Carmine is a brilliant red colouring matter prepared from cochineal, either by precipitating an aqueous infusion with alum, or by extracting with dilute sodium carbonate solution and adding white of egg and sulphuric acid. It occurs in light, bright red pieces, which can readily be reduced to powder. Carmine contains about 50 per cent. of carminic acid, and should not leave more than 9 per cent. of ash, consisting chiefly of calcium and aluminium oxides. Carmine gives on heating an odour of burnt feathers. It should dissolve readily in dilute solution of ammonia, leaving only a slight residue of aluminium hydroxide. It may be compared with a pure standard specimen by rubbing 1 decigram of each with 10 grammes of starch and noting the depth of colour produced. Carmine should not be exposed to light. It is sometimes adulterated with inorganic matter (e.g., aluminium oxide or barium sulphate), or with aniline dyes.
Insoluble in water or diluted acids.
Uses.—Carmine is used to colour ointments, tooth powders, tooth washes, dusting powders, and other pharmaceutical preparations. If used in solid form prolonged trituration with powder is necessary to obtain a good colour and even distribution. To obtain the maximum of colour the carmine should be dissolved in a small quantity of strong solution of ammonia and triturated with the powder. A good solution for colouring neutral or alkaline mouth washes and mixtures is Liquor Carmini, an ammoniacal aqueous solution, of which 3 or 4 drops to the fluid ounce of liquid is sufficient. The colouring matter of Liquor Carmini is precipitated in acid solutions. Carmine is much used for staining histological specimens; it is less fugitive than the aniline colours.
- Glycerinum Carmini, B.P.C.—GLYCERIN OF CARMINE. 1 in 16.
- Glycerin of carmine is but slightly alkaline, and has a pure carmine tint. It is a suitable colouring agent for neutral or alkaline liquids, 3 decimils (0.3 milliliters) (5 minims) being added to 30 mils (1 fluid ounce) of mixture. For many purposes, however, Glycerinum Cocci will be found more suitable.
- Liquor Carmini, B.P.C.—SOLUTION OF CARMINE. 1 in 16.
- Solution of carmine is a suitable colouring agent for alkaline mixtures, month washes, lotions, etc., in the proportion of about 1 per cent. The red colouring matter is precipitated by acids; for colouring acid mixtures tincture of cudbear should be employed. The "Liquor Rosae Dulcis'' of commerce is usually prepared from carmine.