Arsenic Pentiodide, AsI₅

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. 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.

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. The absorption spectrum of the solution in carbon disulphide is similar to that of a mixture of the triiodide and iodine.

A series of double compounds of arsenious oxide and the iodides of bivalent light and heavy metals has been described. The compounds are obtained in crystalline form by saturating hot moderately concentrated solutions of the iodides with arsenious oxide and allowing to cool. The following have been prepared:

BeI₂.3As₂O₃.8H₂O
Mg (or Ca or Sr)I₂.3As₂O₃.12H₂O
BaI₂.3As₂O₃.9H₂O
ZnI₂.3As₂O₃. 10H₂O
Mn (or Fe or Co)I₂.4As₂O₃.12H₂O
NiI₂.4As₂O₃.10H₂O
Also AlI₃.6As₂O₃.18(?)H₂O

These compounds resemble in properties the arsenites of the metals, the characters of the iodides being suppressed. They are moderately stable in dry air but tend to become oxidised on keeping. With the exception of the magnesium compound they are slightly soluble in water, the solution apparently containing a complex salt with a simple metallic cation. When heated with water they undergo partial decomposition into the iodide and arsenious oxide.

Similar compounds of the alkali metals are also known, but except in the case of the lithium salt, LiI.2As₂O₃.3H₂O, prepared as above, the crystals are anhydrous. Thus a solution containing ammonium dihydrogen arsenite, ammonium iodide and arsenious oxide yields hexagonal prisms of composition NH₄1.2As₂O₃, stable up to 180° C. The sodium salt, NaI.2As₂O₃, crystallises from a hot solution containing sodium iodide and arsenious oxide or sodium arsenite; it is decomposed by water. Two potassium salts are known: KI.As₂O₃, which separates in yellow plates from a hot solution containing potassium iodide and arsenic triiodide in dilute hydriodic acid, and also KI.2As₂O₃, prepared analogously to the sodium salt. The rubidium salt, RbI.As₂O₃, and the ccesium salt, CsI.As₂O₃, have been obtained by the method used for the corresponding potassium salt.

A complex bromiodide of mercury and arsenic, AsHg₆Br₁₂I₃, presumably the arsenic salt of the acid HI(HgBr₂)₂, prepared by addition of hydriodic acid to a benzene solution of mercuric bromide, has been obtained.