Prescriptions containing quinine sulphate and potassium iodide are not uncommon, and precipitation can rarely be avoided for the reasons given on page 75. Even when just sufficient acid is added to dissolve quinine sulphate precipitation frequently occurs owing to the presence of a trace of iodate in the iodide. A few examples are here given of other quinine and iodide combinations.

The liquor (1 to 7) added to a solution of the quinine in the acid gives a copious finely divided precipitate of 'quinine iodide,' but if the syrup be added before the liquor, a clear solution is obtained, from which the quinine iodide gradually crystallises. The former method should be adopted. Note the solubilities of the iodides (page 70).

The amount of acid used to dissolve the quinine exerts a certain influence on the nature of the precipitate formed, as in the following:

By using just sufficient dilute acid, mxxv., to dissolve the quinine, a yellow precipitate of iodide of quinine is formed ; but if a large excess of acid be added, it liberates iodine from the potassium iodide, and the liberated iodine combines with the sulphate of quinine to form the insoluble iodosulphate of quinine, or herapathite, which is gradually deposited as a greenish-brown sediment.

In some cases of this kind it is not herapathite which is formed, but a brown compound of sulphate of quinine, iodide of quinine, and iodine. This is especially the case with mixtures containing quinine sulphate, potassium iodide, and spirit of nitrous ether, such as the following:

Numerous futile attempts were made to combine mixture a in a presentable manner; the prescriber being near, he was consulted, and gave directions to dispense the iodide and sal volatile in a separate mixture. The writer of prescription b was interviewed. He was quite aware that iodine might be liberated in his mixture, and on being informed that the whole of the iodine would be freed and that it would precipitate the quinine as an iodo-compound, he suggested the addition of enough acid to dissolve it. Learning, however, that chemistry would not accommodate him in this manner, he directed the addition of 1 ounce of mucilage to suspend the precipitate. The mixture was therefore dispensed in this way, and produced a reddish-brown, muddy compound evolving nitrous fumes. The mixture was never repeated.

The Bromides form perfectly clear mixtures with quinine, as the quinine hydrobromides are amongst the most soluble salts of the alkaloid. The following gives a clear mixture:

Dissolve the quinine hydrobromide in 4 ounces of water, and add to it the chloric ether. Mix the tincture, acid, and 2 ounces of water together, and filter into the quinine solution, if an absolutely clear mixture is desired. Sometimes precipitates are induced by salting out, as in the following instance:

When the quinine sulphate is dissolved in the water and acid, and the potassium salts are added, a quinine salt separates in white flocks, but there is no precipitation if the potassium salts are dissolved in 3 ounces of water and the quinine sulphate in 2 ounces with the acid and tincture, then mixed. There is such a thin partition, as it were, between solubility and precipitation that little is required to break through it. Apart from that, it is always bad compounding to add a solid salt to a solution of another salt.