Rate of the Dushman Reaction in Iodic Acid at Low Iodide Concentration. Complexity of Iodic Acid

Three rate laws have been postulated for the Dushman reaction, 6H++5I−+IO3−=3H2O+3I2, with total reaction orders of four, five and six, at iodide ion concentrations below 10−8 M. New direct measurements of the rate and an examination of thermodynamic data for iodic acid solutions have eliminated some of the confusion. The complexity of iodic acid solutions, with and without added salt, makes it impossible to establish a single reaction order from the radioactive iodine exchange measurements. The new results are best expressed by a quasiempirical rate equation, −d[IO3−]⁄dt=8.59×104[H+]2(I−)[IO3−] mol l−1 min−1, which is closely approximated by −d[IO3−]⁄dt=γ±p4(H+)2(I−)(IO3−){k′+k″(HIO3)} mol l−1 min−1, with k′=2.83×105 mol−3 l3 min−1 and k″=6.60×106 mol−4 l4 min−1 for 25 °C. The second rate law is interpreted in terms of a reasonable mechanism that takes into account the thermodynamically established polymerization in iodic acid solutions.