Model for the outer-sphere association of heteropoly anions and the electroreduction kinetics of ensembles of ionic associates

The most probable positions of cations, successively added during a multistage association in electrolytes, are determined from positions of hydrated cations in crystals of sodium salts of heteropoly anions (HPA) with the Anderson structure. For the structurally close HPA [Co-2(III)Mo10O38H4](6-), equilibriums in solutions are analyzed at widely diverse concentrations c of the supporting electrolyte, using the Co-59 NMR technique. Stability constants, calculated from these data with the Fuoss equation, lead to the model parameters (separations between centers of HPA and sodium cation) that are close to relevant separations in sodium salts of HPA, i.e., the model satisfactorily describes bulk properties of associates. The current density j of the HPA central ion reduction in solutions of sodium salts of [Co(III)Mo6O24H6](3-) and [Mn(IV)W6O24](8-) is studied at a positively charged mercury electrode as a function of the concentration of the I,l-valence supporting electrolyte (CH3COONa + CH3COOH). The considerable differences of the derivative of logj over logc in the sign and absolute value for HPA with charges of -3 and -8 may be caused, along with some other factors, by different effective electrostatic interactions that are due to a nonuniform charge distribution in particles of the reagents. Models of the reaction layer are considered, along with relevant approaches to a calculation of basic kinetic parameters.