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silver halide[′sil·vər ′ha‚līd]
any of the chemical compounds of silver and the halogens. Halides of monovalent silver have been studied thoroughly; these include the fluoride AgF, the chloride AgCl, the bromide AgBr, and the iodide AgI. The halides Ag2F and AgF2 are also known to exist; the latter is a strong oxidizing agent. AgF crystals are colorless, those of AgCl are white, and AgBr and AgI crystals are yellow. The melting point of AgF is 435°C, of AgCl is 457.5°C, of AgBr is 430°C (decomposes upon heating to 700°C), and of AgI is 555°C (melts with decomposition). Crystal hydrates with the formula AgF • x H2O, where x = 1, 2, or 3, are known. AgF cannot be stored in glass vessels since it attacks the glass.
All silver halides except fluorides have very poor solubility in water. For example, the solubility of AgCl is 1.55 × 10-3 gram per liter (g/l), of AgBr is 3.5 × 10-4(g/l, and of AgI is 3.4 × 10-5g/l. Solubility increases substantially in the presence of the corresponding halogen acids or salts thereof owing to the formation of complex compounds of the [AgX2]- type, where X represents Cl, Br, or I. All silver halides dissolve in ammonia with the formation of complex ammoniates; this property is used in the purification and recrystallization of silver halides. In the solid state, silver halides combine with gaseous ammonia to yield the complex compounds AgX • NH3 and AgX • 3NH3.
Silver halides are easily reduced to metallic silver by the action of Zn, Mg, Hg, alkali metals, and H2. The halides AgCl and AgBr can be reduced to metal by smelting with Na2CO3. Silver halides are obtained by the direct interaction of halogens with silver at high temperatures. Sparingly soluble silver halides can also be obtained by precipitation from an AgNO3 solution with the aid of the corresponding halogen acid or (soluble) salt thereof, whereas AgF is prepared by the interaction of Ag2O or Ag2CO3 with HF.
Silver chloride serves as a detector of cosmic radiation and is also used in medicine; AgBr is used as a catalyst in organic synthesis. Silver chloride, bromide, and iodide are important in the manufacture of such photosensitive materials as photographic paper and motion-picture and photographic films. The silver halides in these materials are reduced to metallic silver during photographic development.
S. I. GINZBURG [23–875–]