Measurement of TI(III/I) Electron Self-Exchange Rates Using Enriched Stable Isotope Labels and Inductively Coupled Plasma Mass Spectrometry

An approach is described for measuring electron selfexchange rate constants (An) in solution based upon stable isotope-labeled reactants, chemical separations, and inductively coupled plasma mass spectrometry. The technique is demonstrated for the exchange between Tl^III and Tl¹ liaquo ions in aqueous HClO4. Tl^IIIis prepared using ²⁰³Tl-enriched Tl₂O₃(²⁰³Tl abundance, ~36%), and Tl^Iis prepared from natural abundance Tl reagents (natural ²⁰³Tl abundance, 29.52%). The exchange is monitored by mixing the labeled and unlabeled reactants and performing timewise separations through selective precipitation of 11 as TlBr. Isotope abundances are measured in the TlBr precipitate and solution phases using ICPMS with minimal sample preparation; an NIST 981 (common lead) spike is added, and tibe ²⁰⁸Pb/²⁰⁶Pb is measured as an internal standard to correct for mass discrimination. The self-exchange rate constant is determined from a McKay plot obtained from the ²⁰⁵Tl abundances of either oxidation state. A An of (1.0 ± 0.1)×10 4 M^-1s^-1was obtained in 1.5 M aqueous HClO₄at 25 °C. The obtained An compares favorably to a value of 1.1×10-4 M^-1s^-1based upon a previously published study of this exchange reaction using radiolabeled (204 ) reactants.