On filtering, an almost colorless filtrate is obtained, which, by means of sulphide of hydrogen, is freed from lead, and, when sufficiently concentrated, yields caffeine in colorless crystals. Some of the alkaloid still remains in the mother liquor, and, after treatment with bone charcoal, colorless crystals may again be obtained. Description and Tests. Also soluble in 9.5 parts of boiling water, and very soluble in boiling alcohol. When ignited, caffeine is completely volatilized without charring or leaving a residue.
Caffeine is neutral to litmus paper.
On dissolving a small quantity of caffeine in concentrated sulphuric acid, and adding a minute fragment of potassium dichromate to the liquid, the latter will acquire a yellowish-green color, which gradually becomes green. If a small quantity of caffeine be dissolved in about 1 Cc. of hydrochloric acid, a little potassium chlorate added, the whole evaporated to dryness on a water-bath, and the capsule then inverted over a vessel containing a few drops of ammonia water, the residue will acquire a rich purple color, which is destroyed by alkalies. At an elevated temperature it sublimes in needles similar to those of benzoic acid. Tannic acid precipitates it from aqueous solution. According to the analysis of theine from tea by M. Jobst, caffeine proves to be identical with it.