Cyanide of potassium is prepared by igniting together ferrocyanide of potassium and carbonate of potassa. The carbonic acid escapes; a portion of the cyanogen combines with a portion of the potassium to form cyanide of potassium; the liberated oxygen of the potassa combines with another portion of cyanogen to form cyanic acid, which then unites with an undecomposed portion of potassa; and the iron which is separated falls to the bottom of the melted mass. The liquid portion, being now poured off, and allowed to cool, constitutes the salt in question, mixed with a small and insignificant proportion of cyanate of potassa.


Thus made, cyanide of potassium is in white, opaque lumps, with a sharp somewhat alkaline taste combined with that of hydrocyanic acid, deliquescent, very soluble in water, and slightly soluble in alcohol. Exposed to the air, it exhales an odour of hydrocyanic acid, owing probably to its decomposition by the carbonic acid of the atmosphere. it has an alkaline reaction. Almost any acid will decompose it, liberating the hydrocyanic acid. Hence, by the addition of an acid to it at the time of exhibition, the effects of hydrocyanic acid can always be obtained; and the same result would almost always follow its administration, even without previous admixture with an acid, as there is usually sufficient acid for the purpose present in the stomach.

Effects. it is highly poisonous in over-doses, acting precisely as hydrocyanic acid. Whether it would produce these effects without the reaction of an acid is not certainly known; but it should never, under any circumstances, be ventured upon in poisonous doses. Some years since, a physician in France prescribed a three-ounce mixture containing 72 grains of cyanide of potassium; a tablespoonful, containing about 12 grains of the salt, to be taken three times a day. At the first dose, the patient fell as from a stroke of lightning, and died in three-quarters of an hour. The prescriber was condemned to pay a fine of 50 francs, and to be imprisoned for three months. (Journ. de Pharm., 3e sÚr., III. 82.)


The dose is one-eighth of a grain, dissolved in a fluidounce of distilled water, with the addition of a little dilute acetic or citric acid, either pure or in the form of vinegar or lemon-juice. it may be repeated and increased, in the same manner as the diluted acid itself. it was proposed as a substitute for hydrocyanic acid, on the presumption of its greater certainty and uniformity of strength; and this recommendation would be well founded, could we always be assured of the purity of the preparation used. But this is by no means the case. it may be used externally also, as a sedative lotion, in the proportion of from one to four grains to a fluidounce of water; and the stronger solution may be employed for removing stains left on the conjunctiva by nitrate of silver, a drop being applied every other day. Care must be taken not to apply this solution, or any other liquid preparation of hydrocyanic acid, too freely to abraded or mucous surfaces.