Anhydrous Hydrogen Iodide and Hydriodic Acid Anhydrous hydrogen iodide is a pungent, colorless, corrosive gas at room temperature and atmospheric pressure. It is supplied as a compressed liquefied gas in cylinders which should be stored at less than 34°F (1°C). In the presence of heat or light, HI decomposes into hydrogen and iodine. A check valve, trap or vacuum break should be in the delivery line to block suck-back of foreign material which could cause dangerous pressure to develop in the cylinder. Anhydrous HI is used primarily in the manufacture of hydriodic acid, in the preparation of organic and inorganic iodides, and as a reducing agent. In the presence of moisture, hydrogen iodide will corrode most common metals. Piping suitable for HCl should handle HI satisfactorily from the standpoint of corrosion resistance. Avoid fragile, brittle materials if impact damage and mechanical abuse are likely to occur. Commercial 47% hydriodic acid is a clear, colorless to slightly yellow liquid which may take on a brownish color from the formation of free iodine. Hydriodic acid is used in the preparation of iodine salts, pharmaceuticals, and as a disinfectant.

Typical properties of hydrogen iodide
Density of liquid	2.80 gm/ml at boiling point
Density of gas	5.7245 gm/liter (0°C, 1 atm.)(32°F)
Specific gravity of gas (Air = 1)	4.4 (0°C, 1 atm.) 32°F
Boiling point	-35.5°C (1 atm.) (-32°F)
Freezing point	-50.8°C (-59°F)
Solubility	Very soluble in water and slightly soluble in lower alcohols at room temperature
Specific gravity of 57% hydriodic acid (constant boiling solution at 127°C) 261°F	1.70