The acids usually present in liquor consist of several members of the fatty or acetic group, which distil over with boiling water, of other non-volatile organic acids, and sometimes sulphuric acid, which is added to assist the swelling of the leather.
To determine the acids of the acetic group, Kohnstein and Simand proceed as follows:—100 c.c. of the liquor are distilled, in a flask or retort with a good condenser, to about 30 c.c., allowed to cool a little, made up again to 100 c.c., and again distilled; and this is repeated till about 300 c.c. have passed over. The distillate is then made up to 300 c.c., well mixed by shaking, and the acid is determined with standard soda. Methyl orange and sodic carbonate is not so suitable for this titration, as caustic soda and litmus, since methyl orange is not very sensitive to vegetable acids. If it be desired to ascertain what quantity of acids of the acetic group exist in combination with lime and other bases in the liquor, small excess of sulphuric acid may be added to the residue in the retort, and the distillation repeated, when the organic salts will be decomposed and the volatile acids come over.
To determine the total free organic acids, Kohnstein and Simand shake about 80 c.c. of the liquor with 3-4 grm. of freshly ignited magnesia, quite free from carbonate and from lime, and allow to stand for some hours with frequent vigorous shaking, till the liquor, which at first is brown or dirty green, becomes almost colourless and gives no reaction of either acid or tannin. The mixture is then filtered, and the tannin and colouring matter are retained on the filter in combination with magnesia, while the organic salts of magnesia, which are mostly soluble, pass through with the filtrate. 10-30 c.c. of the filtrate, according to the amount of acid present, is evaporated to dryness, and gently ignited so as not to decompose any magnesic sulphate present. The residue is moistened with water saturated with carbonic acid, to convert any magnesic oxide into carbonate, and then dried, in order to make the mass powdery, and easier to wash, It is next taken up with hot distilled water, filtered and well washed. Any sulphate which is present passes into the filtrate, while the carbonate, which corresponds to the organic salts present before ignition, remains on the filter, and after solution in hydrochloric acid, is estimated as magnesic pyrophosphate. To the hydrochloric solution is added excess of ammonia and sufficient ammonic chloride to redissolve the precipitate formed, and prevent the precipitation of the magnesia; the solution is heated and then ammonic oxalate solution, first dilute, and then concentrated, is added to precipitate any lime which may be derived from lime salts present in the liquor. After filtering out and washing the precipitate, 10-15 c.c. of 10 per cent. sodic phosphate solution is added, and the liquid is stirred with a glass rod without touching the sides of the beaker, and allowed to stand 12 hours. The crystalline precipitate is then rinsed on to a filter, and washed with a mixture of 1 of ammonia and 3 of water, till the washings no longer give any milkiness with silver nitrate. The filter is then dried and the precipitate is placed in a platinum crucible and first gently, and then strongly ignited with the cover on; the filter paper, freed as much as possible from the precipitate, is burnt in the usual way on the crucible lid, the ashes are added to the precipitate in the crucible, and the whole is again ignited and allowed to cool in the desiccator, and finally weighed. 111 parts of magnesia pyrophosphate correspond to 120 parts of acetic, or 180 parts of lactic acid. Kohnstein and Simand calculate the pyrophosphate corresponding to the acetic acid already found by distillation, and after deducting it reckon out the remainder as lactic acid. Of course the volatile acids are really a mixture consisting of acetic, propionic, butyric and other members of the fatty group; but it would be difficult if not impossible to separate them. Similarly other fixed acids exist in mixture beside the lactic acid, but as their action is similar and lactic acid is always the most abundant, these acids are to be reckoned as lactic.
It has been mentioned that when sulphuric acid is present in the liquor it is found in the filtrate from the magnesia carbonate as sulphate. After removal of the lime as oxalate, as previously described, the magnesia may be similarly determined as pyrophosphate, and reckoned out as sulphuric acid (111 parts of pyrophosphate being equal to 98 parts sulphuric acid, H2SO4). It may also be estimated with barium chloride, but in this case regard must be had to the sulphates originally present in the liquor.
Since waters invariably contain both lime and magnesia salts, a portion (50 or 100 c.c.) must be evaporated, ignited, and after precipitation of the lime, the magnesia must be estimated as already described, and deducted from the amount found in a similar amount of liquor after saturating with magnesia. If, together with the organic acids, the liquor contains sulphuric acid, the correction may be divided equally between the two.
The method is not applicable in presence of phosphoric, tartaric, or oxalic acids. To overcome this difficulty, Messrs. Kohnstein and Simand are at present investigating a method dependent on decolorisation of the liquor with bone charcoal, completely free from mineral salts, and subsequent titration with soda.
It may be interesting to add the determinations of a complete set of handlers in a Continental upper-leather tannery, in which larch bark is used. 100 c.c. of liquor contained as follows, in grm.:—
| No. of Handler. | Total Acids reckoned as Acetic. | Volatile Acids reckoned as Acetic. | Fixed Organic Acids reckoned as Lactic. |
| 1 | 0·205 | 0·050 | 0·232 |
| 2 | 0·628 | 0·237 | 0·586 |
| 3 | .. | 0·372 | .. |
| 4 | 0·688 | 0·426 | 0·393 |
| 5 | 0·569 | 0·432 | 0·206 |
| 6 | 0·509 | 0·453 | 0·084 |
| 7 | 0·487 | 0·456 | 0·047 |
QUALITATIVE DETECTION OF TANNINS.
It is often desirable to determine from what tanning materials an extract or liquor is made, or with what a sample of leather is tanned. The following table gives reactions of the principal tanning materials, which will enable any one of them to be recognised with certainty, and in many cases will determine the constituents in a mixture of several, though this is naturally far more difficult. In such cases, colour reactions are apt to mislead, that of one tannin being modified by another, and it is safest to rely on the categorical test of precipitate or no precipitate, coloration or no coloration, without regard to the tint. The infusions must be very weak, not exceeding 1-2° Bktr., or precipitates will be formed where mere coloration or clouding is noted. In some cases only negative peculiarities can be given, and the material cannot be positively determined in mixture with materials where these peculiarities are present. Thus myrobalans could not be distinguished from divi with certainty, where any other material, such as gambier, was present, which gave a deep coloration with concentrated sulphuric acid. The writer will feel greatly obliged by the communication of more distinctive reactions.