T H E J O U R N A L OF I N D C S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
Dec., 1 9 2 0
I ORIGINAL NATURE OF THE HIDE-TANNIN COMPOUND AND ITS BEARING UPON TANNIN ANALYSIS’ By John Arthur Wilson and Erwin J. Kern LABORATORIES
OP
A. F. GALLUN& SONS CO., MILWAUKEE, WISCONSIN Received September 2, 1920
I n an earlier paper2 the authors described a new method of tannin analysis which they believe gives the true t,anning value of vegetable materials. Comparative tests showed the official method of the American Leaeher Chemists’ Association t o be greatly in error, exceeding 2 0 0 per cent for typical samples of gambier extract. (The earlier paper should be referred t o for a working description of both methods.) The much higher percentages of tannin obtained by the A. L. C. A. method for every material examined were attributed t o the formation of readily hydrolyzable compounds between the hide powder and a variable fraction of the non-tannins. A t the 17th annual meeting3 of the A . L. C. A. a formal discussion4 of this paper was staged, and the chief aim of t h e opposition was apparently t o show t h a t the low results obtained by the new method were due t o losses of tannin in the manipulation. I t was contended t h a t a certain proportion of the tannin of a liquor will form a stable compound with hide only after long contact, and, further, t h a t even tannin which has already combined with the hide will be removed t o a n appreciable extent during the washing required by the new method; but no really conclusive evidence was offered in support of these contentions. The object of the present work was t o investigate these points because they are of great importance, not only t o the new method, but to the theory and practice of tanning itself. T A N N I N G -MATERIALS E X A J I I K E D
Certain differences in behavior of the several different tanning materials have caused a widespread belief t h a t some tannins form more stable compounds with hide t h a n others; for example, the tannin from gambier is supposed t o form a compound with hide less stable than t h a t from hemlock bark. I t has also been supposed t h a t mixtures of tanning materials behave differently in this respect from the individual materials. We have therefore chosen for examination typical commercial tanning extracts ranging in properties from the extrernes of gambier and sumac t o those of hemlock and quebracho, and have also included two mixtures, each consisting of a pair of extracts of very different nature. The analyses of the 8 extracts and z mixtures, both by the new method and by the official method of the American Leather Chemists’ Association, are given in Table I, along with the percentage errors involved in the A. L. C. A . method, assuming t h a t the new method is correct. 1 Pi.esented before the Section of Leather Chemistry at the 60th Meeting of the American Chemical Society, Chicago, I!l,, September 6 to 10,
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PAPERS
TABLE I-COMPARATIVE ANALYSESOF EXTRACTS STUDIED, BY N E W METHODAND B Y THE A. L. C. A. METHOD ---PERCENTAGE ANALYSISOF MATERIAL-Per----A L. C. A. METHOD-centage -Soluble MatterNew Error in Method A.L.C.A. Insoluble hTonEXTRACT Water Matter tannin Tannin Tannin Method Quebracho . . . . . . . 2 2 . 2 8 9.61 7.24 60.87 46.84 30 Gambier . . . . . . . 50.17 7.43 16.79 25.61 7.87 225 Quebracho - gam bier mixture . . . 4 4 . 5 3 7.39 14.86 33.22 20.67 61 Oak bark . . . . , . . . 5 2 . 4 3 3.10 18.28 26.19 15.52 69 Chestnut wood ,._. 52.47 3.64 18.19 25.70 13.99 84 Hemlock b a r k . . . 5 6 . 1 0 6.23 10.99 26.68 23.47 14 Chestnut woodhemlock bark 14.53 25.64 18.73 mixture . . . . . . . 5 3 . 8 9 5.94 37 7.42 18.36 22.96 11.29 103 Larchbark ...... 5 1 . 2 6 1.01 22.24 25.51 Sumac. . . . , , , . . . 5 1 . 2 4 16.36 56 14.75 1.50 33.55 24.66 Wattle bark ... .. 5 0 . 2 0 36
It is interesting t o compare the errors found for the sumac and hemlock bark extracts with those for the actual leaves and bark noted in the earlier paper. Extract manufacturers often remove only about 80 per cent of the tannin from the raw materials, because in order t o get the last 2 0 per cent they would have t o extract a large proportion of undesirable non-tannins t h a t would considerably lower the purity of their products. The raw materials used were extracted completely and therefore contained a higher ratio of non-tannin t o tannin t h a n the commercial extract, and this, as the authors have shown, increases the error obtained by the A. L. C. A. method. RESISTANCE O F LEATHER TO WASHING
Three methods of measuring possible losses of combined tannin during the washing of leather were considered. The first was t o wash the leather free from soluble matter and analyze i t for tannin, and then t o wash i t further and analyze it again. The second was t o use the gelatin-salt reagent ( I O g. gelatin plus I O O g. NaCl per liter) in testing the wash waters for tannin; provided all tests were negative, i t would show t h a t only a very small amount of tannin, if any, was washed out. The third was t o collect and weigh the soluble residues from a number of wash waters. Of these, the first method appeared most important and consequently received greatest attention. TABLE 11-SHOWING EFFECTOF EXCESSIVSWASHING OF THE TANNED HIDE POWDERUPON THE PER CENT OF TANNIN POUND BY THE NEWMETHOD Hide Substance in Powder Per Cent Tannin in Extract Used to Extract. Value Obtaiqed Grams Detannize from Analysis of in 200 Cc. -Tanned Powder Washed200 Cc. Soln. 15 25 50 EXTRACT Soh. Grams Times Times Times Quebracho ......... . . 3.80 10.44 46.84 47.25 46.90 Gambier.. . . .. . . . . 1 0 . 0 0 10.44 7.87 7.89 7.67 Ouebracho-gambier mixture’. . . _ ,. . . , 6.90 10.44 20.67 20.34 20.43 Oak bark . . . . .. . . .. . , 1 3 . 6 0 10.40 15.52 15.36 15.35 Chestnut wood. . . . . 13.60 10.32 . . . 3 13.99 13.93 Hemlock bark. . . . . . , 13 .OO 10.32 23.47 23.38 23.50 Chestnut wood-hemlock bark mixturez. 1 3 . 3 0 10.32 .,...3 18.73 19.05 Larch bark . . . . . . . , . . 13.60 10.32 . . . . .3 11.29 11.28 Sumac . . . . . , . . . . 13.00 10.39 16.36 16.29 16.39 Wattle b a r k . . . . . , . 8.00 10.32 24.66 24.16 24.73 1 Mixture of 19 parts solid quebracho extract to 50 of gambier extract. 2 Mixture of 68 parts of chestnut wood extract t o 65 of hemlock bark extract. 3 Calculation not made because 15th wash water gave test f o r nontannin with ferric chloride.
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1920. 2 “The ’ h i e Tanning Value of Vegetable Tanning Materials,” THIS JOURNAL, i a (ISJZO), 465. 8 Atlantic City, N. J., May 27 t o 29, 1920. 4 Printed in full, J . A m . Leather Chem. Assoc., 16 (1920), 451.
P R O C E D U R E A N D RESULTS-Portions of all I O samples were dissolved in water t o give the concentrations noted in Table 11. For each sample 1 2 g. of air-dry
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hide powder, of known hide-substance content, were put into each of five wide-mouth, half-pint bottles, 2 0 0 cc. of t a n liquor were added t o each, and the bottles stoppered and shaken in a rotating box for 6 hrs. A t the end of this time all solutions gave a negative test for tannin with the gelatin-salt reagent. The bulk of detannized liquor was removed from each powder by squeezing through a suitable cloth. Two powders from each set were allowed t o dry slowly, without previous washing, and bottled for use in connection with a n investigation of the “aging” of leather t o be mentioned later. The remaining three powders of each set were washed as follows: Each powder was returned t o its bottle along with 2 0 0 cc. water and shaken for a n hour. The aqueous extract was separated from the powder by squeezing through a cloth, and the washing operation was repeated. Of the three powders of each set, one was washed fifteen times, the second twenty-five times, and the third fifty times. After washing, the tanned powders were allowed t o dry slowly, and were then analyzed for water, ash, f a t (chloroform extract), and hide substance (nitrogen X 5 . 6 2 ) . The per cent of tannin was calculated by difference. The results are given in Table 11. The detannized liquor and fifteen wash waters from one of the gambier samples were evaporated separately t o show the rate a t which the soluble matter is removed by washing. Table I11 shows t h a t only 1 2 mg. of soluble matter were removed by the fifteenth wash water, which appeared colorless and gave no color upon addition of a drop of ferric chloride solution. The TABLE 111-SHOWING SPEED OF WASHINOOF POWDERST A N N E DWITH GAMBIER Weight SOLUBLE RESIDUEFROM Mg. Original detannized solution. 1408 776 1st wash water. 2nd wash water... 396 250 3rd wash water.. 4th wash water.. 160 101 5th wash water.. 6th wash water. ....
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8th 9th 10th 11th 12th 13th 14th 15th
wash wash wash wash wash wash wash wash
water.. water.. water.. water,. water.. water.. water.. water..
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33 31 22 17 16 13
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smallness of the weights of t h e residues made the evaporation of individual wash waters beyond the fifteenth of doubtful value. However, all wash waters from the twenty-sixth t o the fiftieth (5000 cc. in all) were collected from both t h e sumac-tanned and larchtanned powders, and evaporated t o dryness. The total matter extracted froni the former was only 67 mg., and from the latter only 51 mg. Pure hide powder washed in the same way was found t o yield even greater residues because of slow hydrolysis, and i t should be remembered t h a t hide powder was used in great excess in these tests. If the 67-mg. residues from the sumac-tanned powder consisted solely of hide substance, it would contain 12 mg. of nitrogen; a Kjeldahl determination gave 14 mg. Furthermore, the residues from the tanned powders resembled those from pure hide substance in odor and general appearance.
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All wash waters without exception gave negative tests with the gelatin-salt reagent. Baldracco and Camilla1 recently proposed a reagent for which they claimed greater sensitivity t h a n the gelatin-salt reagent. Their reagent was made by dissolving 0.9 g. gelatin in 50 cc. water and adding I O cc. of glacial acetic acid. While this solution appears t o be more sensitive than a gelatin-salt solution which has been kept for some weeks, we found i t much less sensitive t h a n the gelatin-salt reagent immediately after preparation. When the gambier liquor was diluted I O O times and a drop of freshly prepared gelatin-salt reagent was added, a distinct precipitate was formed, whereas a freshly prepared gelatin-acetic acid reagent gave a negative test. We found t h a t the gelatin-salt reagent must be prepared from the highest quality gelatin and used only on the day it was made for the most satisfactory results. The gambier liquor had t o be diluted 2 0 0 times before i t failed t o give a test, and all the other liquors even more. Since no tests were obtained for any wash waters, a t the end oE fifteen washings not more than 8 per cent of the tannin originally present in the t a n liquor could have been removed by washing. I n other words, even if, as is most unlikely, each wash water removed a n amount of tannin t h a t would just fail t o give a test with the gelatin-salt reagent, we should have t o increase the per cent of tannin found for gambier extract, the material least sensitive t o the test, only from 7.87 t o 8.55, whereas a figure of 25.61 is obtained by the A. L. C. A. method. coNcLusIoNs-The results show conclusively t h a t prolonged washing is quite without influence upon the values obtained for the per cent of tannin in various materials by the new method, and, further, t h a t tannin once combined with hide cannot be washed out by any amount of washing t h a t would be practical. Thus the major contention of opponents of the new method is disproved. A N O T H E R VALUABLE P R O P E R T Y O F C E R T A I N NON-
T A N N I N S NOT B E F O R E F U L L Y R E C O G N I Z E D
I n our earlier paper it was shown t h a t aon-tannins like gallic acid are valuable agents in rendering too astringent t a n liquors sufficiently mild for practical use. We now find t h a t these same non-tannins are capable of undergoing chemical change with the formation of substances capable of tanning. During his criticism of the new method, G. W. Schultz2 said, “We have taken the non-tannins and washings and reconcentrated them under a high vacuum t o the original volume of 2 0 0 cc. and have tanned hide powder with it, and, by the calculations employed, we have found a definite percentage of tannin.” He mentioned also t h a t the concentrated liquor gave a positive test for tannin with the gelatin-salt reagent. I t might look a t first sight as though the detannized liquor and wash waters, before concentrating, really had contained tannin and Schultz evidently so regarded it. This view, however, would be difficult t o reconcile with the experimental d a t a noted in the preceding section. 1
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J . SOC.Leather Trades’ Chem., 4 (1920), 101. J . A m . Leather Chem. Assoc., 15 (1920), 455.
Dec.,
1920
THE J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
We confirmed Schultz’s findings while analyzing a sample of gambier extract by the new method. The detannized liquor and 1 5 wash waters, all of which gave n o test with the gelatin-salt reagent, were concentrated t o 2 0 0 cc., whereupon they were found t o give a bulky precipitate with the reagent. But, when dilated back to 3200 CG., they still gave a bulky precipitate with the gelatin-salt reagent, showilzg that a most important chemical change had taken place during the 60%cenfrating. TABLEIV-GAMBIER EXTRACT 200 cc. solution containing 9.00 g. extract were detannized with 12 g. air-dry hide powder containing 10.40 g. hide substance, and then the tanned powder was wished 17 times with 3400 cc. water. The residual liquor and wash waters were evaporated t o 250 cc. and used to t a n 12 g. fresh hide powder which was afterwards washed as usual. Total tannin’ either originally present or formed during the concentratinn of the wash waters. 13.50 Der cent. -HIDE POWDERTANNED INOriginal Concentrated ANALYSISOF AIR-DRY LEATHER Solution Wash Waters Water.. 17.31 16.24 Ash 0.16 0.14 Fat. 0.39 0.42 Hide .substance (N X 5.62). 76.86 79.38 Tannin (by difference). 5.28 3.82, Per 100 g. hide substance: Tannin found, grams.. 6.87 4.81 Material used, grams.. 86.54 86.54 Per cerit tannin in extract.. . . . . . . . . 7.94 5.56
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Another sample of gambier was analyzed by the new method and found t o contain 7.94 per cent tannin. The detannized liquor and 17 wash waters (3600 cc. in all) were evaporated t o 2 5 0 cc., analyzed by the new method, and found t o contain 5.56 parts of tannin per IOO of original extract, giving the extract a total of 13.50 per cent tannin. The detailed results are given in Table IV. I n order t o show t h a t this increased amount of tannin would have combined with the hide powder had it been present in the original solution, we made up a new solution of this extract, concentrated and diluted back several times, and then analyzed it by the new method, finding 12.69 per cent of tannin. If the concentrating had been continued a little longer the figure 13.50 would probably have been reached or passed. The results are given in Table V. TABLE V-GAMBIER EXTRACT (S.ame as noted in preceding table.) Dissolved 60.00 g. extract in 1 liter of water. Concentrated t o 250 cc., and diluted back to 1 liter. Repeated 3 times, the fourth time diluting to 2 liters. 200 cc. diluted solution containing 6.00 g. original extract were detannized with 12 g. air-dry hide powder, containing 10.37 g. hide substance. which was afterwards washed as usual. ANALYSIS O F AIR-DRYLEATHER Water 18.23 Ash 0.18 0.42 Fat.. Hide substance (N X 5.62). 75.62 Tannin (by difference). 5.55 Per 100 g. hide substance: Tannin found, grams.. .............. 7.34 Material used, grams.. 57.86 Per cent tannin in extract.. 12.69
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I n spite of the great change in the t a n liquor produced by concentrating, it is not shown t o any appreciable extent in the analyses by the A. L. C. A. method shown in Table VI. Concentrating the t a n liquor and diluting back caused a rise in per cent of tannin by the new method from 7.94 t o 12.69, but the rise in the A. L. C. A. method is only from 26.14 t o 26.40, which difference is so small as even t o be attributable to experimental error. The reason for this small difference is probably t h a t the non-tannins which are convertible into tannin all combine with the hide
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initially, even though they are easily removed later by washing. TABLE VI-GAMBIER EXTRACT Both the original liquor noted in Table I V and the specially treated liquor noted in Table V were appropriately diluted and analyzed by the A. L. C. A. method. PERCENT OF ORIGINAL EXTRACT Original Treated Liquor Liquor Insoluble matter.. .................... 7.66 8.62 Non-tannin ........................... 18.33 17.57 Tannin. 26.14 26.40
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Just what chemical actions are involved in the conversion of non-tannin t o tannin must remain a matter of speculation until more data are available; oxidation, condensation, and polymerization may all be involved. It is conceivable t h a t gallic acid might be converted into digallic acid under suitable conditions, and i t seems extremely likely t h a t a polymerized form of digallic acid would have tanning properties. A solution of pure gallic acid gives no test for tannin, but after boiling for some time i t gives a bulky precipitate with the gelatin-salt reagent, and apparently will t a n hide. A detannized solution which gives no test for tannin can be made t o give a strong test merely b y passing oxygen gas through it. Long exposure t o air has a similar action. It is evident t h a t the new method furnishes a valuable means of studying the conversion of non-tannins into tannin, and might conceivably be applied t o a study of the formation of tannins in nature and t o the aging of barks. T H E TIME F A C T O R I N T A N N I N G A N D THE “AGING”
OF
LEATHER
The results .of the preceding section suggest t h a t the conversion of non-tannin into tannin is responsible for two factors of great importance t o tanners of heavy leather, namely, the time factor in tanning and the (‘aging’’ of leather. I n the discussion referred to, Alsopl remarked t h a t sole leather tanned slowly not only contains more tannin, but actually consumes less tanning material than the rapid tannages. In a private communication, Professor Procter, of Leeds, has called attention t o the fact t h a t leather stored for a long time, or “aged,” before washing contains more tannin than if i t had been washed immediately after tanning. The extent of this Aging is well shown in the following experiment: TABLE VII-EFFECT
OF
“AGING”UPON PER IN
LEATHER
CENT OF
COMBINED TANNIN
Two 12-g. portions of hide powder were used t o detannize 200-cc. portions of the solutions of tanning materials noted in Table 11. One portion in each case was washed 25 times immediately after tanning: the other portion was allowed t o dry without washing, kept exactly 30 days, and then washed 25 times. TANNIN AS PERCENT -OF ORIGINAL EXTRACTI n Leather I n Leather Washed Kept Immediately 30 Days after before EXTRACT Tanning Washing 47.25 53.00 Quebracho. 7.89 10.49 Gambier. 20.34 23.92 Quebracho-gambie 15.36 17.23 Oak bark.. Chestnut wood. 13.99 18.02 Hemlock b a r k . . 23.38 24.87 Chestnut wood-hemlock bark mixture. . . 18.73 20.45 11.29 13.22 Larch bark.. Sumac.. 16.29 17.94 94.16 25.89 Wattle b a r k . . ....................
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LOC.cii., p, 464.
