Dissolve 1 gram of pure fuchsine (rosaniline hydrochloride) in about half a litre of hot water; then add, little by little and shaking well after each addition, 20 c.c. of a saturated aqueous solution of sodium bisulphite, followed by 10 c.c. of strong hydrochloric acid. Cool, and make the bulk up to 1 litre with distilled water. The reagent should give no colour with pure ethyl alcohol when this is treated as described below.