Hydriodic Acid

HI. Colorle fuming gas, of strong acid reaction and suffocating odor, sp.gr. 4.37. Liquefied -17.8° С., 2.9 atm.; solid-55° C. The solution in water is very unstable; the strongest which can be prepared has a sp. gr. of 1.99. It is made by treating phosphorous tri iodide with water, or by pa ing hydrogen sulphide into water containing finely pulverized iodine in suspension.