Silver iodide, AgI

The silver iodide, AgI, can be synthesized by heating silver with iodine, and is precipitated by mixing solutions of iodides and silver salts. It is also formed by the decomposition of hydrogen iodide by silver, and by the action of iodine on silver chloride or bromide.

Transition-curve of silver iodide

Silver iodide is known in an amorphous form, and also in hexagonal and cubic crystals. Its melting-point is 352° C.; after fusion and resolidification it expands with fall in temperature, a phenomenon explained by assuming that the affinity of silver for iodine diminishes with the temperature. Its density is given as 5.609; 5.71 (native cubic crystals); 5.687 (fused); 5.669 (crystalline at 14° C.); 5.596 (compressed amorphous). The transition-point of the hexagonal to the cubic form is 146° to 147° C., the colour changing from pale yellow to golden yellow. The transition-curve is given in Fig.

The specific heat of the hexagonal form is 0.0544, and that of the cubic form 0.0577. The solubility at 20.8° C. is given as 1.5×10^-8 gram-molecule per litre of water, another value being 1.23×10^-8 gram-molecule. The heat of formation from the elements is given as 13.8 Cal., 14.3 Cal., 14.57 Cal., 15.1 Cal., 15.158 Cal., and 15.17 Cal.

Silver iodide is only slightly soluble in ammonia, but dissolves in sodium thiosulphate, concentrated hydriodic acid, and saturated solutions of potassium iodide. It forms a series of double salts with silver bromide, with mercuric iodide, and with the iodides of the alkali-metals. Double compounds of silver iodide and ammonia of the formulae AgI,3NH₃ (6.92); AgI,1½NH₃ (7.25); AgI,NH₃ (8.56); AgI,2NH₃ (7.05); and AgI,½NH₃ (11.59) have also been prepared, the figures in parentheses indicating the calculated heats of formation in large calories.

Like the other silver halides, silver iodide is sensitive to light, the loss in weight not exceeding 1.1 per cent. The sensitiveness to light is diminished by the presence of potassium iodide, and increased by that of Silver nitrate. In the second instance the liberated iodine reacts with the Silver nitrate:

6AgNO₃ + 3H₂O + 3I₂ = 5AgI + AgIO₃ + 6HNO₃.

The possibility of developing the latent image was discovered by Daguerre, who at first employed a silver plate coated with the iodide, development being effected by exposing the plate to the action of mercury-vapour. Later, he substituted glass for silver, and developed with a mixture of Silver nitrate and ferrous sulphate. His discovery led to the introduction of the wet collodion-process with silver iodide as the sensitive material.