Surface studies of the jalpaite-based copper(II) ion-selective electrode membrane in seawater

Ageing of the jalpaite Cu(II) ion-selective electrode (ISE) membrane in saline solutions and seawater has been studied using X-ray photoelectron spectroscopy (XPS), and it is shown that the membrane surface composition is altered after exposure to these media. XPS evidence shows that the change in membrane surface composition in a saline Cu(II) ion buffer involves the transformation of jalpaite (i.e. Ag1.5Cu0.5S) to a mixture comprising copper-deficient sulfide (i.e. Ag1.5Cu0.5-2xS1-x) and AgCl. A similar process takes place in seawater with the exception that AgI is also formed on the membrane surface. The variation in membrane surface composition in saline solution and seawater has been linked to the electrochemical behaviour of the Cu(II) ISE. XPS analysis of membranes treated in seawater has identified the carboxylic acid-based organic substances of seawater (i.e. humic and fulvic acids) on the surface of the jalpaite membrane. In seawater and saline Cu(II) ion buffers, the chloride induced poisoning of the jalpaite membrane is suppressed through the adsorption of surfactant-like ionic forms of the organic ligands onto the membrane surface. The findings of this work have important ramifications for the application of the jalpaite Cu(II) ISE in studies of Cu(II) complexation in seawater.