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By boiling, however, the ignited residue with spirits of wine this difficulty is said to be overcome, as the ferrocyanate is absolutely insoluble in that menstruum, while the cyanuret, at that heat, freely dissolves, and is as easily re-deposited on cooling. "There is, however, a much better process by which this salt may be formed, viz. by simply transmitting hydrocyanic acid through potassium.
This is from that author's work entitled, "Electro Metallurgy," American edition: "The cyanide of potassium, so often alluded to while treating of the metallo-cyanides, may be formed in several ways. It may be obtained by heating to a dull redness the yellow ferrocyanate of potash, in a covered vessel, filtering and rapidly evaporating it.
Although the modes of making this acid are very numerous, there is but one which is likely to be employed on a very large scale, and that is its formation from the yellow ferrocyanate by means of sulphuric acid.
Chlorine decomposes it and sets the iodine free, and chlorohydric acid converts it into a chloride. It fuses below a red heat. Although the effect of light on the iodide is less rapid than on the chloride, the former sooner turning black, assuming a brown tinge; but when in connection with gallic acid and the ferrocyanate of potash, it forms two of the most sensitive processes on paper.
The objection to this method, however, is that without great care the whole of the ferrocyanate is not decomposed, a circumstance which much reduces its value for electro-metallurgy.
The ferrocyanate of potash may be considered as containing three equivalents of hydrocyanic acid, two of potash and one of iron; but, unfortunately, we can only obtain half the acid from the salt, owing to the formation of a compound during its decomposition which resists the action of the acid.
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