Cyanogen Iodide, CNI

This compound is formed by the action of iodine on mercuric cyanide; if the iodine is dissolved in anhydrous ether the cyanogen iodide goes into solution in the ether, and remains behind after the evaporation of the latter.

Iodide of cyanogen crystallises in long white needles which melt at 146-5° C. and easily sublime, giving a vapour whose density corresponds to the formula CNI, has an unpleasant smell like that of the bromide, and is very poisonous. This compound often occurs as an impurity in commercial iodine. Its heat of formation from its elements is -23,100 calories, and from (CN)₂ and I₂ + 17,900 calories. It is easily soluble in alcohol and ether, sparingly so in water. This compound is unchanged in aqueous solution by hydrochloric or sulphuric acid, but reacts with hydriodic acid thus:

CNI + HI = HCN + I₂.

The reaction between cyanogen iodide and a soluble iodide in aqueous solution is brought about by the presence of hydrogen ions. It has therefore been proposed to use it as a method of acidimetry.