Potentiometric ion-selective electrodes

Department of Chemistry, University of Pittsburgh, 219 Parkman Avenue, 

Potentiometric ion-selective electrodes (ISEs) are one of the most important gronps of chemical sensors. The application of ISEs has evolved to a well-established rontine analytical technique in many fields, inclnding clinical and environmental analysis, physiology, and process control. The essential part of ISEs is the ion-selective membrane that is commonly placed between two aqueous phases, i.e the sample and inner solutions that contain an analyte ion. The membrane may be a glass, a crystalline solid, or a liquid (1). The potential difference across the membrane is measured with two reference electrodes positioned in the respective aqueous phases 

Under equilibrium conditions, the measured potential (or emf of the ceU), E, can be expressed as 

---

Initial attempts at developing precipitation titration methods were limited by a poor end point signal. Finding the end point by looking for the first addition of titrant that does not yield additional precipitate is cumbersome at best. The feasibility of precipitation titrimetry improved with the development of visual indicators and potentiometric ion-selective electrodes. 

Nitrogen, Kjeldahl, Total (Potentiometric, Ion Selective Electrode)... 

Electrochemical ion-selective sensors (ISSs), including potentiometric ion-selective electrodes (ISEs) and potentiometric or amperometric gas-selective sensors (GSSs), attracted the interest of clinical chemistry because they offer fast, reliable, inexpensive analytical results in service-free automated analyzers. In this way, the electrochemical sensors satisfy the present demands of central hospital laboratories and peripheral point-of-care medical service points, such as bedside, emergency or first-contact healthcare centers. 

Lewis, S.C. and Miller, K.W. (1981). Performance evaluation of the Kodak Ektachem 400 analyzer Potentiometric (ion selective electrode) determinations. Clin. Chem. 27, III, Abstr. 448. 

Potentiometric ion-selective electrodes are passive probes, which in contrast to voltammetric sensors do not convert the analyte in the sample. The response of an ISE depends linearly on the logarithm of the activity (concentration) of a potential determining ion (primary ion) in the presence of other ions. The schematic layout of a complete potentiometric cell including an ion-selective electrode is shown in Figure 2. The electrochemical notation of the cell assembly is given as ... 

CA films by using the phase inversion process. These CA films were cast from solvent/nonsolvent solutions to yield size exclusion membranes consisting of a thin permselective outer layer and a more porous sublayer. These membranes permitted the rapid permeation of a 1500-dalton poly (ethylene glycol) ester of ferrocene however the reproducibility of results presents a problem with these CA mem-branes. Christie et demonstrated that thin films of plasticized polyvinylchloride (PVC), normally used for potentiometric ion-selective electrode applications, applied to electrodes over a polycarbonate dialysis membrane offered improved selectivity ratios for the amperometric detection of phenolic compounds and H2O2 in the presence of the common biological interferents, ascorbic acid and uric acid, over those observed at the dialysis membrane alone or at a composite dialysis/membrane. 

In on-colnmn detectors, the working electrode is actually inserted into the tube used to make the column as illustrated in Figure IB. (Technically, these are not truly on-column detectors unless the stationary phase actually extends to the end of the tube however, for this discussion, on-column wiU refer to the case of the electrode inserted into the same tube used for the column.) The first on-column electrochemical detector was a 1 pm tip potentiometric, ion-selective electrode inserted into the ontlet of a 25 pm i.d. OTLC column [15]. The detection cell volume of 20 fL was proportional to the surface area of the electrode and the length of the stagnant diffusion layer from the electrode surface. Unlike the end-column detector, experiments demonstrated that the on-column detector did not cause a measurable loss in theorical plates. Little follow-up on use of micro-potentiometric electrodes as detectors has been reported. 

An older general review by Stefan et al. [2] considers mathematical modeling for data processing (including a variety of chemometric methods such as linear and nonlinear partial least squares, fuzzy neural networks, and multivariate analysis of variance), designs for electrochemical sensor arrays as well as applications in environmental, food and clinical analysis. Arrays of potentiometric ion-selective electrodes, piezoelectric crystal sensors, and voltammetric biosensors, as well as the electronic nose gas-phase sensor arrays are reviewed. 

In this part, different potentiometric ion-selective electrodes for the determination of surfactants will be discussed including liquid membrane electrodes, coti-ventional polyvinyl chloride membrane electrodes (PVC), solid crmtact electrodes, carbon paste electrodes (CPEs), and more recently disposable screen-printed electrodes (SPEs). 

---