Effect of dissolved oxygen on the response of Cu(II) ion-selective electrodes in metal buffer solutions Original Research Article

Effects of dissolved oxygen on the potentiometric responses of precipitate based CuS/Ag2S and CuS, and mineral CuS membranes in metal buffer solutions were investigated. Potentiometric responses of CuS/Ag2S membranes to concentration (activity) of free Cu2+ ion in triethylentetramine (trien) solutions were found to deteriorate in the presence of dissolved oxygen; detection limit of the CuS/Ag2S membranes was higher (10−5.6–10−4.6 M of total Cu2+ concentrations) in oxygenated 1.0 × 10−2 M trien solutions at several pH compared to those in deaerated solutions (10−6.7–10−6.2 M of total Cu2+). On the other hand, dissolved oxygen did not affect the response of the membrane for Cu2+ ions in 0.1 M KNO3 solutions. In the absence of added Cu2+ ions, a higher membrane potential, typically about 50 mV, was observed in oxygenated trien solutions than in deaerated trien solution for all Cu(II) ISE membranes used in this study. This higher potential caused a decrease in the dynamic range and also led to a higher detection limit of the electrode. In the presence of dissolved oxygen, Cu2+ ions were found to be continuously dissolved from the membranes into trien solutions with consumption of dissolved oxygen and with increase in pH in the solution, while amounts of total sulfur dissolved from the membranes was by far less than those of Cu2+ ions. From the measured pH and concentration of Cu2+ ions dissolved, the total amount of OH− released was calculated to be about twice the dissolved Cu2+ ion for synthetic CuS membrane in oxygenated trien solution. In pure water, oxygenation caused no significant difference in the dissolution of copper from the membranes. From these results, the following oxidation scheme was derived for CuS in trien solution in the presence of dissolved oxygen; CuS (solid) + 1/2 O2 (solution) + H2O (solution) → Cu2+ (solution) + S0 (solid) + 2OH− (solution). In fact, elemental sulfur, S8, was detected by liquid chromatography after extracting with CS2 solution from synthetic CuS powder treated with oxygenated trien solution but not from untreated and oxygenated–water treated CuS powders. Adsorption of a cationic dye, Methyl Violet, on the CuS surface confirmed that the CuS surface became negatively charged in trien solution due to the formation of an excess sulfide layer on the solid surface, while Cu2+ dissolution increased. Based on the fact that the membrane corrosion by dissolved oxygen proceeded significantly only when ligands such as trien were added in solutions, it was concluded that the higher membrane potential of the Cu(II) ISE membranes, which made the detection limit of the CuS electrode worse for Cu2+ activity, was due to oxidation of the negatively charged sulfide layer to elemental sulfur, S0, by dissolved oxygen.