SPECIALISTS' WORKSHOP ON IODINE CHEMISTRY IN REACTOR SAFETY September 11th and 12th, 1985, at AERE Harwell, England COMMITTEE FOR SAFETY IN NUCLEAR INSTALLAnONS OECD STEERING COMMITTEE FOR NUCLEAR ENERGY

A number of experimental techniques have been used to study two of the equilibria which control the speciation of solutions of molecular iodine in water. These are the tri-iodide formation reaction; 12 + 1= 13and the iodate formation reaction: 312 + 3H20 = 103+ 51+ 6H+. The eqiulibria were studied at temperatures up to the region 150 to 200.C. The 13formation reaction was studied by a vapour equilibration method, a spectrophotometric method and a redox electrode method. The measured equilibrium constants were all in good agreement with those reported recently by Palmer, Ramette and Mesmer using slightly different methods. The 103formation reaction was studied by two redox electrode systems (Ptl 103-' 12, HCIIIHCl, 12, rIpt up to 200.C and ptl 103-' 12, HCIIIHCl/AgCljAg up to 170.C). The equilibriwn constants were not in good agreement with one another or with the results of Palmer et al. Correction for the presesnce of IC12brought the results into satisfactory agreement among themselves, but not with those of Palmer et al. A number of possible causes for systematic errors were considered and it is concluded that further work is necessary to explain the remaining differences.