The Principles of Leather Manufacture

by H. R. Procter

The action of solutions of sugars, glycerine, and the like is in principle similar to that of alcohol, but more complex, since in general these bodies are soluble not only in the water, but in the gelatine or hide-fibre, so that their effect cannot be foretold, though usually it tends towards contraction rather than swelling. In general terms the equilibrium is a balance of the attraction of the water and the sugar for the gelatine, against the sum of their mutual attraction in the solution outside and the resisting cohesive force of the gelatine; and will depend not only on the nature of the substances, but on temperature and concentration.

The action of acids, alkalies and salts on gelatinous fibre is yet more complex, since not only electrolytic dissociation, but most probably actual chemical combination comes into the question. The chemical constitution of gelatine is as yet quite uncertain, but it is known that the molecule contains both amido-groups capable of linking with acids, and carboxyls which will combine with bases (see [p. 58]). Hence hide-fibre absorbs both acids and bases with great avidity, so much so that the sulphuric acid of a decinormal solution may be completely removed by hide, leaving only water without a trace of acid recognisable by litmus. Alkalies are absorbed in a similar way, and in both cases the gelatine or gelatinous fibre acquires a greatly increased power of absorbing water, and consequently of swelling. Familiar cases of this are the swelling of hide by acid, and by lime, and in neither case can the added substance be removed in any reasonable time by mere washing with water. Hence to free hides from lime or acids it is necessary to neutralise the alkali with acids (see [p. 153]) or the acid with chalk or alkalies ([p. 91]). No accurate determination has yet been made of the amount of acid or alkali with which gelatine or hide-fibre will combine, since the matter is complicated by the volume of acid or alkaline solution which is absorbed mechanically, and by the tendency of the compound to partially decompose on washing with water. Experiments made by the author lead to the conclusion that 1 grm. of air-dried gelatine will combine with about 0·025 grm. of actual hydrochloric acid (HCl) when placed in a very dilute solution of the latter, and this compound will absorb 40 or 45 grm. of water while still retaining the jelly state. The maximum swelling, with both acids and alkalies, is obtained with dilute solutions; and with the stronger acids, the outside solution must be almost neutral when equilibrium is attained, increasing quantities of acid diminishing the amount of water absorbed. The same statement is true of the strong alkalies. Thus in both cases, where swelling is desired, the object is defeated by the use of too strong solutions, and the quantity of acid or alkali should be rigidly adjusted to the weight of pelt, and not to the volume of solution.

As regards a physical explanation of the effect of acid and alkaline solutions upon gelatine, anything which can yet be said must be regarded rather as speculation than as actual scientific knowledge. It must also be admitted that while the view that actual chemical combination takes place between the gelatine and acids (or alkalies) seems much the most probable, difficulties arise from the fact that different acids apparently do not always combine in proportion to their equivalents, though it is probable that these will prove only apparent anomalies when more accurate means are known of determining how much acid is really combined, and how much merely mechanically absorbed.

Leaving out of account for the moment the question of swelling, a few words must be said about a property of these acid- (and alkali-) gelatine compounds, a knowledge of which is essential to understanding the swelling process. If a mass of acid-gelatine be suspended in pure water, a certain portion of it will be decomposed into neutral gelatine and free acid, and the latter will diffuse into the water. Thus acid-gelatine can only exist in presence of a certain amount of free acid. This dissociation by water is a common property of all salts, and necessarily follows from what has been said of ionisation; but it is only where the combining affinity of the constituents is weak, that it becomes practically perceptible. Water to a very small extent ionises to H and OH. If we imagine a salt dissolved in it, such as NaCl, which ionises almost completely to Na and Cl, we see that a certain proportion of NaOH and HCl must be formed by combination with the water-ions. In the case named the quantity is absolutely negligible, since both sodium hydrate and hydrochloric acid are almost completely ionised themselves, but if either the acid or the base is weak (that is little ionised), the process of combination must go on till the acid or basic solution is strong enough to have an ionisation-pressure equal to that of the salt. As this acid or base is no longer in an ionised condition, it may be removed from the solution by volatilisation or diffusion. For instance, if a solution of ferric chloride be confined in a tray of parchment paper, through which it has little power of diffusion, and this tray floated upon water which is frequently changed, the dissociated acid will diffuse through the membrane into the water, and in this way the whole of it may be ultimately removed, leaving nothing but a colloid solution of hydrated ferric oxide in the tray. Actions of this sort, in which the gelatinous fibre of the hide plays the same part as the parchment-paper membrane, have an important share in many of the phenomena of tanning.[55] Thus, in the case of hide swollen with acid, the acid compound with the fibre is somewhat dissociated, and if the hide be hung in water which is constantly changed, the acid diffuses into it, and the whole may be ultimately, though slowly removed. A similar effect is produced in the familiar operation of removing acid from pelt or chromed leathers by paddling with “whitening” (calcium carbonate). The latter is insoluble in water, and therefore cannot penetrate into the hide, but as it instantly combines with any acid which diffuses out, the acid-gelatine compound is rapidly decomposed, since it is only permanent in a solution containing enough free acid to have an ionisation-pressure equal to that of the compound. Similar statements are true of the alkali-gelatine and lime-gelatine compounds.

[55] Probably chrome, aluminium and iron salts are decomposed in this way in mineral tanning, and thus fixed in the hide as insoluble basic salts. Cp. [pp. 186], [215].

It will be easier to follow the results of what has been said if we take a concrete case which has been carefully investigated by the author and others; that of the action of hydrochloric acid solutions on gelatine. If a weighed sheet of gelatine be placed in a very dilute solution of the acid, it swells much more considerably than it does in water, a maximum swelling being attained with a concentration of the outer solution of 0·1 to 0·2 grm. of HCl per litre. The swollen jelly has then a volume of about 45 c.c. per gram of the air-dried gelatine, and a concentration equal to about 0·75 grm. of HCl per litre of swollen jelly, or at least about five times that of the outer solution. As the concentration of the latter is increased, the concentration in the jelly also increases, but in a much smaller ratio, while the volume of the jelly diminishes, till, with a concentration of 5 grm. of HCl per litre in the outer solution, the volume of the jelly is only about 18·5 c.c., and its concentration not quite 6 grm. per litre. These facts cannot be accounted for by any theory of simple solution of the HCl in the jelly, since the law of such solutions is that the concentration in each maintains a constant ratio, unless chemical change takes place. It is possible that they might be explained by adsorption (surface attraction), but as it is known that gelatine contains both amido-groups capable of combining with acids, and carboxyl-groups which can combine with bases, it is much more likely that actual chemical combination takes place, and that the apparent irregularities in the amount of acid fixed are due to partial hydrolysis of the compound.[56]

[56] Cp., however, Walker and Appleyard on the ‘Absorption of Acids by Silk,’ Chem. Soc. Trans. 1896, p. 1334.

The following may be suggested as a working hypothesis. As both water and hydrochloric acid can pass freely in and out of the jelly, it must be in osmotic equilibrium with the outer solution in every respect, and neither the un-ionised hydrochloric acid of the solution, nor the small amount which may be formed by hydrolysis of the gelatine compound can have any effect on the swelling. So long as the outer solution is very dilute, by far the greater part of the acid present is absorbed and fixed by the gelatine, and almost the whole of the outer acid will be ionised, as well as a portion of that in combination with the gelatine. In the latter case, however, the ions will be unable to pass out of the jelly, and will therefore cause an internal osmotic pressure, and the gelatine will swell till the Cl-ions are in osmotic equilibrium with those of the outer solution. At the same time, this internal pressure of Cl-ions will oppose the entry of the Cl-ions (and therefore also of their associated H-ions) from the outer solution, and the acid solution absorbed mechanically will be somewhat less concentrated than that outside. As the concentration of the outer solution is increased, the pressure of the outer Cl-ions will repress the ionisation of the gelatine-chloride, and at the same time its tendency to hydrolyse. Thus the acid actually combined with the gelatine should somewhat increase, but the swelling should diminish, as is actually the case.[57] It is impossible to carry the concentration of the hydrochloric acid much above 5 grm. per liter without causing solution of the gelatine, but the addition of common salt to the outer solution should equally increase the pressure of its Cl-ions, and cause further diminution of swelling, the Na-ions in this case increasing the outside pressure in the same way as the hydrogen ions. In fact the addition of salt in sufficient quantity will reduce the swelling till the gelatine becomes quite solid, and retains only about its own weight of water, while at the same time the apparently combined acid largely increases. This cannot be attributed to any direct dehydrating action of the salt, since concentrated sodium chloride solutions have no dehydrating, but rather a swelling effect on gelatine in the absence of acid, and the concentration of the salt in the outer solution and in the jelly proves precisely the same within the limits of experimental error. Several other facts may be noted, tending to support the explanation which has been given. The tendency to swell gelatine is common to all acids of appreciable strength, and in all cases where the concentration of the acid could be increased to a moderate extent without causing solution of the jelly, the effect of a maximum swelling, diminishing as the concentration of the acid increased, has been observed. Other salts also produce similar effects to sodium chloride; thus the swelling caused by sulphuric acid is repressed by sodium sulphate. Sodium chloride seems to diminish the swelling caused by all acids, but in presence of large excess of sodium chloride, most of the acid in combination with the gelatine will probably be hydrochloric, whatever the acid used to originally produce the swelling. A curious fact observed by the author, is that absolute alcohol, which so effectually dehydrates neutral gelatine, is almost powerless to remove either water or acid from gelatine swollen by hydrochloric acid. HCl is freely soluble even in absolute alcohol, but H- and Cl-ions can only exist in it to a very small extent, so that we may conclude that the acid which causes the swelling and retains the water of the jelly exists either in actual combination with the gelatine, or in an ionised condition.

[57] The acid retained by the gelatine, as measured by deducting from the total contained in the jelly, a quantity equivalent to the volume of solution absorbed, at first rises rapidly to a maximum, then slightly diminishes and remains practically constant. On the theory suggested, it is evident, however, that the absorbed solution must be more dilute than that outside, and the actual combined acid greater than that shown by the above calculation. The “combined” acid, as determined by indicators, shows slight but continuous increase. It is acid to phenolphthalein, but neutral to methyl orange.

Solutions of caustic alkalies are in most respects analogous in their swelling action to those of strong acids. A portion of the alkali is in some way fixed by the gelatine, while another portion is simply absorbed as solution. A maximum swelling effect is also noticed with dilute solutions, which is diminished as the concentration increases. Swelling by alkalies is not diminished by chlorides so far as has been observed, and especially it may be noted that the swelling produced by caustic soda is not diminished by sodium chloride. On the theory which has been suggested there is no reason why alkaline swelling should be reduced by chlorides, since the swelling agent has no Cl-ion, but it is somewhat singular that the sodium salt, having a common Na-ion should produce no repression of the swelling by caustic soda. In the present state of our knowledge no definite explanation can be given, but it is quite possible that the swelling in this case is not produced by the sodium-ion but by some more complex one, or even by the hydroxyl-ion, like most of the characteristic reactions of alkalies. Apparently the gelatin-alkali compound is still strongly alkaline, affecting phenolphthalein indicator like uncombined alkali—an effect which is known to be due to the presence of free HO-ions.