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.