Isotope exchange of strontium and molybdate ions in strontium polymolybdates

The heterogeneous isotopic exchange reactions in strontium polymolybdates of Sr2+ and MoO42- ions in the strontium nitrate and sodium molybdate solutions have been studied using 90Sr and 99Mo as tracers. Electrometric methods have been used to study the compositions of strontium molybdates obtained by adding strontium chloride to a progressively acidified solution of sodium molybdate. It has been found that the exchange fraction increases with increasing chain length of strontium polymolybdate. The exchange equilibrium constant (Kex) has been calculated between 298 and 348 K as well as ΔG deg, ΔH deg and ΔS deg. The results indicate that Sr2+ cations have a much higher affinity for exchangers than MoO42- anions. By fitting the data to the Dubinin-Radushkevich (D-R) isotherm it has been shown that the exchange capacity (Xm) for both ions is affected by the ion adsorption process at low temperatures and by the ion exchange process at high temperatures. At high concentrations, the recrystallization process contributes to on the cation exchange but is ineffective on the anion exchange mechanism. (author)