Yet the behaviour of bromine displays some striking differences. Chlorine alone, as already stated, causes no immediate alteration of somewhat diluted solutions of quinine, whereas they became turbid on addition of bromine as long as there is about 1⁄2500 or more of quinine present. Now, the precipitate which is produced by bromine in solution of quinine does not turn green if a little ammonia is subsequently added, or, at least, the thalleiochine thus obtained is rather greyish. But in more dilute solutions of quinine bromine acts more readily than chlorine. An excess of bromine is to be carefully avoided.
This is easily performed if the vapour of bromine, not the liquid bromine itself, is allowed to fall down on the surface of the solutions of quinine; their superficial layer only must be saturated with bromine by gently moving the liquid. Then a drop of ammonia will produce the green or somewhat bluish zone, which is much more persistent than that due to chlorine.
Consequently, for demonstration of the test under notice, chlorine is to be used in comparatively concentrated solutions. In solutions containing so little quinine (less than 1⁄2500) that it is no longer precipitated by the vapour of bromine, the thalleiochine test succeeds much better with bromine, and goes much further, as shown above.
The author also shows that morphine gives a dark, dingy brown colour with chlorine and ammonia, which is capable of more or less masking the reaction of quinine.
Another test for quinine is the formation of its iodosulphate, the so-called herapathite. For this purpose the quinine is dissolved in 10 parts of proof spirit, acidulated with 1⁄20th part of sulphuric acid, and to this solution an alcoholic solution of iodine is carefully added, and the liquid in the meanwhile stirred with a glass rod. There appears either immediately or after some minutes a black precipitate of iodosulphate of quinine, which if redissolved in boiling proof spirit, forms in cooling the beautiful crystals of herapathite. 100 parts of this herapathite, if dried on a water bath, represent 56·5 parts of pure quinine.
Dr de Vrij prefers the employment of the iodosulphate of chiniodine as a reagent for the detection and estimation of quinine. In a communication to the ‘Pharmaceutical Journal’
he writes:—“In estimating quinine in a mixture of cinchona-alkaloids by means of an alcoholic solution of iodine the reagent requires to be added in slight excess, in order to ensure complete precipitation. An undue excess of the reagent, however, causes the formation of a compound richer in iodine and much more soluble in alcohol than herapathite, and thus renders the determination inaccurate.”[127] For this reason the author suggests the application of an alcoholic solution of iodosulphate of chiniodine (so-called sulphate of amorphous quinine) in place of free iodine. The reagent is made as following:—
[127] 3rd series, vi, 461.
Two parts of sulphate of chiniodine are dissolved in eight parts of water containing 5 per cent. of sulphuric acid. To this clear solution, contained in a large capsule, a solution of one part of iodine and two parts of iodide of potassium, in 100 parts of water, is slowly added with continuous stirring, so that no part of the solution of chiniodine comes into contact with excess of iodine. By this addition an orange-coloured flocculent precipitate is formed of iodosulphate of chiniodine, which either spontaneously, or by a slight elevation of temperature, collapses into a dark brown, red-coloured, resinous substance, whilst the supernatant liquor becomes clear and slightly yellow coloured. This liquor is poured off, and the resinous substance is washed by heating it on a water bath with distilled water. After washing, the resinous substance is heated on the water bath till all the water has been evaporated. It is then soft and tenacious at the temperature of boiling water, but becomes hard and brittle after cooling. One part of this substance is now treated with 6 parts of alcohol of 92 or 94 per cent. until it is completely dissolved, and the solution allowed to cool. In cooling a part of the dissolved substance is separated. The clear dark-coloured solution is evaporated on a water bath, and the residue dissolved in 5 parts of cold alcohol. This second solution leaves a small part of insoluble substance. The clear dark-coloured solution obtained by the separation of this insoluble matter, either by decantation or filtration, constitutes the reagent which the author has used for some time under the name of iodosulphate of chiniodine, both for the qualitative and quantitative determination of crystallisable quinine.
To determine a quantity of quinine contained in the mixed alkaloids obtained from a sample of cinchona bark, 1 part of the alkaloid is dissolved in 20 parts of alcohol, of 90 or 92 per cent., containing 1·6 per cent. of sulphuric acid, to obtain an alcoholic solution of the acid sulphates of the alkaloids.