a. From hydrated ferrocyanide of potassium.—By heating it in a glass retort with oil of vitriol and water, Everitt states that the best proportions are nearly ten parts of the salt to seven of oil of vitriol (diluted with any convenient amount of water). Adopting these proportions, 422·4 parts of ferrocyanide of potassium yield 81 parts of hydrocyanic acid. The greater part of the hydrocyanic acid passes over at the beginning of distillation, at a temperature a little above 212° Fahr.; and when the residual liquid reaches a higher temperature, the water (which then contains but little hydrocyanic acid) is then carried over. It is therefore necessary to employ a good condensing apparatus, or the hydrocyanic acid which passes over at first will for the most part be dissipated in vapour mixed with the air of the apparatus. This loss may also be obviated by placing water in the receiver. The residue need not be boiled down to dryness; it will be found best to distil off from two thirds to three fourths of the liquid, according to the amount of water present.
It is not necessary to dissolve the ferrocyanide in water previous to adding the sulphuric acid, as it readily dissolves in the water during the process of distillation.
Three conditions are important to be observed in the arrangements of the apparatus:—1. The mixture in the retort should not be allowed to spirt over. 2. It should contain but little air. 3. It should present the greatest possible amount of surface to be cooled.
If sulphate of potassium and prussian blue are spirted over into the distillate, this must be carefully rectified over a small quantity of magnesia, chalk, or carbonate of barium.
b. From cyanide of potassium (without distillation).—To a solution of nine parts of tartaric acid in sixty parts of water, contained in a well-stoppered bottle nearly filled with it, four parts of pure cyanide of potassium are added; the vessel is shaken, frequently dipped into cold water, and then left in the cold for twelve hours; and the aqueous hydrocyanic acid, which contains but a very small quantity of tartrate of potassium, is poured off from the crystallised tartrate.[351] This acid contains 3·6 per cent. of anhydrous hydrocyanic acid.
[351] ‘London Med. Surg. Journ.,’ vi, 524.
c. From cyanide of mercury.—126 (accurately weighed) parts of cyanide of mercury are agitated with at least 28 parts of iron filings, in a well-stoppered bottle, containing 49, or rather more, parts of oil of vitriol, diluted with a considerable quantity of water. The agitation is continued until a portion of the liquid taken out is not blackened by sulphuretted hydrogen.
The solution, decanted from the iron and mercury, is then placed in a retort and distilled; the acid coming over at a gentle heat. Excess of iron accelerates the decomposition.
d. From cyanide of silver.—Everitt recommends 200 parts of pure cyanide of silver to be shaken up with 240 parts of hydrochloric acid of specific gravity of 1·129, and when the decomposition is complete, the hydrocyanic acid to be separated from the chloride of silver by decomposition.
This hydrocyanic acid may contain a small quantity of hydrochloric acid, a not very objectionable admixture, since it retards decomposition. It possesses the advantage of definite strength.