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

by T. Frederick Hardwich

Ammonia dissolves Chloride of Silver freely, as do solutions of Hyposulphite of Soda and Cyanide of Potassium. Concentrated solutions of alkaline Chlorides, Iodides, and Bromides are likewise solvents of Chloride of Silver, but to a limited extent, as will be more fully shown in Chapter IV., when treating of the modes of fixing the Photographic proofs.

Dry Chloride of Silver carefully heated to redness fuses, and concretes on cooling into a tough and semi-transparent substance, which has been termed horn silver or luna cornea.

Placed in contact with metallic Zinc or Iron acidified with dilute Sulphuric Acid, Chloride of Silver is reduced to the metallic state, the Chlorine passing to the other metal under the decomposing influence of the galvanic current which is established.

Preparation and Properties of the Subchloride of Silver.—If a plate of polished Silver be dipped in solution of Perchloride of Iron, or of Bichloride of Mercury, a black stain is produced, the Iron or Mercury Salt losing a portion of Chlorine, which passes to the Silver and converts it superficially into Subchloride of Silver. This compound differs from the white Chloride of Silver in containing less Chlorine; the composition of the latter being represented by the formula AgCl, that of the former may perhaps be written as Ag₂Cl(?).

Subchloride of Silver is interesting to the Photographer as corresponding in properties and composition with the ordinary Chloride of Silver blackened by light. It is a pulverulent substance of a bluish-black colour not easily affected by Nitric Acid but decomposed by fixing agents such as Ammonia, Hyposulphite of Soda, or Cyanide of Potassium, into Chloride of Silver which dissolves, and insoluble metallic Silver.

THE CHEMISTRY OF IODIDE OF SILVER.

The properties of Iodine are described in the third division of the Work: they are analogous to those of Chlorine and Bromine, the Silver Salts formed by these elements bearing also a strong resemblance to each other.

Preparation and Properties of Iodide of Silver.—Iodide of Silver may be formed in an analogous manner to the Chloride, viz. by the direct action of the vapour of Iodine upon metallic Silver, or by double decomposition, between solutions of Iodide of Potassium and Nitrate of Silver.

When prepared by the latter mode it forms an impalpable powder, the colour of which varies slightly with the manner of precipitation. If the Iodide of Potassium be in excess, the Iodide of Silver falls to the bottom of the vessel nearly white; but with an excess of Nitrate of Silver it is of a straw-yellow tint. This point may be noticed, because the yellow salt is the one adapted for Photographic use, the other being insensible to the influence of light.

Iodide of Silver is tasteless and inodorous; insoluble in water and in dilute Nitric Acid. It is scarcely dissolved by Ammonia, which serves to distinguish it from the Chloride of Silver, freely soluble in that liquid. Hyposulphite of Soda and Cyanide of Potassium both dissolve Iodide of Silver; it is also soluble in solutions of the alkaline Bromides and Iodides, as will be further explained in Chapter IV.