Sulphate of Quinine
Owing to the high price of this salt, it is frequently adulterated. The substances used for this purpose are: crystalline sulphate of lime, boric acid, mannite, sugar, starch, salicine, stearic acid, and the sulphates of cinchonine and quinidine. These bodies are detected as follows:
a. Upon slightly warming 2 grammes of sulphate of quinine with 120 grammes of alcohol of 21° B., the pure salt completely dissolves; if, however, starch, magnesia, mineral salts, or various other foreign substances are present, they are left as insoluble residues.
b. Those mineral substances that are soluble in alcohol are detected by calcining the suspected sample: pure sulphate of quinine is completely consumed; whereas, the mineral substances present remain behind as a residue.
c. In presence of salicine, the salt acquires a deep red color, when treated with concentrated sulphuric acid.
d. Stearic acid remains undissolved upon treating sulphate of quinine with acidulated water.
e. To detect sugar and mannite, the sample is dissolved in acidulated water, and an excess of hydrate of baryta added: a precipitate, consisting of quinine and sulphate of baryta, is produced. Carbonic acid is then passed through the fluid, in order to precipitate the excess of baryta as insoluble carbonate, the fluid saturated with ammonia, to throw down the quinine which may have been re-dissolved by the carbonic acid, and the mixture filtered. If the salt be pure, no residue will be obtained upon evaporating the filtrate; a residue of sugar or mannite is formed, if these substances are present.
f. Sulphate of quinine invariably contains 2 or 3 per cent. of cinchonine, originating, not from a fraudulent admixture, but from an incomplete purification of the salt. One of the best methods for detecting the respective quantities of quinine and cinchonine, present in a sample of the sulphate, is the following: Several grammes of ammonia and ether (which has previously been washed with water) are added to one or two grammes of the salt under examination, the mixture thoroughly agitated, and then allowed to remain at rest. The supernatant etherial solution contains all of the quinine; the cinchonine, which is almost completely insoluble, both in water and ether, remaining suspended between the layers of the two fluids. The ether is next removed by means of a stop-cock funnel, evaporated to dryness, and the weight of the residue obtained determined. The operation is then repeated, the ether being replaced by chloroform in which both quinine and cinchonine are soluble. The residue, formed by the evaporation of the second solution, will be heavier than the first residue: the difference between the two weighings gives the weight of the cinchonine present.
g. The detection of the presence of sulphate of quinidine is based upon the difference in the solubilities of the oxalates of quinine and quinidine. Oxalate of quinidine is sufficiently soluble in cold water not to be precipitated by double decomposition when solutions of oxalate of ammonia and sulphate of quinidine are mixed. Under the same circumstances, quinine is almost completely thrown down. The test is applied as follows:
The suspected salt is dissolved in water, a slight excess of oxalate of ammonia added, and the precipitate formed separated by filtration. If the salt be pure, the filtrate is scarcely rendered turbid by the addition of ammonia; when, however, sulphate of quinidine is present, it will be entirely contained in the filtrate, in which ammonia will produce an abundant precipitate.