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INDUSTRIAL A N D ENGINEERING CHEMISTRY
VOl. 19, No. 3
Action of Ammonia on Calfskin' By Henry B. Merrill A. F. GALLUN & SONSCo., MILWAUKEE, WIS.
MMONIA occurs in considerable quantities in two classes of tannery liquors. I n lime liquors, especially old ones, i t is present as the final product of protein hydrolysis. I n bates it occurs in larger quantities, owing chiefly to the interaction of lime with ammonium chloride. Ammonia is present also in the soaks, during curing, and indeed in all neutral or alkaline solutions in contact with raw skin. I n the sweating process for unhairing by bacterial action, notable quantities of ammonia are formed and undoubtedly play a part in the removal of the hair. It is a matter of general knowledge that under certain conditions ammonia is a strong unhairing agent. It is a matter of widespread belief that undue exposure to ammonia results in damage to skins. The unhairing properties of ammonia are of interest both because of the possible effect of ammonia in old limes and because of their bearing on the general problem of the mechanism of unhairing. The action of ammonia on collagen is of importance, if, as is generally supposed, losses of hide substance are thereby brought about. This paper reports a study of the action of ammonia on collagen and the unhairing action of ammonia.
A
Action of Ammonia on Collagen
Ammonia, as a weak base, must exert a hydrolytic action on collagen by virtue of its hydroxyl ions. This has been demonstrated by Moeller,2 using hide powder, and has been confirmed by work in this laboratory. Moeller's work indicates that ammonia has less solvent action on collagen than either sodium hydroxide or lime a t the same concentration, which is to be expected from the much lower hydroxyl-ion concentrations of its solutions. The impression prevails, however, that ammonia exerts a specific solvent action on skin, and that great damage is sometimes done in old limes or in the bates from this action. To test this point, the hydrolytic action of ammonia on calfskin was measured under conditions simulating those met in bating and liming. Although the concentration of ammonia used was carried to a point far beyond any that could possibly arise in the tannery, no specific action of the ammonia, apart from its action as a weak base, could be detected. METHoDS-The experimental technic used in this investigation is essentially that used previously by the author in measuring the effect of different variables on the rate of hydrolysis of skin.3 The materials subjected to the action of ammonia mere purified calfskin (unhaired, delimed, brought to pH 8, washed, and dehydrated with alcohol and xylene) and purified hide powder4 from which all the fat and most of the material readily soluble in water had been removed. These materials represent the purest obtainable form of collagen and differ only in their degree of subdivision. The purified skin, which was cut into cubes of about 2 mm. edge, more nearly simulates the state of collagen in skin. It was used for most of the experiments. Samples of the skin or hide powder, containing exactly 2.000 grams of collagen, were placed in stoppered bottles 1 Presented before the Division of Leather and Gelatin Chemistry at the 72nd Meeting of the American Chemical Society, Philadelphia, Pa , September 5 to 11, 1926 2 Collegium, 1821, 265. 8 Merrill, Tms JOURNAL, 16, 1144 (1924). 4 Wilson and Merrill, J A m Leather Chem Assoc , 21, 2 (1926).
with 200 cc. of the solution containing ammonia. A little toluene was added to prevent bacterial action. The bottles were placed in the constant temperature bath and digested for a definite time. The skin or hide powder was then brought on a filter and washed repeatedly to remove the ammoniacal solution. Undissolved collagen was determined by carrying out a Kjeldahl determination on the entire insoluble residue. Dissolved collagen was determined by difference. Blanks containing no ammonia were run for every experiment. EFFECTOF COXEXTRATIONOF A ~ i ~ o ~ ~ a - T h rseries ee of experiments, in each of which the independent variable was the concentration of ammonia, were run. The solutions were made up respectively with water, saturated calcium hydroxide, and 0.05 N sodium hydroxide. The time of digestion was three days, the temperature was 25" C., and the material on which the test was carried out was purified calfskin. The results of these experiments are given in Figure 1. I n Series 1 (Figure 1-a), in which the solutions contained only ammonia and water, the percentage of collagen dissolved increases with the concentration of ammonia. This is to be expected, as the pH value also increases. When the pH value is kept constant, by the addition of sodium hydroxide or lime, the ammonia has no effect upon the hydrolysis of collagen, at least none that cannot be attributed to experimental error. This shows that ammonia has no specific action on collagen. This conclusion was confirmed by running another series with ammonia in saturated limewater, using hide powder instead of skin. Hide powder is attacked much more rapidly than skin; the percentages digested are much larger; and any difference due to the presence of ammonia should be much more pronounced. The ammonia again had no appreciable action on the collagen (Figure 2). EFFECTOF TIME-& it was thought possible that small quantities of ammonia in limewater might have some action on collagen after long contact, a series was run in which all the members contained the same quantity of ammonia, the independent variable being time. A corresponding series was run with limewater containing no ammonia. The two series gave the same results for all periods of digestion (Figure 3). This indicates that ammonia in lime liquors is without action on collagen up to 6 days of contact. EFFECTOF p H VALuE-The possibility remained that ammonia, or the ammonium ion, might exert a specific hydrolyzing action on collagen a t some pH lower than those hitherto employed. This is of importance in connection with possible damage to skins by ammonia during bating. Two series were run a t p H values covering the range from 7 to 12, in one of which all the members contained ammonia while the other served for blanks. The experiment was run both a t 25" and at 35" C., the latter temperature being somewhat above that commonly used in bating. The results (Figure 4) show that a t no p H value in the range covered does ammonia have any appreciable action on skin. TESTSON FRESHSKIN-AS a drastic test of the conclusion previously drawn, that ammonia is without appreciable action on calfskin, three strips of fresh skin were placed in solutions containing, respectively, 0.5 N , 8 N , and con-
March, 1927
I X D USTRIAL AND ENGILVEERING CHEMISTRY
centrated ammonia, and allowed to stand for six weeks. Upon removal from the ammonia, the strips were greatly swollen, but otherwise unchanged except for the loss of hair. The pieces were pickled with acid and salt, and tanned with Tegetable tanning materials. After tanning, they appeared to be good leather, showing no looseness of grain, no emptiness, and no weakness on tearing. So far as can be told from a small strip, the skin had suffered no appreciable damage even from immersion in concentrated aqua ammonia for six weeks. ComLusIoxs-From the experiments described, it is safe to conclude that ammonia has no appreciable action on the collagen fibers that make up the bulk of the skin, and that, therefore, ammonia is not responsible for the large losses of hide substance and resulting low leather yields, for which it is sometimes blamed. On the other hand, it is entirely possible that ammonia may cause damage to skins, in the form of inferior appearance, by acting on some of the minor constituents of the skin. Investigation of this point can best be made by a thorough microscopic investigation of skin exposed to ammonia. Such a study is beyond the scope of this paper. U n h a i r i n g Action of A m m o n i a
The unhairing properties of moderately strong ammonia solutions are well known. If a piece of skin is placed in a solution, say 2 ATammonia, t h e hair slips easily aft'er a (a) few hours, and the unhaired skin is very little swollen. The difficulty and exi b ) pense of working w i t h s t r o n g ammonia precludes the a d o p t i o n of t t h i s m e t h o d commercially. :In the sweat(ci ing process the ammonia formed from the putrefaction of 0.2 0.4 0.6 0.8 the skin probably Ammonia (mols per liter) plays a part in the Figure 1-Action of A m m o n i a o n Calfskin unhairi,lg. Old lime (Time, 3 Days; Temperature, 25' C . ) (a) Ammonia in distilled water; ( b ) ammonia liquors COlltain conin saturated calcium hydroxide; ( c ) ammonia siderable ammonia, in 0.05 A' sodium hydroxide and there is a widespread belief that the improved depilatory qualit'ies of mellow limes are attributable to their ammonia content. The only critical work done on this subject is that of S t i a ~ n ywho , ~ reached the following conclusions: (1) The unhairing is a specific property of ammonia, as it is manifested a t pH values around 11, where the unhairing action of hydroxyl ion is negligible. (2) The unhairing is inhibited by divalent cations. When ammonium salts are added to saturated lime solutions, there is no improvement in the unhairing action of the lime. When calcium salts, or salts of barium or zinc, are added to ammonia, its unhairing power is lost. From this i t follows t h a t t h e presence of ammonia in old lime liquors has no influence on the unhairing. The explanation suggested for the inhibition of ammonia unhairing by calcium was the formation of complex ions, of the type Ca(NH3)8, such as ammonia is known to form with divalent cations.
If this explanation is true, it follows that a given amount of calcium ion can inactivate, as it were, only a limited amount of ammonia, and that if this ratio of ammonia to calcium is exceeded, the excess ammonia should manifest 5 J . SOC.Leather Trades' Chem., 2, 129 (1919).
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its unhairing powers despite the presence of lime. Experiments were undertaken to test this point. EFFECTOF LIVE ON UNHaIRING WITH L ~ ~ . M O ? ; I A - ~ ~ series of solutions was made up, each member of which contained 1.0 gram of lime in 200 cc., and in which the different members contained increasing amounts of ammonia. A parallel series was run witJh the same quantities of ammonia, but no lime. Strips from the head of a fresh calfskin, each about 1 X 4 inches, were placed in the solutions, and examined for unhairing action a t hourly intervals. Unhairing was considered satisfactory when the hair slipped easily under gentle pressure from a blunt knife. The results of these experi- 3 ~ ( ( i ments are given $515 in Figure 5 , in s: which the time c k reauired for satis-
