of the latter, when treated with formaldehyde in the cold, undergoes no change; on heating the mixture on the water bath a brown precipitate is thrown down. If gelatine solution is added to the opaque liquid, a yellow flocculent precipitate separates. If caustic soda is added to the opaque liquid containing the condensation product described above, a clear solution results from which no deposit separates on the addition of acetic acid. Gelatine is precipitated by this solution.

The concentrated hot a-naphtholsulphonic acid, upon addition of sufficient formaldehyde, effervesces strongly and yields a dark brown condensation product insoluble in water, but soluble in caustic soda. If acetic acid is added in excess to the alkaline solution, the resultant solution strongly precipitates gelatine.

A suspension in water of the insoluble condensation product does not precipitate gelatine.

b-Naphthol, dissolved in hot concentrated sulphuric acid and heated for some time, yields the light brown, viscous b-naphtholsulphonic acid. A dilute solution of the latter, mixed with formaldehyde, remains clear; when heated on the water bath, however, it assumes a dark, reddish-yellow colour, and remains soluble in water and precipitates gelatine strongly. This condensation product, on adding excess of caustic soda, assumes a deep blue coloration, the alkaline solution giving no precipitate with gelatine; on adding acetic acid the solution turns brown, remains clear, and now precipitates gelatine.

The concentrated b-naphtholsulphonic acid heated with formaldehyde on the water bath yields as condensation product a dark, reddish-yellow mass, soluble in water, which precipitates gelatine. A dilute solution, when allowed to act upon pelt, gave in a few days a light brown leather, the properties of which are very similar to those possessed by vegetable tanned leathers.

The use of naphtholsulphonic and aminonaphtholsulphonic acids for the manufacture of synthetic tannins is protected by Ger. Pats., 293,640, 293,693, 293,042, and 303,640. [Footnote: Cf. Austr. Pat., 70,162.]

It is a remarkable fact that non-condensed methane derivatives of naphthol, e.g., b-naphthol-a-methanesulphonic acid, dissolved in water and partly neutralised, are devoid of tanning character when allowed to act upon pelt. Neither does this substance precipitate gelatine, but it does give a deep blue coloration with ferric chloride.

The condensation product of b-naphthol above referred to precipitates gelatine and aniline hydrochloride and gives a brown coloration with ferric chloride.

Thionaphtholsulphonic acid, when acted upon by formaldehyde, yields a condensation product of the following constitution:—

HSO_4 ^ ^ SH SH ^ ^ HSO_4 __CH_2__ v v v v

This is a light yellow powder which, dissolved in water, yields an opaque solution; the latter only exhibits any tanning properties when it is not neutralised and even slightly acidified and then precipitates gelatine, aniline hydrochloride and barium chloride; dissolved in alkali, it forms a clear, yellow solution devoid of tannoid properties. Leather tanned with the acidified solution is very similar to those tanned with the phenolsulphonic acid condensation products; its colour, however, is more pronouncedly yellow.

b-Naphthol condensed with hydrochloric acid and formaldehyde yields a methylenedinaphthol, which is insoluble in water; the sodium salt, however, easily dissolves. The same condensation, however, takes place in alkaline solution with direct formation of the sodium salt. The condensation product gives a slight precipitate with gelatine, and a bluish-grey precipitate with ferric chloride; acids re-precipitate the insoluble methylene compound. Towards pelt it exhibits tanning properties, whereby the insoluble product referred to above is deposited, and soft, full, and white leather is obtained, possessing, however, but little tensile strength.

4. Condensation of the Anthracene Group

Anthracene heated with excess sulphuric acid yields the water-soluble anthracenesulphonic acid; the latter, when heated with formaldehyde, yields water-soluble, reddish-brown condensation products, which remain soluble on prolonged heating with formaldehyde. The aqueous solution of the condensation product shows no particular reactions; it gives a flocculent precipitate with gelatine and a green precipitate with copper sulphate, soluble with blue colour in excess of the reagent.

The partly neutralised solution tans pelt—to which it imparts a brown colour—in eight days, but on the surface only; the inner layers are merely pseudo-tanned (white colour). When dried, pelt thus treated yields a full and soft leather with brown grain and flesh possessing but little tensile strength. Hence, this condensation product exerts a pickling rather than a tanning effect.

Anthraquinone heated with sulphuric acid and treated with formaldehyde in the usual manner, yields a substance which, when mixed with water, forms an opaque, milky solution. This is not altered by excess of caustic soda. The aqueous solution precipitates gelatine and aniline hydrochloride; all other tannin reagents give no reaction.

The partly neutralised solution of the condensation product exerts, in the main, a pickling action on pelt; only the surface of which is tanned, with brown colour, the remainder being merely pickled (white colour). During "tannage," bakelite is formed in the liquid, and practically all solubles originally present are deposited. The tannage completed, a light brown, fairly soft and full leather, possessing little tensile strength, results; this leather can be washed only with great difficulty and approaches more the character of a pickled pelt.

1-Hydroxyanthraquinone, 1,5-dichloroanthraquinone, l,5-diaminoanthraquinone, 1-methylaminoanthraquinone, 1-benzoylamino, 6-chloranthraquinone, 1-m-toluidoanthraquinone, when treated with sulphuric acid and formaldehyde, all yield condensation products which are but little soluble in water, and which do not at all precipitate gelatine. Tanning experiments with these condensation products in alcoholic solution yielded empty leathers of pronounced pickle character.

If, however, 1-methylamino-4-bromanthraquinone is condensed with sulphuric acid and formaldehyde, a condensation product is obtained which is but slightly soluble in water, but which precipitates gelatine.

When phenanthrequinone is heated with excess of sulphuric acid for some time, a water-soluble, reddish-yellow coloured condensation product results. The latter, when treated with formaldehyde in the cold and then finally heated, gradually fixes the formaldehyde and forms a substance soluble in water. If the heating, however, is prolonged, insoluble bakelites are formed, which are neither soluble in alkali nor in alcohol.

An aqueous solution of these condensation products gives no reactions with the usual tannin reagents, though it completely precipitates gelatine. When acting upon pelt, the partly neutralised dilute solution of the condensation product pickles the former, and after a few days the pelt is converted into a light brown, full, and rather soft leather possessing good tensile strength.

When the condensation product is acted upon by bromine in hot aqueous solution, an additive compound is formed and the resulting product is soluble in water. The aqueous solution of the brominated product gives no special reactions with the usual tannin reagents, but precipitates gelatine completely. Its tanning action upon pelt is much slower than that of the original condensation product; the surface of the pelt only is tanned with brown colour, the inner pelt being only pickled (light brown colour). When dried, a hard and empty leather of good tensile strength is obtained, possessing mainly the properties of a pickled pelt.

CO OH ^ ^ ^ QUINIZARENE, , treated with sulphuric acid v v v CO OH

and formaldehyde, yields a condensation product which is but little soluble in water and which does not precipitate gelatine.

QUINOLINE, when sulphonated and condensed with formaldehyde, yields a dark coloured condensation product, completely soluble in water; the solution does not precipitate gelatine.

OXYQUINOLINE exhibits similar behaviour.

On the other hand, the use of retene (methylisopropylphenanthrene),

CH3 ^ ^ CH:CH C3H7 v v

for the production of synthetic tannins, is protected by Ger. Pat., 290,965 [Footnote 1: Cf Austr. Pat., 69,194]

5. Di- and Triphenylmethane Groups

If DIPHENYLMETHANE, (C6H5)2CH2, is heated with excess sulphuric acid, a dark blue mass, easily soluble in water, is obtained. The product gently heated with formaldehyde yields a brown, water-soluble condensation product; once condensation is complete, the product will stand stronger heat. If, on the other hand, more formaldehyde is added, brown, water-insoluble bakelites are formed. The water-soluble condensation product precipitates gelatine, but not aniline hydrochloride. Dissolved in water, it possesses tannoid properties: the pelt is, however, tanned on the surface only, the intermediary layers being merely pickled; after four days in the solution, the pelt after drying was found to be converted into a greyish-brown, badly coloured leather, which was empty, hard, and possessed but little tensile strength.

CARBAZOLE (dibenzopyrrole),

^ __ ^ _ _ v v v N_3

on the other hand, was found a suitable base for the commercial production of synthetic tannins; its use is protected by Ger. Pat, 290,965.

TRIPHENYLMETHANE, (C_6H_5)_3CH, heated with excess sulphuric acid, yields a nearly black mass which, when condensed with formaldehyde in the cold, and subsequently heated, yields a mass which is soluble in water. With gelatine and aniline hydrochloride it exhibits reactions similar to those given by the diphenylmethane condensation products; its tanning properties also are similar to those of the latter. The resultant leather is black, but is soft and full and possesses good tensile strength.

Baeyer's observation, [Footnote: Ber., 1872, 5, 280, 1096.] that pyrogallol on condensation with formaldehyde yields an amorphous body soluble in water, which precipitates gelatine and is very similar to tannin, was confirmed by Caro [Footnote: Ibid., 1892, 25, 947.] and Kahl. [Footnote: Ibid., 1898, 31, 114.] These investigators found that by the condensation of phenols and hydroxybenzoic acids with formaldehyde, diphenylmethane derivatives were formed; pyrogallol yields hexahydroxydiphenylmethane—

C_6H_2(OH)_3 CH_2{ C_6H_2(OH)_3

Nierenstein [Footnote: Collegium, 1905, 221.] repeated these experiments, and found that in addition to the insoluble diphenylmethanes, water-soluble bodies were formed, which latter precipitate gelatine. The condensation product yielded by gallic acid was identified as hexahydroxyaurinecarboxylic acid—

_C_6H(OH)_3COOH C{-C_6H(OH)_3COOH }C_6H(OH)_2COOH O

which is formed in addition to hexahydroxydiphenylmethane-dicarboxylic acid—

C_6H(OH)_3COOH CH_2{ C_6H(OH)_3COOH

Baeyer's experiment with pyrogallol probaly also yields, according to Nierenstein, another compound of the following constitution—

C6H2(OH)3 C{-C6H2(OH)3 }C6H2(OH)2 O

Nierenstein considers these bodies confirmation of his hypothesis of the existence of a "tannophor,"—CO—, in the tannins.

This supposition was adopted by Stiasny [Footnote: Gerber, 1905, 233.] and Kauschke [Footnote: Collegium, 1906, 362.] and the latter points out that these easily soluable substances exhibit tanning properties. Nierenstein [Footnote: Ibid., 1906 424.] was further able to show that by all processes of condensation between phenols (or hydroxybenzoic acids) and formaldehyde, compounds of the character of hydroxyaurine (or hydroxyaurinecarboxylic acid) were formed in addition to the insoluble hydroxydiphenylmethanes (or hydroxydiphenylmethanecarboxylic acids), the former possessing the characteristic tannophor group and hence precipitating gelatine, i.e., exerting tanning action. If the formation of leather is viewed in the light of Schiff's base, [Footnote: Ibid., 1905, 159.] one may consider the constitution of a hexahydroxyaurinecarboxylic acid leather as follows:—

_C_6H_2(OH)_3.COOH C{-C_6H_2(OH)_3.COOH _}C_6H_2(OH)_2.COOH R-N

In the preparation of these and similar condensation products, Nierenstein and Webster [Footnote: Ber., 1908, 41, 80.] observed a peculiar steric effect of the carboxyl group. Each 2.5 gm. of the phenol or the acid in question were dissolved in 30 c.c. of water, the solution brought to boil and 5 c.c. formaldehyde (20 per cent.) and 2.5 c.c. hydrochloric acid added drop by drop; the precipitate formed was filtered off after twenty-four hours, dried at 110 C. to constant weight, extracted (in a Gooch crucible) freely with water, and the residue again dried at 110 C. till constant. The following values were obtained:—

Total Insol. Aq. Sol. Aq. Oxy- Precipitate Diphenylmethane aurinecarboxylic in Grammes. Derivatives Acid.

Per Cent. Per Cent. Phloroglucinol 2.4002 100 ... Hydroquinone 2.3716 100 ... " 2.0542 100 ... Pyrogallol 2.5150 100 ... " 2.7940 100 ... Pyrocatechol 2.9805 100 ... " 2.9574 100 ... Resorcinol 2.9954 100 ... " 2.9725 100 ... Gallic acid 2.0706 78.84 21.16 " 1.2240 83.18 16.82 " 1.1405 59.94 41.06 [Greek: b]-Resorcylic acid 2.1040 51.08 48.92 " " 2.2008 47.12 52.88 Protocatechuic acid ... ... ... " " ... ... ... Vanillic acid ... ... ... Tannin 2.0599 ... Nearly all sol. Digallic acid 2.1042 80.16 19.84 Leucodigallic acid 2.0041 1.94 98.06

With the introduction of the carboxylic group the tendency of condensation to diphenylmethane derivatives is lessened; by protocatechuic acid the tendency is nil. Nierenstein considers this reaction analogous to the formation of cork, to the genetic relation of which with the diphenylmethane formation Drabble and Nierenstein have referred in an earlier publication. [Footnote: Biochemical Jour.., 1907, 2, 96.] It is hence possible that the plants may employ formaldehyde as a methylation medium, and produce these insoluble condensation products for the purpose of ridding themselves of the poisonous phenols and aromatic hydroxy acids (and tannins), in addition to oxidising processes whereby phlobaphenes, ellagic acid, etc., are formed.

The reaction between phenols and aldehydes has been further studied by Michael, [Footnote: Amer.Jour., 5, 338; 9, 130.] who prepared a condensation product from phenol and resorcinol with benzaldehyde, and Russanow, [Footnote: Ber.9 1889, 22, 1944.] who also employed benzaldehyde and phenol. Lipp [Footnote: Diss., Bern., 1905.] investigated the action of benzaldehyde and piperonal on phenols, anisoles, cresols, cresylic ether, resorcinol, and the ether of the latter and phenol, and showed that when free phenols are condensed with benzaldehyde the hydroxyls occupy the same position as by the interaction between benzaldehyde and the corresponding phenolic ethers. The resulting dihydroxytriphenylmethane derivatives form beautiful crystals, which on oxidation are converted into benzaurines, the constitution of the latter probably being—

O= ^=__ ^ OH == _ =v v v C C_6H_5

In alkalies, the hydroxylated triphenylmethanes dissolve without imparting any colour to the solution; by concentrated sulphuric acid they are taken up with intense coloration.

If the hydroxyls occupy the ortho-position to methyl, they may form xanthenes by splitting off water—

O ^ ^ ^ CH3 v v v CH3 CH C6H5

In the benzene series this reaction is difficult to establish, and has to be induced by distilling the particular dihydroxy-diphenylmethane at ordinary pressure. In the naphthalene series, on the other hand, the ring closes up by, for instance, the condensation of [Greek: b]-naphthol with benzaldehyde or paraldehyde, and yields the following compounds:—

C_10H_6 C_10H_6 C_6H_5-CH{ }O CH_3-CH{ }O C_10H_6 C_10H_6

These xanthenes are white, silk-glossy needles, which are soluble in water and in alkalies. In concentrated sulphuric acid, they are taken up with beautiful fluorescence.

6. Summary

From the qualitative reactions of the different condensation products described it may be seen that their tannoid properties are not dependent on whether they precipitate gelatine or are adsorbed by hide powder or not. Hydroxynaphthylmethanesulphonic acid, for instance, precipitates gelatine but does convert pelt into leather; on the other hand, sodium dicresylmethanesulphonate does not precipitate gelatine, and neither does it tan pelt; nevertheless it is adsorbed by hide powder as "tanning matter". The author discovered that o-nitrophenol does not precipitate gelatine, but has some tanning action on both hide powder and pelt.

Relatively to the possibilities of forming condensation products possessing tannoid properties, the following may be stated:—

All mono- and polyhydric phenols may be converted into true tanning matters by either condensing them as such, or after their conversion into the corresponding sulphonic acids, by substances capable of eliminating the elements of water. It makes no difference to the final product whether the condensation is the first step followed by sulphonation and consequent solubilisation of the intermediary insoluble product, or whether, vice versa, the sulphonic acid is subjected to condensation. Alkaline solution of phenols may also be condensed, the reaction products, when condensed, constituting tanning matters soluble in water.

Among the substitution products of the phenols, the thio-, chloro-, bromo-, nitro-, and aminophenols as a rule yield tanning matters similar in character.

The quinones are as such—i.e., without being condensed—substances possessing tannoid properties.

The aromatic dihydric alcohols are easily condensed with the different sulphonic acids and yield valuable tanning matters.

Of aromatic acids all those which yield water-soluble sulphonation products seem suitable for the industrial production of tanning matters. If the acids themselves do not yield water-soluble sulphonation products, the alkali salts of the latter may be condensed with formaldehyde, and the resulting products then constitute tanning matters provided their solutions can be neutralised or faintly acidified without the solute being thrown out of solution in insoluble form.

The diphenyl derivatives of the above groups often possess tannoid properties.

The same holds good of those compounds with condensed nuclei (naphthalene, anthracene, etc.), and all their derivatives which satisfy the above conditions.

The choice of condensing agent is, as a rule, of little significance. Elimination of the elements of water by the mere application of heat succeeds in few cases only, since the high temperature required to induce reaction in many cases causes decomposition of the substances. This difficulty is overcome by heating in vacuo. Condensation with formaldehyde always succeeds, with acetaldehyde and benzaldehyde only partly.

The action on hide powder, pelt, and gelatine by these characteristic substances is tabulated below:— Relative Behaviour towards Substance. Gelatine. Hide Powder Pelt Formaldehyde ... ... Tanning Phenol Ppte. ... ... Chlorophenol " ... ... Surface Tribromophenol Slight ppte Tanning tanning _o_ Nitrophenol No ppte " " Bromonitrophenol Slight ppte " " Trinitrophenol Ppte " Tanning Bromotrinitrophenol Slight ppte " " _p_ Aminophenol Ppte ... ... _m_ Dihydroxybenzene " ... ... Orcinol " ... ... _p_ Dihydroxybenzene " Tanning Tanning Monochloro _p_ dihydroxybenzene " ... ... _o_ Dihydroxybenzene " ... ... Pyrogallic acid " ... ... Surface Tribromopyrogallic acid " Tanning Tanning Gallic acid No ppte Not tanning Not tanning Bromophloroglucinol Ppte Tanning " Gallotannic acid " " Tanning Galloflavine Slight ppte " Not tanning Quinone " " Tanning Bromosalicylic acid " " Not tanning Dinaphthylmethanedisulphonic acid Ppte " Tanning Diphenylmethanedisulphonic acid " " " Dicresylmethanedisulphonic acid " " " Sodium dicresylmethanedisulphonate acid No ppte " Not tanning Dixylylmethanedisulphonic acid Ppte " Tanning Naphtholdisulphonic acid " Not tanning Not tanning Methylenedinaphthol " Tanning Tanning Hydroxyphenylmethanesulphonic " " " acid Not tanning Hydroxynaphthylmethanesulphonic Slight ppte " " acid Diaminonaphthylmethanedisulphonic Ppte Tanning Not tanning acid Dihydroxynaphthylmethanedisulphonic acid " " " Dichloronaphthylmethanedisulphonic acid " " Surface tanning Dinitronaphthylmethanedisulphonic acid " " " Dithionaphthylmethanedisulphonic acid " " Tanning Bromo _[Greek: b]_ naphthol [1] Slight ppte " " Rosolic acid_ [1] Ppte " "

[Footnote 1: In alcoholic solution.]



SECTION III

TANNING EFFECTS OF MIXTURES AND NATURAL PRODUCTS

1. Mixture of Phenolsulphonic Acid and Formaldehyde

The most important invention relatively to the search for new tanning materials was that of Weinschenk,[Footnote: Ger. Pat., 184,449.] who first showed that pelt may be converted into leather by the action upon it of mixtures of naphthols and formaldehyde. This process consists of two steps: the pelt is first immersed in a 0.25-0.50 per cent, formaldehyde solution, and secondly in an aqueous solution of -[Greek: a] or -[Greek: b] naphthol; this order may be reversed. If, on the other hand, a pasty mixture is made of formaldehyde and naphthol, and this is allowed to act upon the pelt, the latter is rapidly converted into leather, but the mixture must be administered very gradually or otherwise the insoluble methylenedinaphthol is formed outside the pelt and hinders any tanning effect.

Leather obtained through the action of [Greek: a]-naphthol is, when freshly tanned, pure white and sufficiently soft and firm, but quickly assumes a brown colour on storing; if, however, [Greek: b]-naphthol is employed, a cream-coloured leather results, the colour of which turns only slightly more yellowish even when exposed to the direct rays of the sun.

A similar process has recently (25, xii., 1915) been protected by Ger. Pat, 305,516, granted to the Deutsch-Koloniale Gerb—und Farbstofif Gesellschaft, in Karlsruhe. According to this patent, pelt is treated in separate solutions, one of which is formaldehyde, the other being that of such aromatic compounds or their salts which yield water-soluble condensation products with formaldehyde; for example, pelt is immersed in 2-5 per cent, solution of formaldehyde for a few days, and is subsequently treated with 1-2 per cent neutral or faintly acidified solutions of [Greek: a]-naphthylamine hydrochloride, resorcinol or sodium phenate or cresylate, for several days. The resultant leather is claimed to be soft and full and to possess good tensile strength.

The tanning properties of mixtures of phenolsulphonic acid and formaldehyde have been examined by the author with the following results:—

I. II. III.

Grammes formaldehyde 10 20 40 " phenolsulphonic acid 20 50 100 " caustic soda (sol, 40 per cent.) 10 20 40 " water 500 500 500

The above solutions were made up and allowed to act upon pelt pieces weighing 15 gm.; whereas Solution I. remained clear throughout the experiment, Solution II. became somewhat clouded, and Solution III. assumed a milky appearance. The pelts were tanned through in seven days and yielded leathers which, after drying and finishing, possessed yellow colour, long fibre, and good tensile strength, but a rather empty feel.

To prevent separation of insoluble matter during tannage, another experiment was carried out, in which the pelts were first submitted to the action of formaldehyde (10, 20, and 40 gm. in 500 c.c. water) for three days, being subsequently removed to fresh solutions of partly neutralised phenolsulphonic acid (cf. above). Similar results were obtained, but the leather felt even more empty than those obtained by the former experiment.

Attempts at converting pelt into leather by first immersing the pelt in a partly neutralised solution of phenolsulphonic acid, and subsequently transferring it to fresh solutions of formaldehyde, gave merely negative results; the phenolsulphonic acid effected pickling action upon the pelt, but was subsequently quickly replaced by the formaldehyde, before the latter had penetrated the pelt in sufficient quantity to induce condensation, thereby exerting tanning action.

To explain the tanning effects of these mixtures, the author analysed the leathers resulting from the effects of the latter, and was able to show, that in these cases also, condensation of phenolsulphonic acid and formaldehyde takes place inside the pelt, since on the one hand the analyses left no doubt but that true tannage had been effected, and on the other hand an ammoniacal extract of the leathers gave the typical reaction for condensation products of phenolsulphonic acid, with aniline hydrochloride. [Footnote: Collegium 1913, 516, 142.]

The leather analyses gave the following figures:—

Moisture - - - 18.30 per cent. Fats - - - - 0.47 " _ Ash - - - - 0.98 " Leather { Tannin - - - 26.37 " substance { Hide substance - - 53.88 "

A characteristic feature is the low value of tannin, which is considerably higher [Footnote: Ibid., 1913, 521, 478.] where condensation products of phenolsulphonic acids are used as tanning agents; the action effected by the separate constituents, therefore, is more that of pickling.

2. Mixture of Phenolsulphonic Acid and Natural Tannins

A piece of pelt was immersed in a half-neutralised solution, measuring 6 B., of phenolsulphonic acid, and left sixteen hours in the solution, which completely penetrated the pelt during this time; it was then transferred to a 12 B. solution of a mixture of quebracho and chestnut, which in two days converted the pelt into a light coloured leather possessing good tensile strength.

By using a bath composed of half-neutralised phenolsulphonic acid and quebracho extract in 7 B. solution, another piece of pelt was completely tanned in two days. The same result was obtained by first half neutralising the phenolsulphonic acid and then adding sulphited quebracho extract till a 5 B. solution was obtained.

A piece of pelt received a 2 B. liquor composed of 3 parts of phenolsulphonic acid and 1 part of formaldehyde for sixteen hours, and was then completely penetrated; it was subsequently transferred to a 10 B. liquor composed of chestnut and quebracho, being completely tanned in two days. The same result was obtained on adding sufficient sodium sulphate to the above mixture of phenolsulphonic acid and formaldehyde to raise the density from 2-3 B.

Sixty grammes of phenolsulphonic acid were partly neutralised with 100 c.c. of a 10 per cent solution of caustic soda, and 10 c.c. formaldehyde added to 400 c.c. of the mixture (2 B.): a piece of pelt was completely penetrated by the solution in sixteen hours, and was subsequently tanned in two days, using an extract of 10 B. Similarly, by treating a pelt with 400 c.c. of a half-neutralised solution of phenolsulphonic acid (3 B.) plus 8 c.c. formaldehyde, and adding after eighteen hours sulphited quebracho extract to the same bath, strengthening the latter to 6 B., the pelt was converted into leather in two days; in this case, however, much of the tannin was precipitated by the formaldehyde present in the solution. If, on the other hand, a mixture of 80 gm. dilute phenolsulphonic acid (1:1 aq) and 14 gm. of formaldehyde were cooled for several hours and subsequently strengthened with sulphited quebracho extract to 7 B., no tannin was precipitated in the liquor, and a piece of pelt immersed in the latter was completely tanned in sixteen hours.

To prevent the precipitation of tannin caused by the formaldehyde, sulphite cellulose extract (wood pulp) was substituted for sulphited quebracho extract, and the following experiments carried out:—

To 200 c.c. of a 6 B. sulphite cellulose extract plus 200 c.c. of half-neutralised phenolsulphonic acid solution was added 15 c.c. formaldehyde, and this solution tanned pelt in four days; the resultant leather was light brown, firm, and possessed good tensile strength and long fibre.

Another piece of pelt was immersed in a solution of 400 c.c. phenolsulphonic acid of 3B. plus 15 c.c. formaldehyde for eighteen hours, and was then tanned in a 6 B. solution of sulphite cellulose extract. The resultant leather was extremely light coloured, and possessed qualities similar to those described in the former experiment. Finally, pelt was immersed in a 6 B. solution composed of 140 gm. of a 15 B. sulphite cellulose extract, 10 gm. of formaldehyde, 400 gm. water, 15 gm. phenolsulphonic acid, and 30 gm. of a 10 per cent caustic soda solution, and was tanned in four days. This leather also was coloured light brown, of good tensile strength, and rather firm.

These experiments prove that when pelt is treated with formaldehyde, phenolsulphonic acid, and vegetable tannins, the two former components effect, more or less, actual tannage; it is admittedly a matter of some difficulty to establish whether the effect is one of pickling or pseudo-tannage, or whether the tannage may be considered a true one. The final effect, however, is nearly always that of a true tannage, i.e., by varying the composition of the tanning solutions leather is obtained with properties identical with those tanned with true tannins of vegetable origin. The only difficulty encountered in these combinations is the property of formaldehyde, of precipitating the natural tannins, and it is hence essential, for practical purposes, to so arrange the combination that their value is not reduced by the property referred to. The fact that not only compounds already existing may convert pelt into leather, but that a similar effect is obtained inside the pelt, by their components, is indeed of theoretical interest.

3. Tanning Effects of Different Natural Substances

In addition to the vegetable tannins, Nature has also provided other substances of vegetable origin, which, admittedly, do not effect tannage in their original state, but which may, by suitable treatment, acquire this property. The oldest information on this point is supplied by Resch, [Footnote: Scherer's Jour., 1801, 6, 495.] who carried out tanning experiments, using three parts of peat and one part of oak bark.

By the action of nitric acid on substances of vegetable and animal origin, Hatchett, [Footnote: Gehlen's Jour., 1805, 1, 545.] Chevreul, [Footnote: Ann. Chim., 1810, 73, 36.] and Vogel [Footnote: Jour. Chem. Phys., 1812, 6, 101.] claim to have obtained tanning materials, whilst later, Buff [Footnote: Ibid., 1827, 51, 38.] obtained a material suitable for tanning purposes from indigo.

By subsequent treatment with lime and soot, or tar, Ashmore [Footnote: Dingier's Jour., 1833, 48, 67.] claims to have converted pelt into leather.

By treating peat with nitric acid, Jennings [Footnote: Jahresber. d. Chem., 1858, 666.] and Payne [Footnote: Chem. Centralbl., 1908, ii. 554; Ger. Pat., 200, 539.] have produced artificial tanning materials.

Skey [Footnote: Chem. News, 1866, 206; Zeits. f. Chem., 1866, 753.] obtained a dark brown extract, soluble in water and precipitating gelatine, by treating bituminous coal or lignite with nitric acid; by extracting coal with alkalies, Reinsch [Footnote: Pharm. Centralh., 1887, 141.] isolated a substance (pyrofuscine) which, when partly neutralised with carbon dioxide, was capable of converting pelt into leather.

In addition to these tanning materials the recovery of a substance possessing tanning properties from the so-called acid rosins has been made the subject of a patent; [Footnote: Ger. Pat., 36,019.] this rosin is formed when crude oil is treated with concentrated sulphuric acid in the oil refineries. The greasy substance is partly neutralised with alkali and is claimed to produce a very springy leather.

The waste liquors obtained in the manufacture of cellulose, the so-called sulphite and sodium cellulose waste, have, however, been the subject of numerous investigations, and several hundred publications have appeared and a great number of patents [Footnote: "Literatur beriSulfitablauge" 1910-13. (Reprint from WocheWochenblPapiePapierfabrikation)] taken out, the first one being that of Mitscherlich [Footnote: Jahresber. d. Chem., 1893, 890; Ger. Pat., 72,161.] and Hnig [Footnote: Chem. Centralbl., 1902, ii. 174; Ger. Pat., 132,224.]

The waste liquors contain large quantities of acids and lime, and in order to utilise the liquors for tanning purposes, the excessive sulphuric and sulphurous acids as well as the lime must be removed. The active tannin is no doubt the ligninsulphonic acid, and those cellulose extracts containing the largest amounts of free ligninsulphonic acid may also be considered the most efficient.

According to the author,[Footnote: Technikum, 1912, 20, 156.] such sulphitecellulose extracts precipitate gelatine, aniline hydrochloride, ammoniacal zinc acetate, and basic coal-tar dyes, and give a greenish-black coloration with ferric chloride. These reactions indicate the presence of tanning matters in cellulose extracts.

The official shake method of analysis gives the following results:—[Footnote: Ibid.]

Tanning matters 23.0 per cent. Non-tannins 30.3 " Insoluble matters 0.7 " Water 46.0 " ———————- 100.0 per cent.

Ash 4.3 " Sulphurous acid 0.6 "

Many other substances have been used for tanning experiments, a number of them precipitating gelatine. Zacharias [Footnote: Zeits. f. Ang. Chem., 1907, 1645.] obtained leather by the action of many coal-tar dyes on pelt, similarly Herzog and Adler, by using Prussian blue, Neufuchsin, patent blue V, crystal violet, and colloidal gold.

Most inorganic substances possess tanning properties when in the colloidal state, e.g., sulphur, halogens, chromium salts, iron salts, silver oxide, and the salts of mercury, copper, bismuth, zinc, lead, platinum, cesium, vanadium, and the rare earths (salts of cerium, lanthanum, didymium, neodymium, thorium, and zerconium).

For practical purposes, however, only sulphur, chrome, and alum salts are used, the latter two being of the greatest importance.



SECTION IV

METHODS OF EXAMINING TANNING MATTERS

Whereas the evaluation of vegetable tanning matters necessitates determinations of their practical applicability in addition to qualitative and quantitative analyses, the latter two determinations are of practically no value when dealing with synthetic tannins. The way in which tanning matters obtained by chemical means exert their action, in addition to the intensity with which they convert pelt into leather, is the only criterion of their quality for practical (tanning) purposes; both may be demonstrated by experimental tests.

When dealing with the natural tanning materials it is desirable to know their contents of actual tanning matter, from which their special qualities as tanning agents may be deduced. Where the vegetable tanning materials have already been converted into extracts, it is essential to establish the identity of the original material used by the qualitative reactions of the extract in addition to the quantitative estimation of actual tannin contents. It is frequently necessary to examine whether the extract in question has been actually prepared from the material giving the extract its name, or whether the extract has suffered the addition of other extracts of tanning materials of but low quality. Such determinations may be undertaken by microscopical observations and by means of qualitative and quantitative reactions; for this purpose many colour reactions and precipitation methods are available in addition to the determination of the molybdenum figure (Lauffmann),[Footnote: Collegium, 1913, 10.] the alcohol and ethyl acetate figures and microscopical examination (Grasser).[Footnote: Ibid., 1911, 349.] Of other adulterants tending to reduce the quality of extracts may be mentioned sugars, mineral salts, and coal-tar dyes; [Footnote: Grasser, Collegium, 1910, 379.] for the determination of these, the special literature should be consulted. [Footnote: Grasser, "Handbuch f. gerbereichem. Laboratorien" (Leipzig, 1914); Procter-Paessler, "Gerbereichem. Untersuchungen" (Berlin, 1901).]

Two methods are devised for the purpose of quantitatively determining the tannin contents, both of which employ hide powder, and which are known as the "shake method" and the "filter bell method" respectively: the former is adopted as the official method of the "International Association of Leather Trades' Chemists" (I.A.L.T.C.). [Footnote: And also by the Society of Leather Trades' Chemists.-Transl.]

The original method, [Footnote: Leather Manufacturer, 1894, No. 9 J.S.C.I.,1894, 494.] worked out in the laboratory of the Yorkshire College (now the University of Leeds), essentially consists in introducing 6-9 gm. of hide powder in a shaker, washing it at least twice with distilled water and carefully squeezing out the powder in a linen cloth between each washing. 100 c.c. of the solution to be examined, which may not contain more than 1 per cent, total solids, are introduced into the shaking bottle which is then weighed. About one-third of the washed hide powder is then added, and the bottle shaken ten to fifteen minutes; another third is then added and, after shaking, the third portion. The bottle plus contents is now weighed, and the amount of hide powder introduced ascertained by difference of the two weighings. The liquid is then filtered through filter paper, 50 c.c. of the clear filtrate evaporated in a basin, dried and weighed. The residue in the original solution is then obtained by multiplying the former by 100 (plus weight of water added with hide powder), and dividing by 100.

This method was closely investigated by a large number of leather trades' chemists, was considerably improved, and in its final form presented a method of the highest degree of accuracy; the method was therefore adopted as The Official Method of Tanning Analysis by the I.A.L.T.C., which body, at the same time, gave precise instructions as to the details of the method. The latest instructions, which are reprinted below, permit of any method of analysis which observes the following conditions:—

1. The solution for analysis must contain between 3.5 and 4.5 gm. of tanning matter per litre, and solid materials must be extracted so that the greater part of the tannin is removed at a temperature not exceeding 50 C.

2. The total solubles must be determined by the evaporation of a measured quantity of the solution previously filtered till optically clear, both by reflected and transmitted light. This is obtained when a bright object such as an electric light filament is distinctly visible through at least 5 cm thickness, and a layer of 1 cm. deep in a beaker placed on a black glass or black glazed paper appears dark and free from opalescence when viewed from above. Any necessary mode of filtration may be employed, but if such filtration causes appreciable loss when applied to a clear solution, a correction must be determined and applied as described in paragraph 6.

Filtration shall take place between the temperatures of 15 C. and 20 C. Evaporation to dryness shall take place between 98.5 C. and 100 C. in shallow, flat-bottomed basins, which shall afterwards be dried until constant at the same temperature, and cooled before weighing for not less than twenty minutes in air-tight desiccators over dry calcium chloride.

3. The total solids must be determined by drying a weighed portion of the material, or a measured portion of its uniform turbid solution, at a temperature between 98.5 C. and 100 C. in shallow, flat-bottomed basins, which shall afterwards be dried until constant weight at the same temperature, and cooled before weighing for not less than twenty minutes in air-tight desiccators over dry calcium chloride.

"Moisture" is the difference between 100 and the percentage of total solids, and "insoluble" the difference between "total solids" and "total solubles."

4. Non-Tannins.—The solution must be detannised by shaking with chromed hide powder till no turbidity or opalescence can be produced in the clear solution by salt-gelatine solution. The chromed powder must be added in one quantity equal to 6.0-6.5 gm. of dry hide powder per 100 c.c. of the tanning solution, and must contain not less than 0.2 per cent. and not more than 1 per cent. of chromium calculated on the dry weight, and must be so washed that in a blank experiment with distilled water, not more than 5 mg. of solid residue shall be left on evaporation of 100 c.c. All water contained in the powder should be determined and allowed for as water of dilution.

5. Preparation of Infusion.—Such a quantity of material shall be employed as to give a solution containing as nearly as possible 4 gm. of tanning matter per litre, and not less than 3.5 or more than 4.5 gm. Liquid extracts shall be weighed in a basin or beaker and washed with boiling water into a litre flask, filled up to the mark with boiling water, and well mixed and rapidly cooled to a temperature of 17.5 C., after which it shall be accurately made up to the mark, again well mixed, and filtration at once proceeded with. Sumac and myrabolam extracts should be dissolved at a lower temperature.

Solid extracts shall be dissolved by stirring in a beaker with successive quantities of boiling water, the dissolved portions being poured into a litre flask, and the undissolved being allowed to settle and treated with further portions of boiling water. After the whole of the soluble matter is dissolved, the solution is treated similarly to that of a liquid extract.

Solid tanning materials, previously ground till they will pass through a sieve of sixteen meshes per square centimetre, are extracted in Koch's or Procter's extractor with 500 c.c. of water at a temperature not exceeding 50 C.; the extraction is then continued with boiling water till the filtrate amounts to 1 litre. It is desirable to allow the material to soak for some hours before commencing the percolation, which should occupy not less than three hours, so as to extract the maximum of tannin. Any remaining solubles in the material must be neglected or reported separately as "difficultly soluble" substances.

The volume of liquid in the flask must, after cooling, be accurately made up to 1 litre.

6. Filtration.—The infusion shall be filtered till optically clear (vide 2). No correction for absorption is needed for the Berkefeld candle, or for S. and S. 590 paper [Footnote: Schleicher and Schll, Dren (Rheinland), Germany.] if a sufficient quantity (250-300 c.c.) is rejected before measuring the quantity for evaporation, and the solution may be passed through repeatedly to obtain a clear filtrate.

If other methods of filtration are employed, the average correction necessary must be determined in the following manner:—About 500 c.c. of the same or a similar tanning solution is filtered perfectly clear, and after thorough mixing 50 c.c. is evaporated to determine "Total Soluble A." A further portion is now filtered in the exact method for which the correction is required (time of contact and volume rejected being kept as constant as possible), and 50 c.c. is evaporated to determine "Total Soluble B." The difference between "A" and "B" is the correction sought, which must be added to the weight of the total solubles found in analysis. An alternative method of determining correction, which is equally accurate and often more convenient, is to filter a portion of the tanning solution through the Berkefeld candle till optically clear, which can be generally accomplished by rejecting 300 or 400 c.c., and returning the remaining filtrate repeatedly; and at the same time to evaporate 50 c.c. of the clear filtrate obtained by the method for which correction is required, when the difference between the residues will be the correction sought. An average correction must be obtained from at least five determinations. It will be found that this is approximately constant for all materials, and amounts in the case of S. and S. 605, 150 c.c. being rejected, to about 0.005 gm., and where 2 gm. of kaolin are employed in addition to 0.0075 gm. The kaolin must be previously washed with 75 c.c. of the same liquor, which is allowed to stand fifteen minutes and then poured off. Paper 605 has a special absorption for a yellow colouring matter often contained in sulphited extracts.

7. Hide powder shall be of a woolly texture, thoroughly delimed, preferably with hydrochloric acid. It shall not require more than 5 c.c. or less than 2.5 c.c. of decinormal NaOH or KOH to produce a permanent pink colour with phenolphthalein on 6.5 gm. of the dry powder suspended in water. If the acidity does not fall within these limits it must be corrected by soaking the powder before chroming for twenty minutes in ten to twelve times its weight of water, to which the requisite calculated quantity of standard alkali or acid has been added. The hide powder must not swell in chroming to such an extent as to render difficult the necessary squeezing to 70-75 per cent. of water, and must be sufficiently free from soluble organic matter to render it possible in the ordinary washing to reduce the total solubles in a blank experiment with distilled water below 0.005 gm per 100 c.c. The powder, when sent out from the maker, shall not contain more than 12 per cent. of moisture, and shall be sent out in air-tight tins.

The detannisation shall be carried out in the following manner:—

The moisture in the air-dried powder is determined, and the quantity equal to 6.5 gm. actual dry powder is calculated, which will be practically constant if the powder be kept in an air-tight vessel. Any multiple of this quantity is taken according to the number of analyses to be made, and wet back with approximately ten times its weight of distilled water. Two grammes per 100 of dry powder of crystallised chromic chloride, CrCl3.6aq., is now dissolved in water and made basic with 0.6 gm. of Na2CO3 by the gradual addition of 11.25 c.c. of normal Na2CO3, thus making the salt correspond to the formula Cr2Cl3(OH)3. In laboratories where analyses are continually being made, it is more convenient to employ a 10 per cent stock solution, made by dissolving 100 gm. of Cr2Cl6.6aq. in a little distilled water in a litre flask and very slowly adding a solution containing 30 gm. of anhydrous sodium carbonate, with constant stirring, finally making up to the mark with distilled water and well mixing. Of this solution 20 c.c. per 100 gm., or 1.3 c.c. per 6.5 gm. of dry powder, should be used. This solution is added to the powder, and the whole churned for one hour. At the end of the one hour the powder is squeezed in linen to free it as far as possible from the residual liquor, and washed and squeezed repeatedly with distilled water, until, on adding to 50 c.c. of the filtrate one drop of 10 per cent. K2CrO4 and four drops of decinormal silver nitrate, a brick-red colour appears. Four or five squeezings are usually sufficient. Such a filtrate cannot contain more than 0.001 gm. of NaCl in 50 c.c.

The powder is then squeezed to contain 70-75 per cent, of water, and the whole weighed. The quantity Q containing 6.5 gm. dry hide is thus found, weighed out, and added immediately to 100 c.c. of the unfiltered tannin infusion along with (26.5-Q) of distilled water. The whole is corked up and agitated for fifteen minutes in a rotating bottle at not less than 60 revs. per minute. It is then squeezed through linen, the fitrate stirred and filtered through a folded filter of sufficient size to hold the entire filtrate, returning till clear. Sixty c.c. of the filtrate is then evaporated and calculated as 50 c.c., or the residue of 50 c.c. multiplied by 6/5. The non-tannin filtrate must give no turbidity with a drop of a solution of 1 per cent, gelatine and 10 per cent, common salt. [Footnote: It is convenient for technical purposes to employ the commercially obtainable chromed hide powder as prepared, for instance, by the German Experimental Station at Freiberg, Saxony.]

One gramme of kaolin, freed from all soluble matter, may be added to the filtrate, or it may be used by mixing it with the hide powder in the shaking bottle.

The analysis of used liquors and spent tans shall be made by the same methods as are employed for fresh tanning materials; the liquors being diluted, are concentrated by boiling in vacuo, or in a vessel so closed as to restrict access of air, until the tanning matter is if possible between 3.5 and 4.5 gm. per litre, but in no case beyond a concentration of 10 gm. per litre of total solids, and the weight of hide powder used shall not be varied from 6.5 gm.

The results shall be reported as shown by the direct estimation, but it is desirable that in addition efforts shall be made, by determination of acids in the original solution and in the non-tannin residue, to ascertain the amount of lactic and other non-volatile acids absorbed by the hide powder, and hence returned as "tanning matters."

In the case of tanning materials it must be clearly stated in the report whether the calculation is on the sample with moisture as received, or upon some arbitrarily assumed percentage of water; and in that of liquors whether the percentage given refers to weight or to grammes per 100 c.c., and in both cases the specific gravity shall be reported.

All analyses reported must be the average result of duplicate determinations, which must agree in the case of liquid extracts within 0.6 per cent, and of solid extracts within 1.5 per cent, or the analysis shall be repeated until such agreement is obtained.

All reports shall be marked: Analysed in accordance with the rules of the S.L.T.C. (I.A.L.T.C.)—when the analyses have been carried out according to the method described above.

As has been repeatedly emphasised in this treatise, the synthetic tannins form a special class of substances, and the results obtained by either of the two hide-powder methods do not give figures which are always comparable to those of the natural tannins. An example of the inapplicability of the methods where synthetic tannins are concerned is illustrated by the behaviour towards hide powder of them when partly neutralised to varying degrees: commercial Neradol D of acidity 1 gm.= 10 c.c. N/10 NaOH contains 33 per cent. tanning matters, completely neutralised Neradol D, which exerts no true tanning action on pelt, still contains 20 per cent tanning matter when analysed according to the Official Method; a difference hence exists regarding the adsorption by hide powder of a tannin and the adsorption of the latter by hide. As, however, we are unable to make a distinction between these two different properties by using hide powder only, we are also unable to draw the factor into account.

Another source of error is the swelling influence on hide powder by acids; for instance, an acid extract of vegetable tannins would show higher tannin contents in the analysis than would the same extract when less acid. The free sulphonic acid, however, is the active principle in synthetic tannins, and since the latter always contain other acids (of organic and inorganic origin) devoid of tannoid character, a source of error is thus introduced, which we cannot eliminate by the present method of analysis.

Of other methods of estimating the quality of a tanning material or tanning extract the determination of solubility, ash, colour, and weight-giving properties in addition to the firmness imparted to the leather by the particular material are of importance. As regards the synthetic tannins they are as a rule very soluble and it will generally be found sufficient to subject them to the ordinary qualitative examination. The ash determination in synthetic tannins, on the other hand, is not of such value as in the case of natural tanning extracts. From their composition we know that synthetic tannins contain considerable quantities of mineral salts, the presence of some of which on the one hand emphasises their pickling effect, and that on the other hand the property of dissolving phlobaphenes exhibited by the synthetic tannins is closely connected with their salt contents.

A colour determination of synthetic tannins is not of much importance, since synthetic tannins nearly always impart a white or light brown colour to the hide. In those cases only where coloured decomposition products appear as a result of intermediary reactions, may the former impart greyish or dirty colorations of little beauty to the hide. This is easily ascertained by lightly tanning a pelt.

The determination of the weight and solidity-giving properties is important both for leathers tanned with vegetable tanning extracts and for those treated with synthetic tannins, but the results obtained when using animalised cotton are not directly convertible into figures required for practical purposes. Comparative figures are better obtained by actually tanning pieces of pelt on as practical a scale as is possible, and testing the weights and tensile strengths of the pieces as against those of the original pelts, whereby in the former case the yield (pelt —> leather) is obtained.

Its capability as a tanning agent may be ascertained by submitting the synthetic tannin to an actual test tannage. The latter is carried out by introducing the dilute extract into open glass jars, holding about 400 c.c. at a width of about 8 cm. [Footnote: Accumulator jars are excellent for the purpose.—Transl.] The concentration of the solution is chosen according to acidity and salt contents of the synthetic tannin, the most suitable being 1.5-2.5 B. A piece of bated pelt is suspended in the liquor in such a way that the pelt is completely surrounded by liquor, without, however, being creased or touching the bottom. If the pelt were creased during tannage, the wrinkles would become fixed and would show in the finished leather. Thus an unfair judgment of the extract would be delivered, since similar results are produced by liquors which are either too concentrated or are not properly composed, and naturally this property of an extract would be greatly to its disadvantage.

The various stages of tannage may be judged from various standpoints when examining the pelt as tannage proceeds. On the one hand, the surface of the but slightly porous pelt is altered so as to present a more porous appearance, which is now rendered more capable of absorbing liquids. On the other hand, a similar alteration takes place within the pelt, to the extent to which the tanning matter has penetrated it. How far the penetration has proceeded is easily determined by utilising the different adsorption of coal-tar dyes by untanned and tanned pelt (see p. 121). An indicator for those synthetic tannins, which are derived from the phenols, is ferric chloride, which only colours those parts of the pelt which have been penetrated by the synthetic tannins; clearer and better results are, however, obtained when the dyestuffs referred to above are employed.

As soon as the tanning matter has completely penetrated the pelt, the total time of tannage is noted, and the velocity with which the tanning matter converts the pelt into leather at that particular concentration is thus obtained. The tannage completed, the leather must be well washed in running water to remove excess of synthetic tannin and then dried. On examining the dry leathers, the colour may then be observed, and a cut will give an idea of the tensile strength and the length of fibre of the leather. The tensile strength is, however, not of much value in such a barely tanned leather and cannot be compared with that obtained in leathers tanned on a practical scale. The length of fibre is, however, of some importance, since a special feature of finished leathers tanned with synthetic tannins is the beautifully long fibre—a property which manifests itself when the leather is torn and in which an expression of the quality of the synthetic tannin may be found.

Similarly, tanning experiments combining synthetic and natural tannins may be carried out, the most interesting features of these being the different proportions in which the two products are mixed. Such experiments may be done, for instance, by preparing 2 B. solutions of each extract and then mixing them in proportions of, say, 10:90, 20:80, 30:70, etc. Here it is again possible to infer the tanning intensity of the synthetic tannin from the concentration and the time used for tannage.

A further determination of the quality of a synthetic tannin is the capability of the latter of dissolving or precipitating the natural tannins. As is well known, synthetic tannins frequently possess the practically important property of rendering natural tannins easily soluble in water. In some cases, however, synthetic tannins appear to solubilise natural tannins in concentrated solutions; when, however, the latter are diluted, the natural tannin is precipitated with varying completeness, the reason of which is often the presence of excessive acid or the presence of such salts as have no phlobaphene-solubilising properties.

For practical purposes this determination may be carried out by mixing, in different proportions, concentrated tannin solutions and the synthetic tannin; heating the mixture on the water bath for a short time, cooling and finally diluting 10, 20, and 30 gm. of the mixture to 100 c.c., which are then left in measuring cylinders for twelve to twenty-four hours; the amount deposited will then be an indication of the solubilising or precipitating effect exhibited by the synthetic tannin.

Other properties of the synthetic tannins connected with their practical application will be discussed in Part II. of this treatise.



PART II

SYNTHETIC TANNINS: THEIR INDUSTRIAL PRODUCTION AND APPLICATION

With regard to their industrial production, but few synthetic tannins are, to-day, of practical and commercial interest. In addition to simplicity in the method of manufacture a certain degree of purity of the raw materials constitutes the criterion of their suitability. The methods of manufacture, of which nearly all are the property of the B.A.S.F., have been so worked out that the production of synthetic tannins presents no difficulties on a practical scale. Cresols, naphthalenes, and higher hydrocarbons are used as starting materials in the production of synthetic tannins; the former substances or their oxidation products are sulphonated by means of concentrated sulphuric acid, and the tanning matter produced by condensing the sulphonic acids with formaldehyde. The crude synthetic tannin thus obtained has yet to be diluted and partly neutralised before it can be applied in practice, and this is carried out by mixing the crude product with strong caustic lye. By these means the high acidity is reduced to a suitable degree learned from experience on the one hand; on the other hand, the salts of the sulphonic acids form valuable components of the commercial synthetic tannins.

The first product placed on the market was named Neradol D; this represents the condensation product of cresolsulphonic acid. The second synthetic tannin was Neradol N, which represents the condensation product of naphthalenesulphonic acid; when diluted and neutralised to the same extent as is done in the case of Neradol D, the product is named Neradol N D. The latest synthetic tannin has been called Ordoval G, the starting material of which is a still higher hydrocarbon.

The tannoid-chemical properties of these synthetic tannins have been exhaustively determined by the author, who employed Neradol D, which is most suitable for such a purpose, and the investigations relating to it will now be treated fully in the following chapters. The two other synthetic tannins exhibit very similar properties, but their few characteristics shall be shortly dealt with.

The condensation product obtained by the method described on p. 55 forms a viscous, dark coloured mass, the analysis of which by the shake method gives the following figures:-

Tanning matters 62.6 per cent. Non tannins 6.4 " Insolubles 0.0 " Water 31.0 " ———————- 100.0 per cent.

Acidity: 1 gm. = 40 c.c. N/10 NaOH.

According to its chemical constitution, this product may be considered to be dinaphthylmethanedisulphonic acid.

Samples of this crude, strongly acid material were partly neutralised, and the following figures obtained on analysis:—

Acidity. Tanning Soluble Water. Matters. Non-tans.

Per Cent. Per Cent. Per Cent. 1 gm. = 35 c.c. N/10 NaOH 61.8 7.0 31.2 1 " = 30 " " 58.9 7.1 34.0 1 " = 25 " " 50.1 7.9 42.0 1 " = 20 " " 42.2 8.9 48.9 1 " = 15 " " 37.4 10.4 52.2 1 " = 10 " " 31.6 13.6 54.8 1 " = 5 " " 26.3 16.6 57.1

Experimental tanning tests which were carried out with the various partly neutralised samples yielded leathers which, on an average, were nearly white, but which in comparison with a leather tanned with Neradol D appeared rather more greyish and were much harder.

A solution of the half-neutralised substance (1gm. = 20 c.c. N/10 NaOH) gives the following reactions:—-

Gelatine—Precipitate, partly soluble in excess tannin solution. Ferric chloride——-No coloration. Barium chloride——-Precipitate, insoluble HNO_3. Bromine water——-No reaction. Silver nitrate——-No reaction. Aniline hydrochloride——Precipitate, dissolves when solution is heated.

This condensation product is very soluble in water, but insoluble in most solvents, excepting methyl and ethyl alcohols. The above reactions show the similarity of this dinaphthyl derivative to the dicresyl derivative, and the absence in the former of characteristic reactions with iron salts is mainly accounted for by its lack of phenolic groups. The absence of this reaction does not, of course, influence the tannoid character of dinaphthylmethanedisulphonic acid in the least, and is of no importance in practice, since the various stages of tannage may be demonstrated by means of a solution of indigotine.

From a technical point of view the absence of this reaction is advantageous to this extent, that it eliminates the exceedingly great care to avoid the contact of tan liquors and tanned pelt with iron particles which has to be observed when tannins of phenolic character are employed.

In a chemical and technological evaluation of this tanning matter, all those details apply which will be described when discussing Neradol D. The most important advantage possessed by this tanning matter, from a commercial point of

view, is the lower price which it owes to the greater ease with which naphthalene may be obtained.

By treating the non-condensed crude product with barium chloride, a product completely devoid of sulphuric acid is easily obtained; the contents of sulphuric acid calculated as BaSO_4 is about 9.5 per cent. This value is higher than that found by Neradol D, and may be explained by the fact that a slight excess of sulphuric acid is necessary for the preparation of [Greek: b]-naphthalenesulphonic acid.

Comparative tanning tests using products containing sulphuric acid and products free from sulphuric acid (neutralised to the same degree of acidity) yielded leathers which were very similar; the liquor containing no sulphates yielded slightly softer leather than that obtained from a liquor containing sulphates.

An experiment was also carried out, using a liquor containing the tannin completely neutralised with caustic soda and subsequently acidified with acetic acid till the acidity of 1 gm. = 10 c.c N/10 NaOH; here, again, no essential difference could be detected in the leather as compared with that from a liquor containing sulphates.

One of the most striking properties of this tanning matter is its solubilising effect on natural tannins and the phlobaphenes; this property may mainly be compared to the similar one of other condensed sulphonic acids in their behaviour towards natural tannins.

If, therefore, natural tannins are mixed with this product and the solution used for tanning purposes, the resultant leather will possess a dark colour owing to the presence of solubilised phlobaphenes; if, on the other hand, a dark coloured leather, which has been tanned with natural tannins, is washed over with a 5 B solution of this synthetic tannin, or immersed for some time in the solution, the leather assumes a lighter colour owing to the phlobaphenes being dissolved and removed from the leather by the synthetic tannin.

The presence of Neradol ND in leathers is detected by methods to be described under Neradol D (cf. p. 108). The oxyazo reaction only succeeds when the solution has been boiled with a few drops of hypochlorite solution, quickly cooled and excess of ammonia added. When applying the indophenol reaction, the solution must be treated as follows: 3-4 drops of hypochlorite solution is added, and the solution heated for a short time; or 5-6 drops hypochlorite solution may be added, and the solution left for some time, in which case the heating may be omitted. The solution is then made distinctly ammoniacal, 1-2 drops of dimethyl-p-phenylenediamine solution and a layer of alcohol poured on the top. In most cases a blue coloration will appear; the addition of 1-2 drops of potassium ferricyanide solution with formation of a blue coloration indicates the presence of Neradol ND without fail.

The fact that a product possessing tanning properties may be obtained by condensing [Greek: b]-naphthalenesulphonic acid makes it interesting to investigate the behaviour of a non-condensed [Greek: b]-naphthalenesulphonic acid towards pelt. The following solutions were allowed to act upon pelt for twelve days:—

(1) Concentrated solution of [Greek: a]-naphthalenesulphonic acid (10 B). (2) " " [Greek: b]- " " (6 Be.) (3) " " 2,7- " " (18 B.).

Solution 1 swells the pelt to a considerable extent without, however, solubilising it. Solution 2 produces a similar effect. Solution 3 dissolves the pelt appreciably on the first day; after six days, solubilisation is complete. The reason of this different behaviour of the mono- and disulphonic acids is mainly to be sought in their difference of solubility; the monosulphonic acids are not very soluble, and are only capable of giving solutions measuring 10 and 6 B, respectively, whereas the disulphonic acid yields an 18 B solution, in addition to which the much higher acidity of the latter quickly gelatinises the pelt.

As regards the capability of the naphthalenesulphonic acids of dissolving phlobaphenes, the following results were obtained:—solid Argentine quebracho extract was mixed with—

5 percent, [Greek: a]-naphthalenesulphonic acid: opaque sol., large quantity of insolubles. 10 " " " lesser " " 20 " " " no insolubles. 30 " " " " 5 " [Greek: b]-naphthalenesulphonic acid: opaque sol., lesser quantity of insolubles. 10 " " " " 20 " " " clear solution, no insolubles 30 " " " " 5 " 2,7-naphthalenedisulphonic acid: opaque sol., large quantity of insolubles. 10 " 2,7 " " as above. 20 " 2,7 " " slightly opaque, some insolubles. 30 " 2,7 " " nearly clear solution, no insolubles.