Determination of Total Solids.—Shake the solution, which should be at a temperature of about 20°, and immediately remove 100 cubic centimeters with a pipette, evaporate in a weighed dish and dry to constant weight at the temperature of boiling water.
Determination of Soluble Solids.—Filter a portion of the solution through a folded filter, returning the filtrate to the filter twice and adding a teaspoonful of kaolin, if necessary. Evaporate 100 cubic centimeters of the filtrate and dry as above.
Determination of Tanning Substances.—Extract twenty grams of hide powder by shaking for five minutes with 250 cubic centimeters of water, filter through well washed muslin or linen, repeat the operation three times and dry as much as possible in a suitable press. Weigh the wet powder and determine the residual moisture in about one-fourth of the whole by drying to constant weight at 100°. Shake 200 cubic centimeters of the unfiltered solution of the tannin with the rest of the moist hide powder for about five minutes, add five grams of barium sulfate, shake for one minute and filter through a schleicher and schüll folded filter, No. 590, fifteen centimeters in diameter, returning the first twenty-five cubic centimeters of the filtrate. Evaporate 100 cubic centimeters of the clear filtrate and dry the residue to constant weight at a temperature of boiling water. The difference between the soluble solids obtained in the filtered tannin solution and the residue as obtained above is the amount of tanning material absorbed by the hide powder. This weight must be corrected for the water retained by the hide powder. The shaking must be conducted by means of a mechanical shaker, in order to remove all the tannin substance from the solution. The simple machine used by druggists, and known as the milkshake, is recommended.
Testing the Hide Powder.—Shake ten grams of the hide powder with 200 cubic centimeters of water for five minutes, filter through muslin or linen, squeeze out thoroughly by hand, replace the residue in the flask and repeat the operation twice with the same quantity of water. Pass the last filtrate through paper until a perfectly clear liquid is obtained. Evaporate 100 cubic centimeters of the final filtrate in a weighed dish, dry at 100° until the weight is constant. If the residue amount to more than ten milligrams the sample should be rejected. The hide powder must be kept in a dry place and tested once a month.
Prepare a solution of pure gallotannic acid by dissolving five grams in one liter of water. Determine the total solids by evaporating 100 cubic centimeters of this solution and drying to constant weight. Treat 200 cubic centimeters of the solution with hide powder exactly as described above. The hide powder must absorb at least ninety-five per cent of the total solids present. The gallotannic acid used must be completely soluble in water, alcohol, acetone and acetic ether and should contain not more than one per cent of substances not removed by digesting with excess of yellow mercuric oxid on the steam bath for two hours.
Testing the Non-Tannin Filtrate. For Tannin:—Test a small portion of the clear non-tannin filtrate with a few drops of a ten per cent solution of gelatin. A cloudiness indicates the presence of tannin, in which case the determination must be repeated, using twenty-five grams of hide powder instead of twenty grams.
For Soluble Hide:—To a small portion of the clear non-tannin filtrate, add a few drops of the original solution, previously filtered to remove reds. A cloudiness indicates the presence of soluble hide due to incomplete washing of the hide powder. In this case, repeat the determination with perfectly washed hide powder.
593. The Permanganate Gelatin Method.—This process, which is essentially the method of Löwenthal, as improved by Councler, Schroeder and Proctor and as used by Spencer for the determination of tannin in teas, is conducted as described below.[612] The principle of the process is based on the oxidation of all bodies in solution oxidizable by potassium permanganate, the subsequent precipitation of the tannin by a gelatin solution, and the final oxidation, by means of permanganate, of the remaining organic bodies. The difference between the total oxidizable matter and that left after the precipitation of the tannin represents the tannin originally in solution.
Reagents Required.—The following reagents are necessary to the proper conduct of the potassium permanganate process:
(1). Potassium permanganate solution containing about one and a third grams of the salt in a liter: