Theine contains a very large proportion of nitrogen (almost 29 per cent.), and Wanklyn[6] has suggested the application of his ammonia process (see p. [205]) to the analysis of tea. Genuine tea is stated to yield from 0·7 to 0·8 per cent. of total ammonia, when tested in this manner.
Volatile Oil.—Ten grammes of the tea are distilled with water; the distillate is filtered, saturated with calcium chloride, then well agitated with ether, and allowed to remain at rest for some time. The ethereal solution is subsequently drawn off, and spontaneously evaporated in a weighed capsule. The increase in weight gives approximately the amount of oil present. A sample of good black tea yielded by this method 0·87 per cent. of volatile oil.
Tannin.—Two grammes of the well-averaged sample are boiled with 100 c.c. of water, for about an hour, and the infusion filtered, the undissolved matter remaining upon the filter being thoroughly washed with hot water, and the washings added to the solution first obtained. If necessary, the liquid is next reduced to a volume of 100 c.c. by evaporation over a water-bath. It is then heated to boiling, and 25 c.c. of a solution of cupric acetate added. The copper solution is prepared by dissolving five grammes of the salt in 100 c.c. of water, and filtering. The precipitate formed is separated by filtration, well washed, dried, and ignited in a porcelain crucible. A little nitric acid is then added and the ignition repeated. One gramme of the cupric oxide thus obtained represents 1·305 grammes of tannin. For the estimation of spent leaves (especially in black tea), Mr. Allen suggests the following formula, in which E represents the percentage of spent tea, and T the percentage of tannin found:—
E = (10 - T) 100 8.
The Ash.—a. Total Ash.—Five grammes of the sample are placed in a platinum dish and ignited over a Bunsen burner until complete incineration is accomplished. The vessel is allowed to cool in a desiccator, and is then quickly weighed. In genuine tea the total ash should not be much below 5 per cent., nor much above 6 per cent., and it should not be magnetic. In faced teas the proportion of total ash is sometimes 10 per cent.; in “lie-tea” it may reach 30 per cent.; while in spent tea it frequently falls below 3 per cent., the ash in this case being abnormally rich in lime salts, and poor in potassium salts.
b. Ash insoluble in water.—The total ash obtained in a is washed into a beaker, and boiled with water for a considerable time. It is then brought upon a filter, washed, dried, ignited, and weighed. In unadulterated tea it rarely exceeds 3 per cent. of the sample taken.
c. Ash soluble in water.—This proportion is obtained by deducting the ash insoluble in water from the total ash. Genuine tea contains from 3 per cent. to 3·5 per cent. of soluble ash, or at least 50 per cent. of the total ash, whereas in exhausted tea the amount is often but 0·5 per cent. The following formula has been proposed for the calculation of the percentage of spent tea E, where S is the percentage of soluble ash obtained:—
E = (6 - 2S) 20.
A sample prepared by averaging several good grades of black tea, was mixed with an equal quantity of exhausted tea-leaves. The proportion of soluble ash in the mixture was found to be 1·8 per cent. According to the above formula, the spent tea present would be 48 per cent., or within 2 per cent. of the actual amount.