Bismuth Thioiodide, BiSI

Bismuth Thioiodide, BiSI, is obtained by dissolution of bismuth trisulphide in molten bismuth triiodide, by heating together bismuth trisulphide and iodine, by heating together bismuth trisulphide, iodine and sulphur for a long time, or by the action of thioacetic acid on bismuth triiodide in the cold:

BiI₃ + CH₃COSH + H₂O = BiSI + H₃COOH + 2HI

It crystallises in small, steel-grey, metallic needles resembling bismuth glance. It is decomposed on heating to the melting point into bismuth trisulphide and triiodide. It is not acted upon by water or dilute mineral acids even on boiling; concentrated hydrochloric acid decomposes it with evolution of hydrogen sulphide, concentrated nitric acid with evolution of iodine and sulphur, iodine also being liberated by the action of warm potassium hydroxide (ammonia and other weak bases act in a similar manner, but more slowly and less completely).