Iron (II) and iron (III) isotope exchange in water–dimethyl sulfoxide mixtures

Abstract

AbstractThe rate of the isotope exchange reaction between iron(II) and iron(III) perchlorates has been measured in a solvent mixture containing a 3:2 mole ratio of water to dimethyl sulfoxide over the temperature range from 25° to −98°C. In this temperature range, the reactants can diffuse together faster than they can undergo isotope exchange. The activation enthalpy and entropy for the acid‐independent reaction were 6.0 ± 1.2 kcal/mole and −38 ± 17 cal/deg mole, respectively. Below −22°C, the acid‐dependent exchange reaction did not contribute significantly to the exchange. In liquid media at −112° and −117°C and in a solid glass at −136°C, no isotope exchange was observed over the period of a calculated half‐life for the reaction. At these temperatures, the rate at which the reactants diffuse together is slower than the calculated rate of isotope exchange. In a solid glass at −196°C, no isotope exchange was observed over the period of one week.