Triiodide, and iodination

Arsenic Pentiodide ( ), Asls.—When a mixture of arsenic and iodine in the requisite proportions is heated in an atmosphere of carbon dioxide in a sealed tube at 150° C., a brown crystalline product is obtained.3 The crystals, which melt at 70° C. and have density 3-93, are soluble in water, carbon disulphide, alcohol, ether and chloroform. The solution in carbon disulphide yields, when allowed to crystallise, a mixture of arsenic triiodide and iodine. The latter is readily lost from the pentiodide, and heating at 100° C. in nitrogen in a sealed tube brings about the decomposition. Like the triiodide, the pentiodide dissolves boron tribromide.4... 

The fusion curve of mixtures of arsenic and iodine shows no evidence of the formation of a pentiodide, but there is a eutectic, of freezing point 71-5° C., which has the approximate composition of this substance.5 The absorption spectrum of the solution in carbon disulphide is similar to that of a mixture of the triiodide and iodine.3... 

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