Studies on the Electrode Reactions of Lead(II) Complexes with Polyamino Polycarboxylic Acids by Heyrovsy-Forejt Type Oscillopolarograph

Abstract

A new type of oscillopolarograph (HeyrovskST-Forejt type) with varying frequencies (102000Hz) was designed and constructed. This apparatus was used for the study of the electrode reactions of lead( IC ) complexes with polyamino polycarboxylic acids. Each Cut-in of lead (II) complex with the ligand (nta, edta(OH)2, ttha, edta, dtpa or cydta) appears only in the cathodic process and its Cut-in potential (vs. SCE) was measured accurately. It was found that with decreasing stability constant of complex, the Cut-in height varies sensitively with change in frequency. On the other hand, the Cut-in potential is affected in the case of complex with large stability constant. The rate constant of electrode reaction kg, was determined. The rate constant increases with increasing E pKt(negative logarithm of acid dissociation constant). The value of k: increases with the sequence of the ligands: ntaedta(OH)2cydta dtpaedtattha. In lead(II)-ttha system, the potential of the Cut-in M2 attributable to the reduction of complex for lead(II)-ttha complex does not change with ttha concentration. On the other hand, the Cut-in M2 is remarkably affected by pH. It may be assumed that the number of proton taking part in the electrode process of leadE-ttha complex changes at pH nearly equal to 5. The Cut-in M2 may be due to the reduction of PbH₂L2- at lower pH than 5 and that of PbHL3- at higher pH than 5. As an application, the determination of bismuth (III) in the presence of lead (II) was examined.