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 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	-5	0	+10	+22	+27
Solubility	80.5	80.77	81.16	82.06	83.12	89.52

ZnI2.
Temperature, ° C.	0	+18	+40	+60	+80	+100
Solubility	81.11	81.20	81.66	82.37	83.05	83.62

[ZnI₂] +Aq. = ZnI₂.Aq. + 11.300 Cal.

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.

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.