March, 1926
I S D r S T R l A L S S D ESGI,VEERISG CHESIISTRY
259
Sulfur Tannage’ By Arthur W. Thomas C O L I J M B IUNIVERSITY, A NEW YORK,N. Y .
sulfur (prepared by the oximate. H e was impressed dation-reduction r e a c t i o n by the apparent tanning Colloidal sulfur-i. e., sulfur hydrosol prepared by between sulfur dioxide and effect obtained. Odkn’smethod-is shown to be fixed by hide substance hydrogen sulfidein aqueous A p o s t o l o 3 claims that in a manner analogous to tannin but does not yield a 1 solution). H e found tanthis process actualXy yields true leather. Although most of it is subsequently exning action in every case, leather. The method used tractable by carbon disulfide, the between as judged by the appearance by him Tas the drumming Of the colloidal particles and the collagen apparently does of the treated skins. skins in ‘Oncentrated hypo not result from a mere deposition of the sulfur owing These experiments with to ’vhichlacticacid to coagulation of the sulfur hydrosol. It is due to skins indicated the desiraprogressively a d d e d . the combination of collagen cations and the anion, bility of a q u a n t i t a t i v e He found that a better sulthe complex xS.ySSO6HP. study of fixation of sulfur fur leather was obtained in by hide powder. this manner than by adding a large quantity of lactic acid Preparation of Sulfur Hydrosol init,ially. He describes a turbid solution due to the formation of colloidal sulfur upon the addition of the acid, which is Experiments were conducted in which the Wilson-Kern completely clarified by the skin after drumming for one-half met’hod of measuring the amount of tanning agent fixed by hour. The total time consumed in his progressive acidi- t’he hide powder was used. As tanning agent an aqueous fication and drummings was 10 to 12 hours. The tanned solution of colloidal sulfur prepared by OdBn’s method6 was skins were staked out t o dry, resulting in a beautifully white employed. OdCn’s sulfur hydrosol is prepared by the slow and soft leather, which was unaffected by subsequent soaking addition of 3 N sodium thiosulfate to 17 M sulfuric acid in in cold water for 24 hours. Extraction of the: leather with an ice bat.h. The colloidal sulfur is precipitated by sodium carbon bisulfide removed about 1 per cent sulfur, which chloride. The supernatant liquid is poured off. The colApostolo called mechanically bound sulfur. Analysis of loidal sulfur is repeatedly redissolved in water, reprecipitated the leather showed that a total of 2.5 to 3.5 per cent sulfur by salt, etc., until a clear yellow solution of colloidal sulfur was held by it as tanning agent. This leather was not re- is obtained. sistant to hot water. This sol was selected because it has been extensively The formation of colloidal sulfur through the acidification studied and it is known that the dispersed phase consists of sodium thiosulfate is due to the instability of thiosulfuric of sulfur combined with polythionic acids. Representing acid, which decomposes instantly according to the reaction the mixture of polythionic acids by pentathionic acid, the composition of the dissolved or dispersed phase of this sulfur H:S203 F1 HjS03 SJ hydrosol is zS.yS6O6H2,where z and y represent mols, r Recently Gallardo4and Thuauj have again called attention being, of course, much larger than y. This complex ionizes to the possibility of sulfur tanning. Gallardo recommends into zS.yS&&-- and 2yH+, thus accounting for the negative that the skins be drummed in a solution containing 5 per cent charge of the colloidal sulfur. hydrochloric acid, 30 per cent salt’, and 100 per cent water Nofe-The previous investigators regarded colloidal sulfur as pure sulfur. (referring to weight of raw skins) for 2 hours. The acid This is erroneous. A colloidal solution of a n insoluble substance free from skins are laid out overnight and then drummed for 4 t o “impurity” does not exist. There must be some soluble substance present 6 hours in a bath containing 25 per cent of sodium thiosulfate combined with t h e substance called t h e colloid. Vide the “Complex Theory” of colloids. I n O d h ’ s colloidal sulfur, t h e soluble p a i t of the dispersed and 100 per cent water heated to 100” F. The tanned skins phase is composed among other things of polythionic acids. This is also are described as “surprisingly white” and extraordinarily true for Wackenroeder‘s colloidal sulfur. soft. Gallardo warns that sulfur leather is not strong. Analysis of the sulfur hydrosol used in these experiments He recommends a retannage with chrome. It is interesting showed that 100 cc. contained 0.980 gram of nonvolatile 1 Presented before the Division of Leather a n d Gelatin Chemistry a t solids (mainly sulfur). The p H of the sol could not be meast h e 69th Meeting of t h e American Chemical Society, Baltimore. Md., ured electrometrically owing to “poisoning” of the electrode. April 6 t o 10, 1923. Contribution No. 492 from t h e Chemical Laboratories, I t s total acidity was measured by adding an excess of 0.2 N Columbia University. * This view is questioned by Gustavson, J . Am. Leafher C h t m . Assoc., sodium hydroxide and titrating back with 0.2 X hydrochloric 20, 382 (1923), who ascribes t h e quality of t h e leather t o (differences in t h e acid using phenolphthalein as indicator. This showed that nature of t h e chromium solution. the sol was acid to the extent of AT/40. 3 Collegium, 1913, 420. 4 H i d e and Leofhcr, 68, 4 0 (1924). 6 “ D e r Kolloide Schwefel,” Nova Acia Reg. SOC.Scient. Uprala, 141 3, 5 Cui?, 13, 127. 402 (1924). X o . 4 (1913).
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260
INDUSTRIAL AiVD ENGINEERING CHEMISTRY
Effect of Time a n d Concentration The first experiment was arranged to find the effect of time and concentration upon the fixation of sulfur by hide powder. Portions of hide powder (freed from fats) equal to 2.000 grams of absolutely dry substance were used in every case. The volume of the solutions was always 100 cc. I n the series given in Table I the amounts of water indicated were placed on the hide powder; the sulfur hydrosol was not added until the hide powder had thoroughly imbibed the water. Upon addition of the sulfur hydrosol, the bottles were rotated a t room temperature for the intervals noted. The contents were then filtered in Wilson-Kern extractors and washed free of all uncombined matter with distilled water (4 gallons or 15 liters for each sample). The washed “leathers” were then dehydrated with alcohol and ether, dried a t 100’ C. in vucuo for 16 hours, and weighed. The increase in weight was taken as the amount of sulfur fixed by the hide powder. All these are a trifle low owing to a small loss of hide substance through hydrolysis. Colloidal sulfur does not precipitate gelatin. All the filtrates contained sulfur hydrosol, showing that all the available sulfur had not been removed by the hide powder. Table I-Fixation Sulfur Water Sample Cc. 75 75 50 50 25 25
sol
Cc. 25 25 50 50 75 75
of Sulfur by Hide Powder
Available sulfur Gram 0.245 0.246 0,490 0.490 0.735 0.735
Time Hours 1.5 12 1.5 12 1.5
12
Sulfur fixed Gram
Sulfur extracted by CSa
Gram
0.010
0.038 0.057 0.100
0.061 0.191
The extraction of the dried samples by carbon disulfide apparently removed more sulfur from Samples 2 and 4 than was present. This seeming anomaly is due to the fact that part of the hide powder was lost through hydrolysis in the tanning operation and hence the values for “sulfur fixed” are low. Sample 6 it is seen did not release all of its combined sulfur to the solvent. It is evident that sulfur fixation is slow and increases with the amount of colloidal sulfur present. The reaction is in this respect like other tanning processes. It is apparent that the sulfur was not precipitated from solution on account of the presence of the hide powder. Effect of Acid in Sol O d h ’ s sulfur sol is stable only in acid solution, however, and since the hide powder could readily combine with the acid contained in the sol, the question might be raised whether this possibility might not account for the apparent fixation of sulfur by the hide substance. The nature of the values shown in Table I would indicate that the sol was not precipitated by the hide powder. If it were, one would expect to find more sulfur fixed by Samples 1 and 2. T o render this situation more certain the following experiment was performed: Three portions of hide powder (equal to 2.000 grams absolutely dry substance) were each soaked in 200 cc. N/40 sulfuric acid for 10 hours a t 0” C., then all but the amounts of solution noted in Table I1 as water present was drained off, the noted amounts of sulfur hydrosol added and rotated in bottles for 12 hours a t room temperature. The specimens were filtered, washed, and dried as before. Table 11-Fixation Sample 7 S
9
Water cc. 75 50 25
of Sulfur by Acidified Hide Powder Sulfur Available Sulfur sol sulfur fixed cc. Gram Gram 25 0.248 0.019 0.490 0.106 50 75 0.735 0,158
These values show a gradual increase in the amount of sulfur fixed with increase in amount of colloidal sulfur present.
Vol. 18, N o . 3
It is apparent that the deposition of sulfur is not due to coagulation of the sulfur hydrosol. One would expect a lower figure for the fixation of sulfur in this instance, as compared with the first, owing to the possibility of greater loss by hydrolysis in Sample 7. Furthermore, if the combination of zS . ySsOeH, with hide substance follows the mechanism of the Procter-Wilson theory, the rate of tannage would decrease if the hydrogen-ion concentration were increased beyond that for optimum fixation. While the pH of OdBn’s sulfur hydrosol cannot be measured electrometrically, i t can be estimated roughly. A solution of sulfuric acid of X/40 concentration has a pH value of about 1.8. While the acid in sulfur sol is not sulfuric acid, it cannot be stronger than sulfuric. Hence the sulfur hydrosol is not more acid than the region of p H = 2 . Consequently, increase in acidity should result in a decreased rate of tannage. Sample 8 shows a greater fixation than Sample 4 by 6 mg. This is within the error of the experiment found to be * 5 per cent of the total. Effect of Decreased Acidity The fixation of a colloidal acid (which OdBn’s colloidal sulfur actually is) would be expected to follow the mechanism of the Procter-Wilson theory. Since the acidity appears to be at or near the optimum for the maximum rate of tannage, it was believed that the applicability of the ProcterWilson theory could be tested by studying the tanning in solutions of decreased acidity. A series of solutions of diminishing acidity were prepared by adding 0.1 N sodium hydroxide as noted in Table 111. The addition of 13.1 cc. 0.1 N alkali would just neutralize 50 cc. of the sulfur sol to phenolphthalein. The 2-gram portions of hide powder were soaked overnight a t 0” C. in the amounts of water noted. Upon addition of the colloidal sulfur solutions, the tannage was carried out as before for 12 hours. Table 111-Effect of Acidity u p o n Sulfur Fixation (50cc of sulfur sol in each case) 0.1 N IiaOH Water Sulfur fixed cc. cc. Gram Sample None 50.0 0.112 10 11 2.5 47.5 0.082 5.0 45.0 0.074 12 7.5 42.5 0.062 13 10 0 40.0 0.032 14
The decrease in amount of “tannin” fixed in a given time a t steadily diminishing hydrogen-ion concentration is in accord with the Procter-Wilson theory. Decreasing the hydrogen-ion concentration by the addition of sodium hydroxide diminished the positive charge of the collagen and consequently its reactivity for the weak colloidal acid, sulfur hydrosol. It is difficult, if not impossible, to judge whether a given process yields good leather by experience with hide powder. As a consequence, this paper cannot contribute anything further concerning quality of leather than did Thuau or Gallardo. Resistance to Action of Water The sulfur “tanned” hide powder specimens were resistant to the action of cold water. Occasionally on drying, the fibers meshed and stuck together, which is not a favorable sign, although this may mean that the tannage had not been carried far enough. It was considered of interest to compare the resistance of two specimens of sulfur “tanned” hide powders with that of raw hide powder to the action of an aqueous solution at p H = 3.5, a t 80’ C. The p H = 3.5 solution consisted of the proper mixture of 0.1 N phosphoric acid and 0.1 sodium
