The potassium permanganate solution is set by titration against the decinormal oxalic acid solution mentioned below. The end reaction with the indicator must be of the same tint in all the titrations, i. e., either golden yellow or pink.
(2). Tenth-normal oxalic acid solution for determining the exact titer of the permanganate solution:
(3). Indigo-carmin solution to be used as an indicator and containing six grams of indigo-carmin and fifty cubic centimeters of sulfuric acid in a liter. The indigo-carmin must be very pure and quite free of indigo-blue.
(4). Gelatin solution, prepared by digesting twenty-five grams of gelatin at room temperature for one hour in a saturated solution of sodium chlorid, then heating until solution is complete, cooling and making the volume up to one liter:
(5). A salt acid solution, made by adding to 975 cubic centimeters of a saturated solution of sodium chlorid, enough strong sulfuric acid to bring the volume of the mixture to one liter:
(6). Powdered kaolin for promoting filtration.
The Process.—Five grams of the finely powdered tea (or other vegetable substance containing tannin) are boiled with distilled water in a flask of half a liter capacity for half an hour. The distilled water should be at room temperature when poured over the powdered tea. After cooling, the volume of the decoction is completed to half a liter, and the contents of the flask poured on a filter. To ten cubic centimeters of the filtered tea infusion are added two and a half times as much of the indigo-carmin solution and about three-quarters of a liter of distilled water.
The permanganate solution is run in from a burette, a little at a time, with vigorous stirring, until the color changes to a light green, and then drop by drop until the final color selected for the end of the reaction, golden yellow or faint pink, is obtained. The number of cubic centimeters of permanganate required is noted and represented by a in the formula below. The titration should be made in triplicate and the mean of the two more nearly agreeing readings taken as the correct one.
One hundred cubic centimeters of the filtered tea infusion, obtained as directed above, are mixed with half that quantity of the gelatin reagent, the first named quantity of the acid salt solution added, together with ten grams of the powdered kaolin, the mixture well shaken for several minutes and poured on a filter. Twenty-five cubic centimeters of the filtrate, corresponding to ten of the original tea solution are titrated with the permanganate reagent, under the conditions given above, and the reading of the burette made and represented by b. The quantity of permanganate solution, viz., c, required to oxidize the tannin is calculated from the formula a - b = c. The relation between the permanganate, oxalic acid and tannin is such that 0.04157 gram of gallotannic acid is equivalent to 0.063 gram of oxalic acid. The relation between the oxalic acid solution and the permanganate having been previously determined the data for calculating the quantity of tannin, estimated as gallotannic acid, are at hand.
594. The Permanganate Hide Powder Method.—Instead of throwing out the tannin with gelatin it may be absorbed by hide powder. The principle of the process, save this modification, is the same as in the method just described. As described by Trimble, the analysis is conducted according to the following directions:[613]