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Volume 4, 1871
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Art. LVIII.—New Process for. the Manufacture of Sulpho-cyanide, of Potassium.

[Read before the Wellington Philosophical Society, 25th November, 1871.]

The methods by which this salt is now prepared for laboratory and other purposes necessitates several purifying processes, in order that the ultimate

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product may have the degree of purity required; thus we have first the formation of the crude article, by heating to tranquil fusion dry ferro-cyanide of potassium with sulphur, and this product has to be treated in various ways in order to remove sulpho-cyanide of iron, alkaline sulphides, etc., from the salt in request.

The processes, therefore, necessary to accomplish this purification must largely add to its cost. To save this expenditure in labour and material I have sought to effect the economical manufacture of this salt by the application of a process or reaction which I have taken advantage of in discriminating the state in which sulphur exists on the surfaces of sulphurized gold.* This process consists in applying cyanide of potassium to such compounds at common temperatures, when any sulphur present in a free state would combine with it to form a sulpho-cyanide, but if present as a sulphide would only be transformed into a soluble sulphide.

In my first experiments, however, for the purpose of preparing this salt direct from cyanide of potassium and common flour of sulphur, I found that unless the temperature of the mixture was raised considerably, only a very small portion of the sulphur was taken up, and the product was then contaminated with the impurities I designed to omit.

This refusal of sulphur to combine freely with the cyanide at common temperatures was, I found, entirely due to the presence of some gas, probably air, which substance may readily be got rid of, as will suggest itself, by pouring the sulphur into boiling water and keeping up ebullition for a few minutes. When the water and sulphur is quite cooled down the cyanide may be added to the sulphur in an equivalent proportion.

The quickest way to effect the combination is to suspend the wet sulphur in a porous bag near the top of the cyanide solution, when in a few days the combination will be complete, and a product obtained comparable in purity with that of the cyanide used. I need not state that the operation should be carried on in an air-tight vessel.

It is absolutely necessary that the cyanide used should be free from caustic alkali, otherwise sulphides would be generated; it is but rarely, however, that cyanide is thus contaminated.

If the precautions indicated are taken, the product is sufficiently pure for use in all the ordinary applications to which this salt is put in the laboratory.

[Footnote] * See Trans. N.Z. Inst., Vol. III., p. 216.