Development of a converging wall-jet instrument and its use to measure the dynamic response of the fluoride ion-selective electrode to fluoride and halide interferent ion

Briefly, the dynamic response of an ion-selective electrode is the voltage response of the electrode following a rapid activity change of the primary and/or interferent ion(s) and is usually within the time domain of milliseconds to seconds. Experiments measuring the dynamic response of a fluoride ion-selective electrode to activity steps in fluoride, chloride/bromide, hydroxide, and chloride/bromide/hydroxide/cyanide are presented. An advanced converging wall-jet instrument capable of initiating activity steps within 7 milliseconds was built for this purpose and is described in detail. The dynamic response of a new fluoride ion-selective electrode for full scale voltage changes (t$sb{100})$ has been measured and found to be 240 milliseconds to increasing (forward) decade steps in fluoride ion at constant total ionic activity. Reverse (decreasing) activity steps showed a response time of 350 milliseconds. In both of these cases Nernstian voltage changes were seen. Decade steps in hydroxide ion, the only previously reported interferent species, resulted in complex dynamic voltage behavior. The voltage changes were sub-Nernstian and non-monotonic for both forward and reverse decade hydroxide activity steps. Surprisingly, large activity steps of chloride/bromide showed moderate non-monotonic voltage responses for increasing steps and small voltage responses for decreasing activity steps. The response time of the forward chloride/bromide activity steps was about 700 milliseconds and was much slower than when compared to the response to fluoride. The "four-ion" activity step consisted of an activity step of chloride, bromide, hydroxide, and cyanide simultaneously. The four-ion forward activity steps showed a dynamic response attributed in part to cyanide, while the reverse four-ion activity steps may have had a weak response due to cyanide. This research is one of the first efforts to measure the dynamic response of any ion-selective electrode in the presence of static case interferents, to show that non-interferents may give a dynamic voltage response, and to show that they may have response times differing greatly from the primary ion.