On the standard reduction potential of the MnO4-/MnO42- and MnO42-/MnO43- couples

Using a potentiometric methodology, the formal standard reduction potentials of the MnO4-/MnO42- and the MnO42-/MnO43- couples have been determined in alkaline solutions of high ionic strength, at 25 degrees C, varying either volumetrically or coulometrically the concentration ratios of the manganese oxyanions. By using the Specific ion Interaction Theory (SIT),(1-8) the formal constants, valid in the ionic medium, have been extrapolated to zero ionic strength. Data from previous studies have also been reinterpreted. For the standard potential E-7,6(0) of the MnO4-/MnO42- couple, and E-6,5(0) related to the MnO42-/MnO43- couple, the proposed values are : E-7,6(0) = 0.5434+/-0.0004 V and E-6,5(0) = 0.176+/-0.009 V. Hydrolysis of the MnO43- ion has been accounted for by assuming for HMnO42- a value of logKa(3) = -12.0+/-0.3 at infinite dilution. Analogies in the chemistry of MnO42- and CrO42-, on one side, and of MnO43- and PO43-, on the other, are pointed out.