XANES spectroelectrochemistry: a new method for determining formal potentials

Proof-of-concept experiments are presented outlining the use of X-ray absorption near-edge structure (XANES) in the determination of formal potentials. Two redox couples of Np in 1 M perchloric acid are used for the demonstration. XANES data obtained as a function of applied potential are used to quantitatively determine the relative concentrations of Np(VI)/Np(V) and Np(IV)/Np(III). Bulk electrolysis is done in the X-ray beam with the solution maintained under electrochemical control. Relative concentrations are extracted from the data using a principal component (factor) analysis. The formal potentials of the two redox couples determined using this method agree with those previously published. The results establish in situ XANES spectroelectrochemistry as a viable method for determining formal potentials. Examples, taken from environmental or heteropolyanion chemistry, are discussed for which this methodology will provide information currently unavailable from other established techniques