Chloride Interference in Cupric Ion Selective Electrode Measurements

A theory based on the establishment of a diffusion layer between the electrode surface and the bulk of solution, and the attainment of chemical equilibrium at the electrode surface is used to explain quantitatively the behavior of solid-state cupric ion-selective electrodes in chloride media: Cu(II) from the bulk of solution is reduced at the electrode surface to Cu(I), which is stabilized by chloride complexation. The theory was experimentally verified: in solutions with different concentrations of Cu2+, Cl-, and Cu2+-complexing ligands; with an alternate Cu(I) stabilizing agent, CH3CN; with cupric ion selective electrodes of different standard potentials (a sulfur-rich graphite contacted electrode and a silver contacted electrode). The onset of interference of chloride ion is described as a function of concentrations of Cu2+, Cl-, and Cu2+-complexing ligands, and the standard potential of the electrode. The general unsuitability of the electrode for measurement of cupric ion activity in seawater is accounted for by this theory.