Effect of membrane composition on the flow-injection response of an ion-selective electrode: A comparative study based on selectivity coefficients calculated using a data linearization technique

Commercial iodide- sulfide- and cadmium-selective electrodes and four electrodes based on laboratory-prepared pressed membranes, the latter composed of either silver iodide or a mixture of silver iodide and sulfide, were assessed for their flow-injection response towards iodide in the absence and presence of bromide, thiocyanate and thiosulfate. With the exception of a pressed membrane based on a 3:1 mixture of AgI and Ag2S, which gave rise to erratic, irreproducible potentials. all the electrodes responded rapidly to iodide. While detection limits were generally similar, slope factors ranged from -44.6 mV/decade for a pressed AgI membrane to -61.2 mV/decade for a 1:3 AgI/Ag2S membrane. Selectivity coefficients were determined for each of the electrodes by intercalating mixed iodide-interferent solutions into a 10(-6) M iodide carrier stream, then processing the resultant data by both the accepted IUPAC graphical method and a proposed data linearization technique. Although the two methods returned comparable results for the electrodes and interferents examined, the linearization procedure would be the preferred option in cases where the selectivity coefficients determined are greater than unity. Using electrode membranes composed of either AgI or AgI/Ag2S, a trend in selectivity was observed which suggests a mechanism of response towards the interferents involving the oxidation of the sulfide in the membrane to either sulfate or elemental sulfur.