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 sul fide, were assessed for their flow-injection response towards iodide i n the absence and presence of bromide, thiocyanate and thiosulfate. Wi th the exception of a pressed membrane based on a 3:1 mixture of AgI a nd Ag2S, which gave rise to erratic, irreproducible potentials. all th e electrodes responded rapidly to iodide. While detection limits were generally similar, slope factors ranged from -44.6 mV/decade for a pre ssed AgI membrane to -61.2 mV/decade for a 1:3 AgI/Ag2S membrane. Sele ctivity coefficients were determined for each of the electrodes by int ercalating mixed iodide-interferent solutions into a 10(-6) M iodide c arrier stream, then processing the resultant data by both the accepted IUPAC graphical method and a proposed data linearization technique. A lthough the two methods returned comparable results for the electrodes and interferents examined, the linearization procedure would be the p referred 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.