On the anomalous behaviour of two types solid-state copper(II)-ion-selective electrodes in presence of complexones-a possible explanation

Summary

A hypothesis for the reason of the anomaly, observed in Cu(II)-complexone systems with two types of solid-state homogeneous Cu-ISEs, one based on pressed pellets of ternary CuAgSe and the other on thin-layer electroplated Cu₂-xSe is presented. It is grounded on the results of systematic investigations of these sensors in 13 different Cu(II)-L systems (where carboxylic acids, amino-acids and polyaminopolycarboxylic acids have been taken as ligands) and on the electrodes' response to copper(I). Its main point is that the electrodes being primary for Cu(I) as suggested by the selectivity coefficients, K_{Cu+, Cua+}, act as Cu(II)-electrodes through the fast exchange reaction:

$$Cu_2 Se + Cu^{2 + } \rightleftharpoons CuSe + 2Cu^ + $$

and that at very low Cu(II) concentration the [Cu⁺]0^E-activity, generated by the exchange reaction becomes commensurable with the native [Cu⁺]0^α-surface activity characteristic for the membrane material. The hypothesis has been verified experimentally. By blocking the native Cu(I)-surface activity with glycine, the anomaly, observed in Cu(II)-NTA solution disappears and decreases considerably in Cu(II)-EDTA and Cu(II)-DTPA solutions.