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Silver Iodide

soluble, cent and nitrate

SILVER IODIDE (Pr., lodure d'argent ; Ger., lodsilber) AgI. Molecular weight, 235. Solubilities, practically insoluble in water, alcohol, and ether, soluble in potassium cyanide, and " hypo." It is obtained by adding a soluble iodide to silver nitrate solution ; when precipitated in the pre sence of excess of alkaline iodide it is a pale yellow powder, whilst with excess of silver nitrate it is a deep orange, curdy precipitate; the latter darkens in light, whilst the former undergoes no visible change. It is but very slightly soluble in strong ammonia, r in 2,50o parts, but it is i readily soluble in potassium iodide solution, forming a double salt AgIKI, or AgI2KI, both of which are decomposed by the addition of water depositing pale yellow powdery silver iodide. It is also soluble in silver nitrate solu tion, forming the double salt which is more soluble in cold than in hot solutions. A io per cent. solution of the nitrate

dissolves .053 per cent. AgI, an 8 per cent. solu tion -077 per cent., and alcoholic and ethereal solutions dissolve more. The negative silver bath for sensitising wet collodion plates should always be saturated with silver iodide before use, otherwise the iodide will be dissolved from the film.

Silver iodide is chiefly employed for the wet plate process, and as an addition to gelatino bromide emulsions, in which it acts as a restrainer of fog during digestion and produces greater sensitiveness. It is usually mixed with the alkaline bromide, and it is supposed to form a double salt generally known as bromo-iodide of silver, and this is confirmed by the fact that such an emulsion only shows one maximum of sensi tiveness in the spectrum, whereas the two salts separately emulsified and then mixed show two distinct maxima.