Kinetics and mechanism of pyrophosphate metal complexes electroreduction

Abstract

The results obtained in the study of the labile dipyrophosphate complexes of lead (II) and tin (II) and other pyrophosphate complexes electroreduction in weak alkaline solutions have been considered and discussed. It has been shown that an increase of the Pb(P 2O 7) 6− 2 electroreduction rate at the dropping mercury electrode ( dme) obtained with the increase of the alkali metal cation concentration and in the sequence Cs + < K + < Na + < Li + is determined by alkali metal cation participation in a reversible chemical step. A monopyrophosphate complex of alkali metal and specifically adsorbed lead(II) monopyrophosphate complexes participating in a slow electrochemical step are formed as a result of preceding chemical step. Similar mechanism takes place in the case of Sn(P 2O 7) 6− 2 electroreduction. It has been shown that the change of ψ o potential value at the outer Helmholtz plane connected with the change of the sodium perchlorate concentration (from 0.05 to 1 M) practically does not affect the rate of electrochemical step of Pb(P 2O 7) 6− 2 electroreduction process and relatively weakly affects the rate of the electrochemical step of Sn(P 2O 7) 6− 2 electroreduction process. It has been explained by participation in a slow electrochemical step of specifically adsorbed monopyrophosphate complexes whose metal ions are in contact with the electrode metal atoms. Dipyrophosphate complexes of tin(II) are reduced at more negative potentials in comparison with similar lead(II) complexes and monopyrophosphate complexes of tin(II) are adsorbed with participation of alkali metal cations.