I N D U S T R I A L A N D ENGINEERING CHEMISTRY
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Vol. 17, No. 11
The Destructive and Preservative Effect of Neutral Salts upon Hide Substance'" By Arthur W. Thomas and Stuart B. Foster COLUMBIA UNIVERSITY, NEWYORE, N. Y.
Concentrated solutions of alkali halides catalyze the hydrolysis of hide substance, whereas nearly saturated solutions of sodium or magnesium sulfate inhibit hydrolysis in addition to preventing bacterial action, a property common to all strong salt solutions. By virtue of this double preservative action sodium sulfate is indicated to be a better hide preservative than sodium chloride.
ODIUM chloride is a time-honored hide preservative. It is accepted as a proper reagent for the purpose on account of its low cost and the apparent belief that it is the best. The purpose of this investigation was to provide answers to three questions:
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1-From a practical point of view is there any salt other than sodium chloride that is a better hide preservative? 2-What is the effect of impurities such as chlorides of calcium and magnesium commonly present in common salt?' 3-From purely scientific interest what is the comparative action of solutions of the halides and of the common alkali salts upon hide substance?
The answers to these questions are given in the tables and corresponding curves, where the effect of solutions of various concentrations of the chlorides of sodium, potassium, lithium, calcium, and magnesium, of the bromide and iodide of sodium, and of the sulfates of sodium and of magnesium are given for a period of about fifteen weeks. Hydrolysis of Hide S u b s t a n c e -PER CBNT OF COLLAGBN HYDROLYZEDSodium Chloride Molar concentration 7 davs 0.1 3.4 0.5 5.0 5.1 1.0 2.0 5.7 3.0 4.5 4.0 6.1
0.1 0.5 1.0 2.0 3.0 4.0
0.1 0.5 1.0 2.0 3.0 4.0
0.1 0.5 1.0 2.0 3.0 4.0
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Molar concentration 7 days 14 days 28 days 106 days 0.1 2.7 4.8 7.5 10.9 0.5 5.6 7.7 12.6 5:1 1.0 7.6 12.2 6.0 2.0 9.2 9.9 13.4 24.9 8.8 3.0 9.8 13.4 24.9 13.8 4.0 16.0 21.4 38.9 Calcium Chloride 8 days 17 davs 28 davs 70 davs 104 days 112 days 6.7 0.1 18.2 0.5 16:5 1.0 3916 .. 18.8 2.0 48.8 24.7 3.0 47.0 4810 25.1 4.0 40.1
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0.1 0.5 1.0 2.0 3.0 4.0 0.1 0.5 1.0 2.0
0.1 0.5
1.0 1.3
7 days 5.7 6.6 8.2 12.4 16.4 23.7
104 days
5.2(?) 6 . 1 9.8 5 . 6 ( ? ) 6.6 8.8 16:s 9.3 11.8 16.2 18.9 15.8 18.6 30.7 26.0 22.1 24.6 32.5 35.8 27.7 32.4 26.1 35.9 Magnesium Sulfate 7 days 16 days 28 days 70 days 104 days 4.2 5.57.210.6 9.5 3.2 4.0 4.8 8.7 12.4(?) 2.0 2.4 3.5 5.8 1.0 0.6 1.0 1.6 i:4 Sodium Sulfate 8 days 14 days 28 days 70 days 120 days 5.2 8.5 12.0 6.1 14.2 4.9 4.6 7.3 10.8 9.0 1.4 1.8 3.5 3.7 4.7 0.8 0.6 0.9 1.4 2.2
Water
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7 days 16 days 28 days 70 days 104 days 4.3
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Procedure
15 days 70 days 104 days 114 days 10.8 12.07.6 7.2 11.0 12.7 a . 8.9 18.9 16.0 2i:6 18.1 9.4 20.4 14.0 14.6 12.2 8.3 24.5 23.9 19.5 10.3 Sodium Bromide 8 davs 15 days 28 days 70 days 104 days 114 days 3.j 8.i 10.6 11.0 6.5 10.8 li:1 18.3 22.7 .* 6.2 9.5 14.2 14.8 14.3 21.0 40.4 47.0 19.7 26.6 3216 59.5 59:5 19.8 25.6 43:2 51.5 Sodium Iodide 9 days 16 days 28 days 70 days 104 days 114 days 2.8 7.d 9.6 12.3 7.5 8.9 14.7 16.8 2k:9 39.5 47.8 15.0 20.2 21.8 26.9 29.6 43.5(?) 48.5(?) 26.0 27.4 28.7 36.3 37.5 24.6 26.4 32.5 35.6 3516 Potassium Chloride 8 days 17 days 28 days 70 days 104 days 5.7 6.6 15.5 18.8 6.2 6.9 18.9 7.0 7.5 i:s 1i:2 19.8 7.6 9.3 10.5 17.2 19.6 8.5 10.2 19.0 21.6 7.0 10.4 20.6 24.1 ..
..
Hydrolysis of Hide Substance-(Continued) -PER CENT OB COLLAGEN HYDROLYZELithium Chloride
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1 Presented before the Division of Leather and Gelatin Chemistry a t the 69th Meeting of the American Chemical Society, Baltimore, Md., April 6 to 10,1925. 2 Contribution No. 483 from the Chemical Laboratories, Columbia Unive: s i b . 8 Analyses of ten specimens of commercial salt are given by McLaughlin and Theis, J . A m . Leather Chem. Assoc., 17, 376 (1922).
Fifty-gram portions of American standard hide powder were covered with 1-liter portions of the chemically pure salt solutions in stoppered bottles. In view of the danger of complications through bacterial action in the dilute salt mixtures, all solutions were covered with toluene as inhibitory agent. The toluene was added to the concentrated salt solutions merely for uniformity in conditions. The bottles were stored a t room temperature in the dark. They were shaken once daily during the first four weeks, and thrice or twice weekly thereafter. The room temperature was 19' * 2' C. (The experiment was started December 1, 1924.) At the intervals of time noted in the table, specimens of the solutions were withdrawn by pipet, filtered through dry ordinary filter papers, and the filtrates subjected to the Kjeldahl process for determination of nitrogen. From the figures so obtained the percentage of hydrolysis of the hide powder was calculated. Corrections for the decrease in volume caused by these withdrawals were made in all subsequent calculations. There was none of the familiar evidence of bacterial action in any case. Results
From a practical point of view a very interesting fact is shown. Sodium chloride is not the best of the cheap salts for hide preservation. I n the absence of bacteria it catalyzes hydrolysis of collagen, as shown by the relative positions of the sodium chloride and distilled water curves in Figure 10. Sodium sulfate, on the other hand, is as near an actual hide
November, 1925
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1163
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Hydrolysis of Hide Substance in Various Salt Solutions
preservative as can be hoped for in the form of a neutral salt. Another point of practical value is the marked hydrolytic action of solutions of calcium and magnesium chlorides, which are frequently present in commercial salt. Several facts of general interest are revealed in the curves. Sodium bromide solutions show the most marked destructive action. The order of catalysis for the most concentrated solutions a t 100 days (Figure 10) is NaBr > CaClz >
LiCl > NaI = hlgCl2 > KCl = NaCl > H10 > MgSO, = Na2S04. The various concentrations of potassium chloride (Figure 4) show no wide differences in action, whereas in practically all of the other salts the differences in hydrolysis vary greatly with the concentration. Examination of some of the figures reveals inconsistencies. Three molar sodium chloride solution is less hydrolytic than one and two molar (Figure 1 ) . Three molar sodium bromide is more active than four molar, one molar is less active than half molar
I N D U S T R I A L A N D EN(Y I Y E E R I X G C H E M I S T R Y
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(Figure 2 ) . Three and four molar sodium iodide are less hydrolytic than two and one molar (Figure 3). Two and three molar lithium chloride solutions are identical in hgdrolytic activity (Figure 5). There is a complicated order in the case of calcium chloride (Figure 6). Jlagnesium chloride like potassium chloride (Figure 4)acts fairly consistently (Figure 7 ) . The authors have carefully verified these “inconsistencies” and convinced themselves that they are not due to experimental errors. The present lack of knowledge of the physical chemistry of concentrated salt solutions precludes any explanation. 50 1
Vol. 17, No. 11
hide hydrolysk catalysts, as evidenced by the increase in hydrolysis as a function of increase in halide concentration (Figure 9). The appearance of the specimens is interesting. The hide powder samples immersed in the strong sulfate solutions look exactly as they did when first immersed in them. The solution is absolutely clear, whereas in the concentrated halide solutions the hide powder particles are reduced in size and slimy in appearance, surrounded by very turbid liquid. The results obtained in these experiments create a desire for further investigation. Additional work is now in progress for future publication. In conclusion, it may be suggested that a practical trial of sodium sulfate in the salting of hides is worth while. It takes less sodium sulfate than chloride to form a saturated solution and consequently there is no economic obstacle in the way. Acknowledgment
The authors express their appreciation of the aid of A. F. Gallun and Sons Company in this investigation.
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I 2 3 4 MOLAR CONCENTRATIW ff SAL? SOLUTION Figure 9-Effect of Concentration. Time, 70 Days
The results are not attributable to pH variations. The pH values of the most Concentrated solutions were determined electrometrically a t the end of the investigation. They were as follows: sodium chloride, 4 M , pH = 5.1; sodium bromide, 4 M , pH = 5.4; sodium iodide, 4 M , pH = 6.0; potassium chloride, 4 M , p H = 5.6; lithium chloride, A M , p H = 4.9; calcium chloride, 4 M , p H = 4.3; magnesium chloride, 4 M , p H = 3.8; magnesium sulfate, 2 M , pH = 4.7; sodium sulfate, 1.3M, pH = 5.3. It should be recalled that the pH value of distilled water in contact with the normal carbon dioxide content of the air is 5.7.
Figure 10-Hydrolysis
of Hide Substance by Various Salts
The data for the action of the sulfates of sodium and magnesium emphasize a most important fact. These sulfates are hide preseruatities. The hydrolysis of the hide powder decreases with increasing concehtration of sodium sulfate and of magnesium sulfate. On the other hand, the halides are
Addendum After these experiments were written for publication, information was received from Rosalie M. Cobb, chemist, HuntRankin Leather Company, of a practical 26-day trial of sodium sulfate salting. Miss Cobb states t h a t there were no unexpected developments other than the drying out of skins salted down with Glauber’s salt. A saturated solution of Glauber’s salt has a higher vapor pressure than saturated sodium chloride. This change from green-salted t o dry-salted hide would undoubtedly be avoided if a large number of hides were salted and stacked together. Miss Cobb reports that a skin deliberately salted in a bloody condition, soaked back and made clean leather. The writers have also been informed of statements to the efFect that sodium sulfate may have tanning action and thus adversely affect soaking and liming. This hardly seems possible, and in order to get some information on this question, hide powder that had been soaking for five months in molar sodium sulfate solution was removed therefrom, washed with water to remove the sulfate, dehydrated with alcohol, and air-dried. This hide powder was then subjected t o the action of warm water in direct comparison with ordinary hide powder. It hydrolyzed a t a slightly greater rate than the ordinary hide powder thus indicating that sodium sulfate had not tanned it.
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