Hydroiodic acid, 57%

Hydroiodic acid (or hydriodic acid) is a highly acidic aqueous solution of hydrogen iodide (HI) (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.