This mixture is almost colourless when first prepared, but rapidly acquires a reddish-brown colour, and after a few days crystals of iodine are deposited. This is due to the action of potassium chlorate on ferrous iodide; the latter being oxidised by the former, potassium chloride is produced, iodine set free, and, finally, ferric hydroxide precipitated.

Incompatible mixtures are sometimes the result of impurities in the drugs used, thus:

The acid invariably contains some sulphuric acid, which throws out sulphur from the hyposulphite.

Misce pro dose. Mitte

Although an alkaline carbonate forms a precipitate with mercuric chloride, still, if in the above mixture the first and third ingredients be mixed, and the solution of the carbonate then added, no precipitate occurs. If common water be used, a slight precipitate of calcium carbonate forms, but it is free from mercury. As previously mentioned, mercuric chloride forms clear solutions with some tap-waters.