Abstract
The formal potential of the Fe(CN)63−/Fe(CN)64− couple in 1 M NaHCO3 and 1–2 M Na2CO3 solutions was determined. It is equal to 505 and 510 mV, respectively, exceeding the potentials of the Np(VI)/(V) and Np(V)/(IV) couples in carbonate solutions. The equilibrium of the reaction Np(V) + Fe(CN)63− = Np(VI) + Fe(CN)64− was studied. Fe(CN)63− ions oxidize Np(IV) to Np(V) and then to Np(VI). The arising Np(VI) oxidizes Np(IV). The Np(IV) oxidation accelerates in going from NaHCO3 to Na2CO3. An increase in [Na2CO3] or in the ionic strength (by adding neutral salts) decelerates the oxidation. Np(IV) introduced in an HCl solution reacts with Fe(CN)63− or with Np(VI) faster than Np(IV) introduced in a Na2CO3 solution. The activation energy of the reaction of Np(IV) with Fe(CN)64− in the range 20–45°C is 107 kJ mol−1. The reaction mechanism involves formation of the activated complex from ions of Np(IV) hydroxocarbonate and oxidant.
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