Acidum Carbolicum

Although the solubility of phenol in fats and oils is greater than 1 in 10, not more than 1 part dissolves in 25 parts of soft paraffin, so that paraffin ointments containing more than that proportion show separate crystals of the acid on cooling. This peculiarity indicates that the hot mixture is not a true solution, or that the force of crystallisation is stronger than that of solution. It is advisable to prepare such ointments in the cold, by mixing the liquefied acid with the basis, or dissolving the crystallised phenol in glycerin before mixing with soft paraffin.

Acidum Hydrocyanicum Dilutum

This should be mixed on the slab with the cold basis, using a horn or bone spatula, and mixing as quickly as possible, thus:

M. Fiat ung.

A thoughtless dispenser might thin the extract with the acid ; this should be done with a little water, the lard then mixed with it, and the acid added in drops, mixing expertly on a slab.

Acidum Salicylicum

Always use horn or bone spatulas for this acid, which sometimes gives rise to unexpected results. For example, the following became quite lumpy on keeping:

Fiat unguentum.

The lumpiness was due to formation of zinc salicylate, the water of the lanoline sufficing to bring the acid and zinc oxide together. As the lumpiness arose on keeping, it was avoided by completing the reaction before dispensing the ointment. One drachm of zinc oxide and 10 grains of the acid were triturated in a mortar with 80 minims of water. Separately the rest of the ingredients (170 grains of anhydrous wool-fat) were made into an ointment and added to the mortar-contents and well mixed. It is generally advisable, when a chemical change takes place in an ointment slowly, to bring it about quickly before compounding with the fatty basis.