A. Preparation of the Test Liquor. Fifty grams neutral acetate of lead are dissolved in 400 grams of alcohol, of 92 per cent., and distilled water is added to make up 1 litre.
On the other hand, 1 gram of tannin is dissolved in 40 grams of alcohol of the same strength, and the solution is made up with water to the bulk of 100 c.c. This being done, 10 c.c. of the tannin solution are mixed with 20 c.c. of water, and heated to 60°. The lead liquor is then run into the hot solution from a burette, graduated to tenths of a c.c., so long as a precipitate is formed. At this temperature, and with these alcoholised liquids,
the precipitate forms and settles rapidly. Iodide of potassium may be used as an indicator to show excess of lead, proceeding in the same manner as is done with ferrocyanide in titrating phosphates with nitrate of uranium. If we suppose that to precipitate 10 c.c. of the tannin solution 28° of the lead liquor have been required, then 2·8 c.c. of the latter = 0·10 gram of tannin.
B. Preparation of the Sample to be tested. Suppose that chestnut bark is to be examined. It is coarsely powdered, and 10 grams are mixed with an equal volume of washed sand, and exhausted with water at 50° or 60° C. The filtered liquid is evaporated to dryness in a water bath in a tarred porcelain capsule. After evaporation the capsule is weighed, which shows the yield of the bark in aqueous extract. This is taken up in 40 grams of alcohol at 92°, and water is added to make up 100° c.c. The liquid is filtered if needful. In this manner the resinous, albuminoid, pectic, and gummy matters are got rid of.
C. Titration. The liquid thus prepared is divided into two parts. The first, one third of the entire volume, serves for direct determination of the acetate of lead. Suppose that a gram of the dry extract of chestnut has required, for 10 c.c. of the tannin liquor, in three successive experiments, 16°, 17°, and 16° of the burette, which corresponds to 57 per cent. of tannin. Thus figure 57 represents not only tannin, but every other substance capable of precipitating acetate of lead.
The tannin is then absorbed with bone black, previously washed with hydrochloric acid, and dried at 100° C. in the following manner:—We act with bone black upon the tannin liquor, and on a solution of pure tannic, prepared at a standard somewhat lower than that indicated for the extract by the first direct titration. In the present case this solution of tannin should be prepared at 55 per cent.
From one and the same glass tube, about 1 centimètre in diameter, we cut off two lengths of 20 centimètres each, and we draw out each at one of its ends. The two tubes are fixed perpendicularly with the points downwards, and plugged with a little carded cotton. Into each is put 10 grams of the bone black, pouring into one of them the second part of the tannin liquor under examination, and into the other the same volume of the pure solution of pure tannin at 55 per cent.
Twenty c.c. of the tannin liquor (which will be found to have retained its original brown colour in spite of the bone black) are now heated to 60° C., and the standard lead liquor is added from the burette as before. Two successive trials show 16° = 8° for 10 c.c. in place of the 16° found for 10 c.c. on direct titration. On the other hand, 20 c.c. of the solution of pure tannin require 14°, or 7° for 10 c.c. Thus we see that in the tannin liquor (chestnut extract) there is a certain quantity of matter which acts upon the standard lead solution
like tannin, corresponding to 1° of the lead liquor, i. e. to 357 thousandths of a centigram of tannin; 28°, therefore, correspond to 10 centigrams. The figure 57, obtained by direct titration, is, therefore, too high by 3·57 per cent., and the extract contains 57 - 3·57 = 53·43 per cent. of tannin.
Uses, &c. The value of substances containing tannin in the preparation of leather is well known. In its pure form it is used as an astringent in medicine; internally, in diarrhœa, hæmorrhages, as a tonic in dyspepsia, &c.; externally, made into a gargle, injection, or ointment.—Dose, 1 to 10 gr., in the form of pills or solution. See Gallic acid, &c.