Another proposed method of detecting this adulterant is to add excess of precipitated feiric hydrate to the aqueous solution of the sample, and to raise the liquid slowly to the boiling point. The undissolved portion is allowed to settle, and the clear liquid is decanted off and evaporated to a syrup at steam heat. If tartaric acid be present, even in very small proportion, it is said to cause the deposition of a pulverulent precipitate of ferric tartrate, while the liquid obtained from pure citric acid remains clear. Allen has not succeeded in detecting moderate percentagea of tartaric acid by this test.
The beat test for detecting an admixture of tartaric acid is the well-known one of a salt of potassium. It is remarkable how very few of the ordinary works on analysis make any mention of the precautions necessary for the successful detection of tartrates of potassium by their mutual reaction. If aqueous tartaric acid be added to a strong solution ef the chloride or nitrate of potas sium, a precipitate of the acid tartrate will often occur, but its formation is greatly impeded by the mineral acid set free. This may be proved by filtering off the liquid, and treating it with a strong solution of sodium acetate, when a copious additional precipitation takes place, owing to tho replacement of the free hydrochloric or nitric acid by acetic acid and the insolubility of the acid potassium tartrate in the latter. Of course the same object is attained by using excess of acetate of potassium as the precipitant, instead of the nitrate or chloride. The precipitation is greatly promoted by stirring, farming well-defined and characteristic atreaks in the track of the glass rod. Of course the liquid must be quite cold. Thc delicacy of the reaction is greatly increased by the addition of alcohol.
A recognition of this fact suggested the possibility of rendering the best quantitative and delicate by using alcoholic instead of aqueous solutinns of the sample and reagent. Tartaric and citric acids are both soluble in absolute alcohol, but the potassium salts are insoluble. Acid tartrate of potassium is also practically insoluble in proof spirit, while the citrates of potassium are pretty readily aoluble in weak alcohol. In the following experiments a proof spirit (made by diluting orlinary methylated apirit with water till it had a density of 920) was employed.
Some pure potassium hydrogen tartrate was prepared, and its solubility in proof spirit at 15 was shown to be 0.05 per cent., or 1 grim ia 2000 cc. ef spirit.
A saturated cold solution of potassium acetate in proof spirit was prepared ; 100 ec. contained about 36 grm. of the salt, so that 5 cc. .sufficed for the precipitation of nearly 3 grm. of tartaric acid.
A series of samples of citric aeid were prepared, eontaiuing 5 to 50 per cent. of tartaric acid. Quantities of 2 grm. of each of these adulterated samples were dissolved in 20 c.c. of proof spirit. 5 cc. of the saturated spirituous solution of potassium acetate was added, and the solution stirred and left overnight. It was then filtered, the precipitate was washed with proof spirit saturated with acid potassium tartrate, and then once with ordinary proof spirit. The precipitate was then washed off the filter with hot water into a light porcelain dish, the water evaporated off at steam heat, and the dry tartrate weighed and calculated into tartarie acid.
It was hoped that by proceeding in this manner, very accurate estimations of tartaric acid eould be made, as there could be no loss except from the slight solubility of the precipitate in the solution, for which a correction could easily be made on the assumption that the citric acid and potassium acetate present had no influence on the solubility of potassium hydrogen tartrate in proof spirit. It was surprising, however, that the results obtained, even without the correction for solubility, showed sensibly more tartaric acid than had been actually added to the sample. A fresh series of experiMents was made, exactly the same method being employed, except that the 2 grm. of the samples were dissolved in 40 cc. of proof spirit instead of 20 cc. as before, thus making the volume of the solution 45 cc. By this proceeding the following results were obtained without the correction for solubility, which would make the numbers 1.1 per cent. higher still :— In this course of examinations, the results were checked by igniting the precipitate and titrating the ash with standard acid. It was found that the carbon left retained alkali tenaciously, and after dissolving the ash in water it was necessary to ignite the black residue and then add its ash to the main quantity. In the above cases, the amounts of potassium carbonate found by the titration of the ash correspond to the percentages of tartaric acid shown in the last column of the table.