Hydriodic Acid

Once again, although chemically related, hydroiodic acid is not HI but made from it. Commercial "concentrated" hydroiodic acid usually contains 48% - 57% HI by weight. The solution forms an azeotrope boiling at 127 °C at 57% HI, 43% water. Hydroiodidic acid is one of the strongest of all the common halide acids, despite the fact that the electronegativity of iodine is weaker than the rest of the other common halides. The high acidity is caused by the dispersal of the ionic charge over the anion. The iodide ion is much larger than the other common halides which results in the negative charge being dispersed over a large space. By contrast, a chloride ion is much smaller, meaning its negative charge is more concentrated, leading to a stronger interaction between the proton and the chloride ion. This weaker H+---I− interaction in HI facilitates dissociation of the proton from the anion .