A Manual of Photographic Chemistry, Including the Practice of the Collodion Process

by T. Frederick Hardwich

In the solution of Silver compounds by Hyposulphite of Soda a double decomposition always takes place; thus:—

	Hyposulphite of Soda	+	Chloride of Silver
=	Hyposulphite of Silver	+	Chloride of Sodium.

The Hyposulphite of Silver with an excess of Hyposulphite of Soda forms a soluble double salt, which may be crystallized out by evaporating the solution. It possesses an intensely sweet taste, and contains one atom of Hyposulphite of Silver, chemically combined with two of Hyposulphite of Soda. In addition to this there is a second double Salt, differing from the first in being very sparingly soluble in water. It is formed by acting upon Chloride of Silver with a solution of Hyposulphite of Soda already saturated, or nearly so, with Silver Salts; and contains single atoms of each constituent.

The fact that the Silver contained in an ordinary fixing Bath is present in the state of Hyposulphite must be borne in mind, because this salt is liable to undergo peculiar chemical changes, as will be better shown in Chapter VIII.

Iodide of Silver is dissolved by Hyposulphite of Soda more slowly than Chloride of Silver, and the amount eventually taken up is less. This is explained as follows:— During the solution of Iodide of Silver, Iodide of Sodium is formed, and this alkaline Iodide has a prejudicial effect upon the continuance of the process. Chloride of Sodium has not the same action, neither has Bromide of Sodium, consequently the corresponding Silver Salts dissolve to a greater extent than the Iodide.

ALKALINE CYANIDES.

The chemistry of Cyanogen is sketched in Part III.

The Cyanide of Potassium is the salt most frequently employed in fixing. It occurs in commerce in the form of fused lumps of considerable size. In this state it is usually contaminated with a large percentage of Carbonate of Potash, amounting in some cases to more than half its weight. By boiling in proof Spirit the Cyanide may be extracted and crystallized, but this operation is scarcely required as far as its use in Photography is concerned.

Cyanide of Potassium absorbs moisture on exposure to the air. It is very soluble in water, but the solution decomposes on keeping; changing in colour and evolving the odour of Prussic Acid, which is a Cyanide of Hydrogen. Cyanide of Potassium is highly poisonous, and must be used with caution.