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Stannous Iodide, SnI2

Stannous Iodide, SnI2, is obtained by adding potassium iodide in slight excess to a concentrated solution of stannous chloride, or by the action of hydriodic acid on tin; it crystallises in orange-red octahedra, which may be obtained by melting the compound, or evaporating its solution in carbon disulphide; but it is probably dimorphous. It melts at 320° C., and boils at 720° C. The dihydrate SnI2.2H2O is said to exist; 100 parts of water dissolve 0.93 parts of anhydrous stannous iodide at 20° C., and 4.03 parts at 100° C. It is much more soluble in hydriodic acid and alkali halide solutions, owing to the formation of a complex acid or salts.

When a solution of stannous chloride is titrated with iodine it is sometimes assumed that an additive reaction takes place with the formation of stannic iodochloride, SnCl2I2; but most likely a mixture of stannic chloride and iodide is produced thus:

2SnCl2 + 2I2 = SnCl4 + SnI4;

stannic iodide is, however, reduced to stannous iodide by excess of stannous chloride, thus:

SnI4 + 2SnCl2 = 2SnI2 + SnCl4.

Stannous iodide combines with hydrogen iodide to form iodostannous acid, HSnI3; and unstable, pale yellow needles of this substance separate at 0° C. from a saturated solution of stannous iodide in hydriodic acid. To this acid there correspond the iodostannites or stannoiodides: NaSnI3.3H2O, KSnI3.3H2O, NH4SnI3.3H2O, Sr(SnI3)2, and Ba(SnI3)2, which separate when the corresponding saline iodides are added to concentrated stannous chloride solution.

Stannous iodide combines with ammonia to form the compound SnI2.2NH3, which is yellow.

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