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Silver chloride, AgCl

Addition of a solution of hydrogen chloride or a metallic chloride to one of a silver salt gives a heavy, white, amorphous precipitate of silver chloride, which can be converted into a crystalline form by evaporating its solution in concentrated hydrochloric acid or ammonium hydroxide. The native variety is known as horn-silver, and crystallizes in the cubic system. Its density is 5.31 to 5.55. The crystalline form is also produced by slow diffusion of a solution of hydrogen chloride into one of Silver nitrate. Its cubic crystals are isomorphous with those of silver bromide.

The melting-point of amorphous silver chloride is given as 451° C., 452° C., and 455° C., the substance fusing to a yellow liquid. After solidification its density is 5.45 to 5.59. The specific heat is 0.08775. At 18° C. its solubility is 1.7×10-5 gram-molecules per litre of water, and at 25° C., 1.6×10-5. Silver chloride also dissolves in solutions of ammonia, sodium thiosulphate, potassium cyanide, mercuric nitrate, and in concentrated hydrochloric acid and saturated chloride solutions. For the heat of formation from the elements Berthelot gives 29.2 Cal., and Thomsen 29.38 Cal. Wolff considers their results too low, and gives the value 30.612 Cal., that stated by Braune and Koref being 30.41 Cal. The intermediate value 29.940 Cal. is given by Fischer.

Chlorine reacts with an aqueous solution of Silver nitrate to form chloride and chlorate of silver, the hypochlorite being an intermediate product:

6AgNO3 + 3Cl2 + 3H2O = 5AgCl + AgClO3 + 6HNO3.

Silver chloride absorbs gaseous ammonia, forming double compounds of the composition 2AgCl,3NH3 and AgCl,3NH3. The first crystallizes in rhombic plates from ammoniacal solutions of silver chloride; the second is formed in long prisms by heating the chloride with a saturated aqueous solution of ammonia under pressure. The dissociation-pressures of these compounds have measurable values. Other compounds of similar type with the formulae AgCl,3NH3 (9.16), AgCl,1½NH3 (10.52), and AgCl,NH3 (11.11) have also been prepared, the figures in parentheses giving the calculated heats of formation in large calories. The interaction at 200° C. of iodic acid and ammoniacal solutions of the chloride to form silver iodide has been the subject of investigation. A cold ammoniacal solution of sodium peroxide reduces silver chloride, and it is also reduced to the metal by the action of zinc in the course of a few days. With sodium thiosulphate it probably combines to form a soluble double salt, 2Na2S2O3,Ag2S2O3,2H2O, but the compound has not been isolated.

Potassium ferrocyanide and silver chloride react to form insoluble silver potassium ferrocyanide in accordance with the equation

3AgCl + K4Fe(CN)6 = Ag3KFe(CN)6 + 3KCl.

Neither silver bromide nor iodide react thus, and it is possible to estimate silver chloride in presence of iodides by means of this reaction, the excess of potassium ferrocyanide remaining after filtration being determined by titration with standard permanganate.

Silver chloride is sensitive to light, though not in so great a degree as the bromide or iodide. On exposure to light it develops a violet colour, with evolution of 4.1 per cent, of chlorine in air, and of 81.0 per cent, of chlorine in vacuum, and formation of the so-called "photochloride." On removing the liberated chlorine a loss in weight of 8.57 per cent, is stated to have been observed, although Koch and Schrader consider it not to exceed between 1 and 2 per cent. If diffusion of the chlorine is prevented, the reaction is found to be reversible:

2AgClAg2Cl + Cl.

Ordinary photographic printing-out paper contains silver chloride with a basis of albumin, collodion, or gelatin. On exposure to light, the organic material reduces the salt to metallic silver, with intermediate production of the subchloride. The metal is liberated in the colloidal form, and the image is rendered more stable by " toning," gold or platinum being deposited on it from solution.
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