3951. Test for the Strength of Prussic Acid

3951.     Test for the Strength of Prussic Acid. For estimating the strength of the commercial acid the following plan, proposed by Dr. Ure, will be found very exact and convenient. To 100 grains, or any other convenient quantity of the acid contained in a small phial, add in succession, small quantities of the peroxide of mercury in fine powder, till it ceases to bo dissolved on agitation. The weight of the red precipitate taken up being divided by 4, gives a quotient representing the quantity of real prussic acid present. By weighing out beforehand, on a piece of paper or a watch-glass, 40 or 50 grains of the peroxide, the residual weight of it shows at once the quantity expended. The operation may be always completed in five minutes, for the red precipitate dissolves as rapidly in the dilute prussic acid, with the aid of slight agitation, as sugar dissolves in water. Should the presence of muriatic acid be suspected, then the difference in the volatility of prussiate and muriate of ammonia may be had recourse to with advantage; the former exhaling at a very gentle heat, the latter requiring a subliming temperature of about 300° Fahr. After adding ammonia in slight excess to the prussic acid, if wo evaporate to dryness at a heat of 212°, wo may infer from the residuary sal-ammoniac the quantity of muriatic acid present. Every grain of sal-ammoniac corresponds to .6822 grains of muriatic acid.

3952. Cyanic Acid

3952.    Cyanic Acid. A compound of cyanogen and oxygen only known in its hydrated state in combination with 1 equivalent of water. It combines with bases to form cyanates. "When in contact with water for a few hours it suffers decomposition, and is converted into bicarbonate of ammonia. It cannot be preserved for any length of time, as it soon passes spontaneously into a white, opaque, solid mass, to which the name of cyamelide has been given, which may be reconverted into cyanic acid by distillation. It reddens litmus strongly.

3953. To Obtain Cyanic Acid

3953.    To Obtain Cyanic Acid. Distill dry cyanuric acid, or cyamelide, in a retort, and collect the product in a well-cooled receiver. It is also formed when cyanogen is transmitted over carbonate of potassa heated to redness; a cyanate of potassa results.

Or: Pass a current of sulphuretted hydrogen gas through water in which cyanate of silver is diffused. The sulphuretted hydrogen must not bo passed so long as to decompose all the cyanate of silver; for then the cyanic acid is converted into other products by the excess of the sulphuretted hydrogen.

3954. Hydroferridcyanic Acid

3954. Hydroferridcyanic Acid. This is sometimes written hydrqferricyanic acid, and is a compound of ferridcyanogen and hydrogen. With the oxides of metals this acid forms ferridcyanides ; the ferrid-cyanide of potassium is the red prussiate of potash used in the arts.