Chemical elements
    Physical Properties
    Chemical Properties
      Silver fluoride
      Silver subfluoride
      Silver chloride
      Silver subchloride
      Silver bromide
      Silver oxybromide
      Silver subbromide
      Silver iodide
      Silver hypochlorite
      Silver chlorite
      Silver chlorate
      Silver perchlorate
      Silver bromate
      Silver perbromate
      Silver iodate
      Silver periodates
      Silver suboxide
      Silver monoxide
      Higher oxides
      Silver subsulphide
      Silver sulphide
      Silver sulphite
      Silver sulphate
      Silver selenide
      Silver telluride
      Silver thiosulphate
      Silver dithionate
      Silver azide
      Silver hyponitrite
      Silver nitrite
      Silver nitrate
      Silver phosphides
      Silver hypophosphate
      Silver orthophosphate
      Silver pyrophosphate
      Silver metaphosphate
      Silver arsenite
      Silver arsenate
      Silver carbide
      Silver carbonate
      Silver cyanide
      Silver thiocyanate
      Silver borate
    PDB 1aoo-3kso

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
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 AgINH3 (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.

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