Arsenic Diiodide, AsI₂

Arsenic Diiodide, AsI₂ or As₂I₄, may be prepared by heating a mixture of arsenic (1 part) and iodine (2 parts) in a sealed tube at 250° C. for about 8 hours. After cooling, the tube is placed vertically and the dark red product again heated to 150° C., when the diiodide sublimes into the upper part of the tube; or the diiodide may be extracted from the product by means of carbon disulphide and crystallised from the latter in an atmosphere of carbon dioxide. A modification of this method was employed by Karantassis, who heated a finely powdered mixture of the elements in a sealed retort from which the air had been displaced by pure dry hydrogen; the mixture was kept at the boiling point for two hours and pure arsenic diiodide was obtained from the product by careful distillation. The diiodide is also formed by heating in a sealed tube at 150° to 190° C. a mixture of arsenic and arsenic triiodide in carbon disulphide solution; the reaction is not complete, however, some triiodide remaining undecomposed.

Arsenic diiodide crystallises in thin prisms of a deep cherry-red colour. In air they are rapidly oxidised and become opaque and brick-red. The crystals, when purified by careful distillation, melt at 130° C., but the undistilled product, as generally prepared, melts at about 120° C. owing to the presence of dissolved arsenic. The boiling point in an atmosphere of hydrogen or carbon dioxide is 375° to 380° C.

The crystals dissolve in carbon disulphide and ebullioscopic measurements indicate that the molecular formula is As₂I₄; they also dissolve in ether, alcohol and chloroform. When exposed to air, these solutions darken in colour owing to the formation of the triiodide, and the diiodide cannot be recovered by evaporation owing to complete decomposition. Boiling acetic anhydride also dissolves the crystals but, on cooling, a yellow substance which appears to contain a derivative of the triiodide separates. The diiodide is also decomposed by pyridine with separation of arsenic, the triiodide being found in solution.

The addition of water or alkali to the crystals, or to the alcoholic solution, causes blackening, the triiodide and arsenic being formed; the decomposition is accelerated by warming. Concentrated sulphuric acid and fuming nitric acid have little action in the cold but, on heating, iodine vapour is expelled, and with the latter acid, iodic and arsenic acids are formed.