POTASSIUM CYANIDE (Formula, KCN; molecular weight, 65; synonyms, cyanide, cyanide of by fusing in an iron crucible a mixture of eight parts of well-dried potas sium ferro-cyanide with three parts of potassium carbonate.
When effervescence has ceased and the iron which separates has settled down, the clear liquid is poured off on to slabs. On cooling it solidifies, and is broken up into the irregular masses met with in commerce.
The purest form, however, is made by passing vapor of hydro-cyanic acid into a solution of potash in absolute alcohol, when the cyanide is deposited in small octahedral crystals.
Commercial potassium cyanide occurs in white lumps, containing about 6o per cent. of cyanide, the rest being cyanate and carbonate. These impurities only serve to lessen its strength and have no injurious effect.
It is extremely deliquescent, it being soluble in its own weight of water. It is insoluble in alcohol. It is one of the most dangerous of poisons. When cyanide is employed always keep handy an antidote consisting of a mixture of ferrous sulphate and ferric chloride, made as follows : Dissolve 20 grains of potassium carbonate in 2 ounces of water, and into this pour 2 drachms of the pharmaceutical tincture of perchloride of iron ( Tinctura ferri perchloridi), and half fill with water. When using dissolve io grains ferrous sulphate in a little water, add this to the
other mixture, shake up, and drink off at once. In half an hour take an emetic. If treated with an acid it gives off hydro-cyanide, or prussic acid gas, which, when inhaled, causes insensibility, and has been known to cause death. It should, therefore, be used under all circumstances with extreme caution.
It has several uses in photography, principally as a fixing agent in the wet collodion process, on account of its property of dissolving silver chloride and iodide. It cannot conveniently he used as a fixing agent for dry plates, owing to its disintegrating action upon gelatine. It is not, however, superior to hyposulphite of soda, and is very dangerous. It is further employed for reducing the density of negatives.