Hydroiodic acid (or hydriodic acid ) is a highly acidic aqueous solution of hydrogen iodide ( H I )
(concentrated solution usually 48 - 57% HI). It is the second strongest hydrohalic acid , after hydroastatic acid . Hydroiodic acid is a commonly used chemical reagent and is one of the strong acids that ionize completely in an aqueous solution. Concentrated hydroiodic acid has a pH of less than 0. [ citation needed ]
Hydrogen iodide ( H I ) is a diatomic molecule and hydrogen halide . Aqueous solutions of HI are known as hydroiodic acid or hydriodic acid , a strong acid . Hydrogen iodide and hydroiodic acid are, however, different in that the former is a gas under standard conditions, whereas the other is an aqueous solution of said gas. They are interconvertible. HI is used in organic and inorganic synthesis as one of the primary sources of iodine and as a reducing agent .