A22 Preparation of Hydrogen Iodide

Description
The reaction of red phosphorus and iodine is initiated by water which also hydrolyzes any phosphorus triiodide. Gay-Lussac used this method to produce hydrogen iodide.

Equation
2 P + 5 I₂ + 8 H₂O --> 10 HI + 2 H₃PO₄

Reference
B. Z. Shakhashiri, "Chemical Demonstrations", Vol. 2, The University of Wisconsin Press, Madison ,Wisconsin, (1985) p. 221.