This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
Citric acid is frequently adulterated with tartaric acid; oxalic acid and various crystallized salts are said to have been employed for the same purpose. According to Hager, a solution of four parts caustic potash in sixty parts of water and thirty parts of alcohol at ninety degrees is poured upon the suspected crystals; if citric acid alone is present, it will disappear, but the crystals of tartaric acid will speedily become covered with a crystalline coating of cream of tartar. Heated until decomposition sets in, tartaric acid emits a smell of caramel.
Chapman and Smith state that citric acid will turn an alkaline solution of potassium permanganate, if heated to boiling, to a green color, while a solution of tartaric acid will decolorize it.
The adulteration of citric acid with tartaric acid is usually accomplished by mixing the latter in crystalline form with the former, and, therefore, several ounces of the crystals should he triturated in a mortar into powder, in order to make a proper analysis. Some of the admixtures will be found insoluble in alcohol, and may thus be detected.
To distinguish citric from malic and tartaric acids, the following test is given by M. Mean (Jour. Phar. d'Alsace-Lor.): Fuse together in a porcelain crucible one gramme of crystallized citric acid, and 0.70 grammes glycerine; heat carefully until the mixture swells up and emits acroleine vapors; then dissolve in a little ammonia, of which the greater part is afterwards expelled by moderate heat; add two drops nitric acid (one in five) or peroxide of hydrogen (ten per cent.) Citric acid thereupon assumes a beautiful green color, which changes to blue by heating. Malic and tartaric acids give no such reaction.