Cuprous iodide, CuI

The preparation of Cuprous iodide, CuI, can be effected by methods analogous to those adopted for cuprous bromide. Among them are the interaction of finely divided copper and either iodine or concentrated hydriodic acid under the influence of heat.

The iodide is also produced by the action of sulphur dioxide on an aqueous solution of cupric sulphate and potassium iodide:

2CuSO₄ + 4KI = 2CuI + 2K₂SO₄ + I₂; SO₂ + 2H₂O + I₂ = H₂SO₄ + 2HI.

The hydrogen iodide produced reacts with another portion of the cupric sulphate, the whole reaction corresponding with the equation

4CuSO₄ + 4KI + 2SO₂ + 4H₂O = 4CuI + 2K₂SO₄ + 4H₂SO₄.

Ferrous sulphate can also be employed as reducer:

2CuSO₄ + 2KI + 2FeSO₄ = 2CuI + Fe₂(SO₄)₃ + K₂SO₄.

The iodide can also be prepared from aqueous solutions of cupric sulphate and potassium iodide without subsequent reduction, the liberated iodine being removed by agitation with alcohol.

The best method of preparing cuprous iodide depends on the interaction of cold solutions of cupric sulphate (30 grams), potassium iodide (16 grams), and sodium thiosulphate (25 grams):

2CuSO₄ + 2KI + 2Na₂S₂O₃ = 2CuI + K₂SO₄ + Na₂S₄O₆ + Na₂SO₄.

After repeated washing with water and with alcohol, the precipitated iodide is dried in a vacuum-desiccator over sulphuric acid.

The white powder obtained by the last process gradually acquires a slight brown tint, but the salt does not exhibit phototropy. Cuprous iodide crystallizes in white tetrahedra, melting at 590° C. according to Monkemeyer, or 628° C. according to Carnelley and O'Shea. Carnelley and Williams give the boiling-point as between 759° and 772° C. Its density is D₄^14.5 = 5.672. The heat of formation of the simple molecule CuI from solid copper and solid iodine is 16.26 Cal.

Solutions of cuprous iodide in hydrochloric acid and in ammonia readily absorb carbon monoxide, the maximum absorption corresponding with one molecule of the gas to each atom of copper. The salt also dissolves in solutions of sodium sulphate and potassium cyanide, but the liquids produced do not absorb carbon monoxide.

Several complex derivatives of cuprous iodide have been prepared, including CuI,2NH₃; 2CuI,2NH₄I,H₂O₂; CuI,NH₄I,4(NH₄)₂S₂O₃; 2CuI,(NH₄)₂S₂O₃,H₂O; 2CuI,K₂S₂O₃,H₂O; and 2CuI,Na₂S₂O₃,H₂O.