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Zinc Iodide, ZnI2

Vigorous action occurs when water is dropped on a mixture of zinc dust and iodine, and solutions of zinc iodide have usually been prepared in this way. Anhydrous ZnI2 separates at ordinary temperatures from its aqueous solutions that have been saturated at 100° C.

It forms octahedral crystals, of density 4.696, that melt at 446° C. Unlike the chloride and bromide, it is readily oxidised by nitrogen peroxide and by warming in the air - if the air is moist the reaction occurs at ordinary temperatures. Fused zinc iodide conducts electricity.

The formula ZnI2 is indicated by the boiling-points of its solutions in quinoline.

Solubility of zinc halides
Solubility curves of the zinc halides. The temperature appended for the anhydrous salts represent melting-points.
Zinc iodide is very soluble in water, and the dihydrate separates in hygroscopic prismatic crystals when an 83.5 per cent, solution of ZnI2 is cooled to -8° C. They melt at 27° C. and form the anhydrous salt. The existence of a tetrahydrate is doubtful.

The following solubilities have been determined in grm. ZnI2 per 100 grm. solution: -

ZnI2.2H2O.
Temperature, ° C.-10-50+10+22+27
Solubility80.580.7781.1682.0683.1289.52


ZnI2.
Temperature, ° C.0+18+40+60+80+100
Solubility81.1181.2081.6682.3783.0583.62


[ZnI2] +Aq. = ZnI2.Aq. + 11.300 Cal.

Compounds of Zinc Iodide with Ammonia

Dry ZnI2 absorbs ammonia gas to form the white pentammoniate, ZnI2.5NH4, which is decomposed by water. Rhombic prisms of the tetrammoniate, ZnI2.4NH4, decomposed by warmth or moisture, are obtained by dissolving the anhydrous salt in ammonia solution and evaporating. Tassilly obtained crystals of 3ZnI2.5NH4.3H2O by heating zinc oxide with concentrated ammonium iodide solution, filtering, and crystallising.

Zinc iodide hexammoniate, ZnI2.6NH4, can be obtained, even at ordinary temperatures, by allowing zinc iodide to absorb ammonia gas. It decomposes at 65° C. - the tetrammoniate being finally produced which decomposes at higher temperatures and forms the diammoniate at 199° C.

Double Iodides of Zinc

The following double iodides of zinc are known: NaI.ZnI2.1½H2O; NaI.ZnI2.2H2O; 2NaI.ZnI2.3H2O; KI.ZnI2; 2KI.ZnI2; 2KI.ZnI2.2H2O; 2CsI.ZnI2; 3CsI.ZnI2; NH4I.ZnI2.4½H2O; 2NH4I.ZnI2; CaI2.ZnI2.8H2O; SrI2.ZnI2.9H2O; BaI2.ZnI2.4½H2O.

Tassilly has described some oxyiodides of zinc.

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