Inorganic reactions of iodine(III) in acidic solutions and free energy of iodous acid formation

An analysis of the former works devoted to the reactions of I(III) in acidic nonbuffered solutions gives new thermodynamic and kinetic information. At low iodide concentrations, the rate law of the reaction IO + I− + 2H+ ⇌ IO2H + IOH is k+B [IO ][I−][H+]2 – k−B [IO2H][IOH] with k+B = 4.5 × 103 M−3s−1 and k−B = 240 M−1s−1 at 25°C and zero ionic strength. The rate law of the reaction IO2H + I− + H+ ⇌ 2IOH is k+C [IO2H][I−][H+] – k−C [IOH]2 with k+C = 1.9 × 1010 M−2s−1 and k−C = 25 M−1s−1. These values lead to a Gibbs free energy of IO2H formation of −95 kJ mol−1. The pKa of iodous acid should be about 6, leading to a Gibbs free energy of IO formation of about −61 kJ mol−1. Estimations of the four rate constants at 50°C give, respectively, 1.2 × 104 M−3s−1, 590 M−1s−1, 2 × 109 M−2s−1, and 20 M−1 s−1. Mechanisms of these reactions involving the protonation IO2H + H+ ⇌ IO2H and an explanation of the decrease of the last two rate constants when the temperature increases, are proposed.