Bipolar Pulse Conductance Measurements With Ion-Selective Electrodes

217 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1982.A novel conductometric approach to monitoring of the calcium and fluoride ion-selective electrodes (ISEs) is presented as an alternative to the classical potentiometric technique. Bipolar pulse conductometric monitoring of ISEs does not require a real reference electrode; two voltage pulses of equal time duration, equal magnitude, but opposite polarity are applied to a metal (inert) electrode, and the resulting current through the ISE is measured at the end of the second pulse. The performance of the calcium electrode under potentiometric and conductometric monitoring is similar in most respects; the conductometric method simply short-circuits the ISE potential and allows it to generate a current that is measured along with the current induced by the voltage pulse. Therefore, working curve detection limits, linear ranges, and general shapes are quite similar, as is the selectivity of the electrode to calcium in the presence of a variety of interfering species. The measured current is proportional to log (calcium concentration) rather than log (calcium activity), and it is shown that voltage pulses greater than 15 mV in amplitude are responsible for the concentration dependence. Also voltage pulses force the ISE to respond to abrupt changes in Ca('2+) concentration more rapidly than it does in the potentiometric mode.The conductometric response of the fluoride electrode is not quite as straightforward as that of the calcium electrode. In addition to having a potential dependent upon the fluoride concentration, the electrode also exhibits a resistance that is dependent upon fluoride concentration in solution. Furthermore, the electrode resistance is decreased in the presence of very low (ca. 10('-9) M) fluoride concentrations, reaches a minimum in the vicinity of the potentiometric detection limit, and then increases over the potentiometric working range.Bipolar pulse conductometric monitoring of the calcium and fluoride electrodes in a continuous flow apparatus is explored. The reproducibility is improved because drift problems are eliminated. The response time of the calcium electrode is evaluated under a variety of conditions. Interferences due to electroactive species are also evaluated, and it is shown that shifting of the counter electrode potential is possible, along with faradaic reactions.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD