Synthetic Tannins, Their Synthesis, Industrial Production and Application Part 13

Chestnut Quebracho Per Cent. Per Cent.

Tanning matters 32.0 34.0 Soluble non-tannins 12.0 8.0 Insolubles 1.5 2.0 Water 54.5 56.0 ----- ----- 100.0 100.0

Ash 0.4 2.0

This comparison shows that extracts of natural tannins firstly contain certain amounts of "insolubles," whereas Neradol is completely soluble in water, forming a clear solution; secondly, natural tanning extracts contain smaller quant.i.ties of soluble non-tannins, consisting of colouring matter and sugars, in addition to small quant.i.ties of mineral matters (ash). Neradol D contains considerable amounts of soluble non-tannins, derived from salts of sulphonic and sulphuric acids, again offering a satisfactory explanation of the high ash. If, therefore, a mixture of Neradol D and a natural tanning extract was submitted to a quant.i.tative a.n.a.lysis, the higher non-tannins and the high ash would indicate the presence of Neradol D, provided that wood pulp or a highly sulphited extract were not components of the mixture.

The chemical reactions taking place in the preparation of Neradol D may be expressed thus:-

OH OH OH OH H ^ H___O_____H ^ H H ^ H__CH_2__H ^ H | | || | | = | | | | + H_2O | | CH_2 | | | | | | H v CH_3 CH_3 v H v CH_3 CH_3 v H HSO_3 HSO_3 HSO_3 HSO_3

1. Neradol D Reactions

1. The quant.i.tative determination of phenols introduced by Bader, [Footnote: _Bull. soc. scient., Bucarexi_, 1899, 8, 51.] which consists in precipitating them as oxyazo compounds, has been modified by Appelius and Schmidt [FootNote: _Collegium_, 1914, 597.] for the purpose of detecting Neradol D:--To 50 c.c. of the tannin solution (a.n.a.lytical strength) 15 c.c. of diazo solution are added, the mixture filtered, if necessary, and the filtrate made alkaline with caustic soda; in the presence of Neradol D in sufficient quant.i.ty, a blood-red coloration results. If but little Neradol D be present, the procedure is altered as follows:--The tannin solution, to which the diazo solution has been added, is filtered, and the filtrate poured on a piece of filter paper which is then dried; a solution of caustic soda is spotted on the paper, when, if Neradol D be present, a red-edged spot will appear.

According to Tschirch and Edner, [Footnote: _Archiv. d. Pharm_., 1907, 150.] the diazo solution is prepared as follows:--5 gm._p_-nitraniline are introduced into a 500 c.c. measuring flask, 25 c.c. of water and 6 c.c. concentrated sulphuric acid added, the mixture shaken and a solution of 3 gm. of sodium nitrite in 25 c.c. of water plus 100 c.c. of water added, and the whole then filled up to 500 c.c. The solution should be stocked in the dark.

2. A less sensitive reaction for Neradol and wood pulp extract const.i.tutes that of Appelius and Schmidt employing cinchonine, [Footnote: _Collegium_ 1914, 597.] while the presence of the substances in question yields characteristic precipitates.

3. Seel and Sander [Footnote: _Zeits. f. ang. Chem._, 1916, 333.]

recommend the following method of detecting Neradol D:--

_(a) Oxyazo Reaction_.--About 5 c.c. of the tannin solution are rendered alkaline with caustic soda; after cooling with ice, about half the volume of alcohol is added. 3-4 drops of diazo solution are then added. Frequently, this results in the solution a.s.suming a blue coloration. If not, the solution is acidified with hydrochloric acid, ether added, and the mixture well shaken. The water is now separated from the mixture, fresh water added, together with some caustic soda solution, when, if Neradol D be present, the salt of the colour acid formed dissolves in the water with a beautiful green or bluish-green colour. At the place of contact of the water and the ether a bluish-green ring appears.

The diazo solution is prepared by dissolving _p_-aminophenol or its hydrochloric in a little dilute hydrochloric acid, cooling in ice and carefully diazotising in the cold till a slight excess of nitrous acid is present. It is essential that this solution should be tested before use, and this is carried out by coupling it with an alkaline phenol solution; if a dark blue oxyazo colour is formed, the solution may be used. It must be kept cool by surrounding it with ice.

_(b) Indophenol Reaction_.--To 5 c.c. of the solution to be tested, a drop of a solution of dimethyl-_p_-phenylenediamine is added, the solution rendered alkaline with caustic soda and 1-2 drops of a 5 per cent. solution of pota.s.sium ferricyanide added. If Neradol D be present, a blue colour appears, either immediately or after some time. The reaction is rendered more sensitive if alcohol is carefully poured on the solution after it has been rendered alkaline, and pota.s.sium ferricyanide is then added. At the place of contact a blue layer is formed, which ultimately diffuses into the alcohol.

According to Lauffmann [Footnote: _Collegium_, 1917, 233.] the presence of natural tannins as well as that of wood pulp diminishes the sensitiveness of the reactions described above; [Footnote: _Zeits. f. ang. Chem._, 1916, 333.] this investigator recommends a modification of these reactions.

2. Electro-Chemical Behaviour of Neradol D

The author's investigations of the electro-osmosis of an aqueous solution of Neradol D [Footnote: _Collegium_, 1920, 597, 24.] proved that dicresylmethanedisulphonic acid exhibits anodic migration; hence this product possesses negative charge and acidic character. The impurities accompanying the synthetic tannin, _i.e._, salts, free sulphuric acid, and some phenols, migrated anodic and cathodic respectively, according to their charges. A Neradol D purified by electro-osmosis finally yielded a pure solution of dicresylmethanedisulphonic acid, which precipitated gelatine and exhibited p.r.o.nounced tanning effects, but gave a greenish-black coloration with iron salts. This conclusively proves that the blue coloration given by Neradol D with iron salts is no characteristic feature of the _pure_ synthetic tannin, but is caused by the phenolic impurities accompanying the latter. Especially the first stage of the electro-osmosis produces a cathodic migration of the phenols, which may then be detected at a cathode by means of the iron and bromine reactions.

It is characteristic of a dicresylmethanedisulphonic acid purified by electro-osmosis that it does not precipitate aniline hydrochloride. It appears, therefore, that this reaction--which is characteristic of most synthetic tannins--is again caused by the presence of impurities.

3. The Influence of Salts and Acid Contents on the Tanning Effect of Neradol D

Chemical a.n.a.lysis of crude Neradol revealed a natural dicresylmethanedisulphonic acid (the tanning agent) contents of about 68 per cent, which agrees fairly well with the calculated amount. Like other "strong" and "weak" acids this sulphonic acid exercises a strongly swelling influence on pelt. Whereas the effect of acid present in solutions of Neradol D of medium concentration and its tanning effect both influence the pelt and are fairly well balanced, this is not the case as regards highly concentrated and very dilute solutions. If, for instance, a very dilute solution of crude Neradol (about 0.25 Be.) is used, the tanning effect of this solution is exceedingly small and does not show itself till after several hours. The relatively high dissociation of the acids at this high degree of dilution causes an extremely rapid and strong swelling of the pelt, which has therefore absorbed its maximum amount of water (maximum swelling) before the tanning effect of the sulphonic acid comes into play and by fixing the surface of the pelt is enabled to prevent the excessive swelling effect of the acids.

The addition of neutral salts to the tan liquor diminishes the effect of the acids on pelt (dehydrates the pelt) and prevents "drawing" of the grain. If, for instance, common salt be added to a solution of crude Neradol, the original quant.i.ty of sulphonic acid present would remain constant, but the presence of salt would diminish the degree of dissociation and consequently the swelling. This effect is still more p.r.o.nounced when the absolute amount of free sulphonic acid is diminished. Hence, if crude Neradol is treated with increasing amounts of caustic soda, a series of products containing increasing quant.i.ties of salt and decreasing concentrations of sulphonic acid is obtained.

The acidity of the Neradols may be determined by t.i.tration with N/10 caustic soda; this procedure hence establishes a means of determining the (unknown) acidities which may be expressed in terms of c.c. N/10 NaOH. The acidity of crude Neradol was found to be--

1 gm. = 50 c.c. N/10 NaOH

_i.e._, 1 gm. of crude Neradol requires 50 c.c. N/10 NaOH for complete neutralisation; the decrease in acidity causes a decrease in contents of tanning matters and the quant.i.ties of salts increase. The following table gives the figures obtained by differently neutralised neradols:--

Acidity. Tanning Matters. Na_2SO_4.

Per Cent. Per Cent.

1 gm. = 50 c.c. 68 ...

1 gm. = 40 c.c. 59 4 1 gm. = 30 c.c. 50 8 1 gm. = 20 c.c. 41 12 1 gm. = 10 c.c. 33 17 1 gm. = 5 c.c. 28 20 1 gm. = 0 c.c. 20 ...

Tanning experiments with these different neradols (employing solutions of 1 Be. strength) demonstrated that neradols of acidity 50, 40, and 30 exerted strong swelling and gave comparatively hard leathers; neradols of acidity 20, 10, and 5 exert no swelling, yield quick tannage and soft leather. The swelling (hardening) effect of the acid and the dehydrating (softening) effect of the salts in this case, therefore, are well balanced, and this fact affords an explanation of the rapid change from hardening to softening effects exhibited by partly neutralised Neradol where less acid and a greater quant.i.ty of salts respectively are present.

It may finally be noted that the acidity of Neradol D, 1 gm. = 10 c.c. N/10 NaOH, has been found to be the most suitable one for practical purposes. The author has, however, successfully employed some neradols of considerably higher acidities. The acidity above mentioned is possessed by a Neradol D containing 17 per cent. ash and 30 per cent.

sodium sulphonates and Glauber's salts crystals respectively. This large quant.i.ty of salts present on the one hand effects the rapid pickle and tanning effect exhibited by Neradol D, on the other hand it also effects the softness in the leather resulting from its use either alone or in admixture with natural tannins.

4. Phlobaphene Solubilising Action of Neradols

A special feature of Neradol D is its property of solubilising phlobaphenes, which may be ascribed to its contents of sulphonic acids or their salts. In order to demonstrate whether the sulphonic acids and their salts are capable of solubilising the insoluble or sparingly soluble anhydrides of the tannins (the phlobaphenes) before and after condensation, the following experiments were carried out:--

Crude Argentine solid quebracho extract was converted into a highly viscous liquid by treating it for several hours with water at 100 C., and the anhydrides rendered insoluble by diluting the liquid with a large volume of cold water. The precipitate formed, consisting of quebracho phlobaphenes, was separated from the liquid by decantation, and purified by washing it several times with water. Each 10 gm. of this moist paste were treated in the cold with (_a_) free phenolsulphonic acid; (_b_) sodium phenolsulphonate; (_c_) crude Neradol and (_d_) Neradol D, 20 c.c. of water at 45 C. added, and the mixture allowed to cool slowly; the following solutions resulted:--

(_a_) Opalescent solution, much deposit, (_b_) Clear solution, no deposit.

(_c_) Nearly clear solution, very little deposit.

(_d_) Clear solution, no deposit.

This clearly proves that free and condensed phenolsulphonic acids as such are not capable of completely solubilising phlobaphenes, whereas the sodium salts of free and condensed phenolsulphonic acids possess this property. The salt contents of Neradol D, therefore, const.i.tute an advantage in this respect, that not only may Neradol D be mixed with solutions of any natural tannin without insolubles being thereby deposited, but it may also be added in large quant.i.ties to a tannin solution with the result that the sparingly soluble and wholly insoluble const.i.tuents (phlobaphenes) are completely brought into solution.

The practical importance of the solubilising effect of Neradol D relating to solid Argentine quebracho extract is demonstrated in the following series of investigations carried out by the author:-- [Footnote: _Collegium_, 1913, 478; Austr. Pat., 68, 796.]

Solid Neradol Matters Tanning Abs. Increase Argentine D. Calc. of Mixture Increase per Quebracho Found. in Tanning 100 gms.

Extract. Matters. Extract.

Gm. Gm. Per Cent. Per Cent.

100 0 66.0 66.0 ... ...

0 100 32.5 32.5 ... ...

90 10 62.7 64.7 2.0 2.2 80 20 56.1 58.7 2.6 3.3 60 40 52.6 56.9 4.3 7.1 50 50 49.3 55.2 5.9 11.8 30 70 42.6 47.3 4.7 15.6 20 80 39.2 42.3 3.1 15.5

The maximum solubilising effect is exhibited in the mixture of 70 parts of Neradol and 30 parts of quebracho, with an additional percentage of tanning matters in the mixture of 15.6 per cent.--a figure which is very nearly identical with that of the insolubles present in the original Argentine quebracho extract.

The phlobaphene-solubilising property of Neradol D is closely connected with the influence of the latter on the colour of leathers tanned with natural tannins. If, on the one hand, a pelt is tanned with natural (_i.e._, non-treated) quebracho extract, a rather light coloured leather results, the fleshy colour of which is characteristic of quebracho. The dark coloured phlobaphenes present, on account of their insolubility, will have no influence on the colour of the leather. If, now, the quebracho extract be treated with sulphite and bisulphite in the usual way, the phlobaphenes are solubilised, but the reducing effect of the bisulphite tends to brighten the colour of the otherwise dark coloured phlobaphenes as well as that of the soluble tannins, and a reddish-yellow coloured extract results, imparting its own colour to the pelt. When, on the other hand, the quebracho extract is solubilised by means of Neradol D, the phlobaphenes are brought into solution without reduction taking place, and a dark brownish-red extract results, which imparts a similar colour to the finished leather. This darkening effect of Neradol D is most conspicuous in the case of mangrove, maletto, and chestnut, but is absent in the case of algarobilla, dividivi, gambir, sumac, and valonea. The varying phlobaphene contents of the tannins easily afford an explanation of the different properties above alluded to: the mangrove phlobaphenes are dark coloured bodies, those of mimosa, maletto, and chestnut are of lighter colour, and the last-named tanning materials enumerated above are either devoid of phlobaphenes or possess them only as very light coloured bodies. Algarobilla, sumac, gambir, dividivi, and valonea, on the other hand, are a.s.sociated with large amounts of sparingly soluble ellagic acid, known as "bloom" or "mud"

which imparts a light colour to the finished leather, and conveniently covers the dark colour imparted to the leather by other tanning materials; for this reason the former are often used in the lay-aways or in the finishing processes.

Similar effects to those of Neradol D are exhibited by other salts of sulphonic acids, _e.g._, sodium benzylsulphanilate (Solvenol B.A.S.F., or solution salt ("Solutionsalz") Hoechst); the author prepared mixtures of such salts and untreated quebracho extract in order to determine their solubilising effects, and arrived at the following results:--

30 parts Solvenol plus 70 parts quebracho extract: clear solution, no deposit.

25 parts Solvenol plus 25 parts quebracho extract: clear solution, very little deposit

20 parts Solvenol plus 80 parts quebracho extract: nearly clear solution, very little deposit.

15 parts Solvenol plus 85 parts quebracho extract: slightly opaque solution some deposit.