I N D U S T R I A L A N D ENGINEERING CHEAUSTRY
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Vol. 21. No. 3
The State of Combination of Acid Sulfate in Chrome Leather’ Henry B. Merrill and Joseph G. Niedercorn A. F. GALLUN & SONSCOMPANY, MILWAUKEE, Wis.
Previous work has shown that much of the acid sulHROME leather, as it Gustavson4then foundthat fate of chrome leather is attached to chromium and emerges from the tanpart of the acid sulfate of that the diffusion-neutralization titration with methyl ning drum, contains a chrome leather is more resistred is the most reliable method for determining prolarge percentage of so-called ant to hydrolysis than is the “acid” sulfate. This percenttein-bound sulfate. From the behavior of the leather acid combined with collagen on neutralization, it is deduced that the difference in age is somewhat reduced by in pickled skin. If pickled partiaI neutralization before hydrolyzability between protein-bound and chromiumskin, in which, of course, all or during fat-liquoring, but bound sulfate is so small that, if all the protein-bound the acid is combined with the even in the finished state the sulfate is removed by neutralization, more will be protein, is suspendedin water, leather contains from 1 to 4 formed at once from acid liberated by hydrolysis of and alkali is added so as to the sulfato-chromi complex. From this it follows or 5 per ceqt hydrolyzable keep the solution just neutral sulfate as HZS04. On susthat the percentage of protein-bound sulfate existing to methyl red (pH = 5.0 to pending the leather in water, in any leather is a function of the chromium-bound 5 5 ) , all the acid is removed a portion of the acid sulfate acid content. This has been shown to be the case for from the skin and neutralized leathers containing a chromium complex of acidities hydrolyzes, giving a faintly in about 48 hours. But if acid solution. On adding an between one-sixth and one-third. When the acidity chrome leather is treated in excess of alkali, all the acid of the chromium complex is less than one-sixth, no the same manner, most of the sulfate is gradually neutralprotein-bound acid is present. When acid is added acid is still retained by the ized and removed. The acid to a leather containing a chromium complex of oneleather after 48 hours. Gussulfate resists washing, but third acidity, the acidity of the complex does not intavsonsuggested that the permay be removed com‘pletely crease, and all the added acid is protein-bound. centage of chromium-bound by extraction with water for acid in leather could be deterseveral weeks. I n what state of combination does this acid mined quantitatively by determining the percentage of acid sulfate exist in the skin? left in the leather after it had been subjected to this diffusionFrom its behavior as outlined above, it is evident that neutralization process for 2 days. The percentage of acid this “acid” sulfate is really sulfate combined with a weak combined with the protein can then be taken as the difference base, from which it is readily liberated by hydrolysis. There between total acid sulfate, obtained by the phosphate displaceare two constituents of chrome leather that possess weakly ment method, and chromium-bound sulfate. basic properties-collagen by virtue of its amino groups, and Gustavson6 later proposed the use of a dilute solution of chromium. Part a t least of the acid sulfate must be combined pyridine for the removal of protein-bound sulfate, instead with amino groups, unless these amino groups are all satu- of the tiresome diffusion-neutralization procedure. A 4 per rated by combination with chromium, for we know that a t the cent solution of pyridine, serving as a well-buffered, mildly pH values encountered in tanning amino groups of collagen alkaline solution of pH = 8.0, does remove all the acid from do combine with large amounts of acid. Until about four pickled skin (hence all the protein-bound acid from chrome years ago the leather chemist was free to believe, if he chose, leather), and leaves part of the acid sulfate of chrome leather that all the acid sulfate was collagen-bound, since there was untouched. Gustavson assumed that the undisplaced acid no evidence either way concerning the combination of sulfate was the entire chromium-bound acid, but Merrill, Niedercorn, with chromium in leather. As the simplest explanation, it and Quarck6 showed that the amount of sulfate left in the was suggested2 that all three valences of chromium are com- leather by pyridine is determined by such arbitrary factors bined with collagen, forming a normal chromium collagenate. as the volume of solution employed, the concentration of The sulfuric acid, set free from chromic sulfate, was naturally sulfates in the solution, and the total percentage of acid sultaken up by the amino groups as a side reaction. We now fate in the leather. If 2 grams of leather containing 5.5 know, however, from the work of Gustavson, that a large part grams acid sulfate per 100 grams collagen are digested with of the acid sulfate of chrome leather is combined with chro- 200 cc. of a 4 per cent solution of pyridine for 4 hours, about mium, from which it follows that chrome leather is a sulfato- half the acid sulfate is removed. Very little additional sulchromi-collagen compound, not a simple chromium collagen- fate is removed by further digestion in the same solution for 24 or 48 hours more. But if the leather is removed from the ate. Gustavson3 found that permutite combines with chromium solution and treated with fresh pyridine solution of the same salts very much as collagen does, and that the resulting com- strength as the first, about half the remaining acid sulfate is pounds contain hydrolyzable sulfate. But permutite, unlike speedily displaced. Merrill, Niedercorn, and Quarck concollagen, is in itself unable to combine with acids, and hence cluded that pyridine displaces part of the chromium-bound the acid sulfate taken up must be in combination with chro- sulfate, and that the apparent end point obtained by GUSmium. By analogy, part of the acid sulfate of chrome leather tavson is due to an equilibrium being set up when the tendis attached to chromium. ency of pyridine to displace sulfate is balanced by the tendency for sulfate in solution to displace pyridine. 1 Presented by H. B. Merrill before the Division of Leather and Gelatin Investigation of the diffusion-neutralization method for Chemistry at the 76th Meeting of the American Chemical Society, SwampScott, Mass., September 10 to 14, 1928. determining chromium-bound sulfate in leather led Merrill,
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Wilson, “The Chemistry of Leather Manufacture,” p. 308, New York,
1923.
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Gustavson, J . Am. Leather Chem. Assocn., 19, 446 (1924); Collegium, 1926, 97.
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Gustavson, J . Am. Leather Chem. Assocn., 21, 559 (1926). Gustavson, Ibid., 22, 60 (1927). Meirill, Siedercorn, and Quarck, Ibid., 23, 178 (1928).
March, 1929
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Kiedercorn, and Quarck to the conclusion that the method gives useful and reproducible results, provided a carefully standardized procedure is followed. They found, however, that the difference in hydrolyzability between protein-bound and chromium-bound sulfate is not very great. All the acid sulfate is removed from pickled skin in 2 days, if the skin is left in contact with a solution kept just neutral to methyl red. Presumably all the acid sulfate attached to protein of leather is removed in the same time by the same treatment. But after the 2 days are over, the leather still continues to give up acid to the solution, and alkali must still be added to maintain the solution a t the neutral point of methyl red. This continued in one experiment for 15 days, and apparently would have continued until most of the acid of the leather had been neutralized. This behavior is illustrated by Figure 1, which shows that the sulfato groups attached to chromium hydrolyze sufficiently to give a solution more acid than p H = 5.5. But a t pH = 5.5 or less, protein combines with acid. We must conclude, then, that, even though all the proteinbound acid of a leather is neutralized by alkali, more will be regenerFigure 1-Rate of Removal of Sulfuric Acid from Skin and from Chrome ated in a short-time, Leather in Contact with a Solution of through the liberation pH=5.4 of free HzS04byhydrolysis of sulfato groups, and the recombination of this acid with the basic groups of the protein. Analyses of many leathers a t different stages of the process bear out this conclusion. During the fat-liquoring operations, chrome calf leather may lose an amount of acid sulfate that far exceeds the small percentage of protein-bound sulfate present after tanning, and yet, when the fat-liquored and colored leather is analyzed, protein-bound sulfate will be found. As a general rule, protein-bound acid and chromiumbound acid seemed to diminish in the same proportion. This led to the suspicion that the so-called protein-bound sulfate found by analysis may be only a measure of the hydrolyzability of the chromium-bound acid, which would be expected to diminish with decreasing acidity of the chromium complex. T o test this hypothesis, experimental leathers were prepared by partially neutralizing the acid sulfate in a stock leather. After being washed and dried, these leathers were analyzed for total acid sulfate and chromium-bound acid sulfate. It was found that with a chromium complex of acidity between 33.3 and 17.4 [16.67(?)] the protein-bound acid as determined by analysis is a linear function of the total acid sulfate of the leather. When the acidity of the chromium complex falls below 17.4, the complex can no longer hydrolyze sufficiently to give a solution acid to methyl red, and the proteinbound acid found is zero. If acid is added to a leather containing a complex of 33.3 per cent acidity, all the acid taken up is bound by the protein, and the chromium-bound acidity is unaffected. This is shown by the data plotted in Figure 2. Details of the experiment are given below. Experimental
Two hundred grams of standard hide powder were tanned for 24 hours with 200 cc. of solution containing 16.2 grams
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Cr203per liter. The, solution was made by dissolving the requisite amount of a commercial basic chrome sulfate in water and filtering. After tanning, the powder was alternately treated with water and wrung out for six times. Eighty-gram portions of the damp powder (about 20 grams leather) were suspended in 200 cc. of water, and treated with different amounts of 1 N sodium hydroxide or 1 iV sulfuric acid. The samples were shaken with the acid or alkaline solutions for 2 hours, then transferred to Wilson-Kern extractors, and washed with running distilled water for 1 hour to remove neutral sulfates and the excess of alkali or acid solution. The powders were then dried a t room temperature and analyzed. Moisture, chromic oxide, and collagen were determined by the usual methods. Total and neutral sulfates were determined by the A. L. C. A. (phosphate displacement) method. Chromium-bound sulfate was determined by the diff usion-neutralization niethod using the procedure described by Merrill, Xedercorn, and Quarck. Total acid sulfate was taken as the difference between total and neutral sulfate, calculated as HzS04,and protein-bound sulfate was taken as the difference between total acid sulfate and chromium-bound sulfate. The acidity of the chromium complex was calculated: as the ratio of equivalents of Cr203to equivalents of chromium-bound HzS04. The results of this experiment are given in Figure 2. It will be noted that the leather as tanned contained a complex of 32.3 per cent acidity. Since, as pointed out by Merrill, Niedercorn, and Quarck, the diffusion-neutralization method for chromium-bound sulfates always gives results that are slightly low, this ind i c a t e s t h a t t h e : 10 50 w chromium complex fixed by the protein 2 ti was exactly one-third 2 acidic, containing s u l f a t o g r o u p per ti 6 atom of chromium. 2 Treating the leather 3. w i t h a c i d d i d n o t =$ z appreciably increase IO 2 the acidity of this complex. Partially neutralizing the leather with alkali de'e% IlmH creased the total acid E 20o . IrGs l Bdry a l d ar lenther. allrtll addeb to sulfate in direct pro- Figure 2-Relation between Total Sulportion to the amount furic Acid, Acidity of Chromi-Complex of alkali added, until and Protein-Bound Acid In Chrom; Leather the acidity of the complex had fallen to about half its initial value. Over this range the chromium-bound and protein-bound acid both decreased linearly with increasing neutralization of the leather. When the chromium-bound acid had been reduced to an amount corresponding to a complex of 17.4 per cent acidity [one-sixth acid complex (?)I the leather no longer contained proteinbound sulfate. Further neutralization merely reduced the acidity of the chromi-complex. Since this paper was written, Mr. Gustavson has kindly called our attention to the fact that our findings agree very nicely with some that he obtained in 1924.a His work, too, indicates that the complex fixed initially is a one-third acid sulfato-chromi compound, and that this complex is altered on washing to a more stable complex of approximately one-half the initial acidity. After being aged for 4 months the leathers were'again analyzed. Within the limits of experimental error, no change took place in the distribution of acid between protein and chromium during the aging period.
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