Acidum Hydrocyanicum

Dispensers should never think of dispensing any other preparation than the pharma-copceial one, unless Scheele's acid is specially ordered. Owing to the extreme volatility of the acid, it should be the last ingredient added, the mixture being made up and sufficient space left in the bottle for the acid, which is best measured with a graduated syringe. Mixtures containing hydrocyanic acid should be labelled 'Shake the bottle.'

Acidum Salicylicum

Considering that this acid is really ortho-hydroxybenzoic acid, C6H4(OH).COOH, benzoic acid being C6H5.COOH, it is not surprising that the two closely resemble each other in physical characters, especially in solubility. On pages 71 and 72 the properties of salicylic acid are given, and the question of what kind to use in dispensing is discussed. Mention is there made of the solubility of the acid in alkaline acetates. This is sometimes taken advantage of, as in the following:

There is not sufficient ammonium acetate in this to dissolve the salicylic acid (a drachm of the solution being required for 5 grains of the acid), so that the mixture contains undissolved acid from the first, and at the end of twelve hours quinine salicylate separates. It is therefore advisable to treat the acid as an insoluble substance from the first by rubbing it to fine powder in a mortar, with some of the syrup and water, adding 1/2 ounce of acacia mucilage, then the liquor ammon. acet., tincture, and syrup, and making up to 6 ounces with water.

The mixture contains free acetic acid. For each drachm (60 grains) of salicylic acid there are required.

A similar change to the combination of quinine and salicylates occurs in the following :

Salicylate mixtures are very apt to change colour owing to oxidation (compare page 243 and page 299). The following is an example of a different change :

This produces a deep reddish mixture, due to the formation of salicylate of iron. Salicylates are about the most delicate test for ferric salts, and, as it is impossible to preserve ferrous sulphate absolutely from oxidation, it naturally follows that the mixture should be dark-coloured, whether it is prepared from salicylate made with natural or artificial acid. The sulphate of iron should be dissolved in the syrup and 1 ounce of chloroform-water; the salicylate and compound powder made into a mixture with the rest of the water, and the iron solution then added to it. In this way the most lightly coloured mixture is obtained.

The combinations of salicylic acid, such as aspirin (acetyl-salicylic acid) and diuretin (sodio-theobromine salicylate), behave in many instances like the acid itself, and difficulties with them should as a rule be considered as salicylic difficulties. See the index for references to some of them.