ON THE STANDARD REDUCTION POTENTIAL OF THE MNO4- MNO42- AND MNO42-/MNO43- COUPLES/

Using a potentiometric methodology, the formal standard reduction pote ntials of the MnO4-/MnO42- and the MnO42-/MnO43- couples have been det ermined in alkaline solutions of high ionic strength, at 25 degrees C, varying either volumetrically or coulometrically the concentration ra tios of the manganese oxyanions. By using the Specific ion Interaction Theory (SIT),(1-8) the formal constants, valid in the ionic medium, h ave been extrapolated to zero ionic strength. Data from previous studi es have also been reinterpreted. For the standard potential E-7,6(0) o f the MnO4-/MnO42- couple, and E-6,5(0) related to the MnO42-/MnO43- c ouple, 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 accounte d for by assuming for HMnO42- a value of logKa(3) = -12.0+/-0.3 at inf inite dilution. Analogies in the chemistry of MnO42- and CrO42-, on on e side, and of MnO43- and PO43-, on the other, are pointed out.