Ultrasonic absorption in the lanthanide sulphates

Abstract

The rates of formation of the complexes MSO⁺₄ have been measured using the ultrasonic pulse technique for the trivalent ions of yttrium, lanthanum, cerium, samarium, europium, gadolinium, dysprosium and ytterbium. Some justification for Eigen's three-step complex formation mechanism is reported. For metal ions with a noble gas configuration, the rates show a linear dependence on the inverse cation radius and, for transition metal ions, a similar dependence is observed after corrections for ligand field effects have been made. In the lanthanide sulphates series, ligand field effects are negligible. A more favourable comparison is possible with the alkaline earth metal ions, but only after allowance has been made for certain specific solvation effects in the lanthanide cations. The rates of formation are faster than for divalent cations of similar size. From the weight of evidence in favour of an increase in hydration number across the series, a kinetic model is proposed in which increased lability in the centre of the series is associated with a reaction path of lower activation energy relative to that for the lighter or the heavier elements.