International Union of Pure and Applied Chemistry Analytical Chemistry Division Commission on Electroanalytical Chemistry* Recomendations for Nomenclature of Ion-selective Electrodes

The rapid growth of interest in the field of ion-selective electrodes (ISEs), within the larger field of electrochemical sensors based on potentiometric, amperometric, and conductometric principles, makes it highly desirable to achieve improved standardization of nomenclature in this area. The previous document, W A C Recommendations 1975 (1) has been corrected using recent experimental and theoretical findings. This report does not define or recommend activity standards or calibration procedures using activity standards. 1. GENERAL RECOMMENDATIONS Definitions of Terms 1. Activitv, Activitv Coefficient and Concentration. Refer to Keference (2). 2. Calibration Plot. This is a plot of the cell emf (electromotive force measured as ion-selective electrode potential minus external reference electrode potential) of a given ion-selective electrode cell assembly (ion-selective electrode combined with an identified external reference electrode) vs. the logarithm of the single ionic activity (concentration) of a given species (3,4,5). For uniformity, it is recommended that the cell emf be plotted on the ordinate (vertical axis) with the more positive emfs at the top of the graph and that pa^ (-log activity of the species measured, A) or PCA (-log concentration of the species measured, A) be plotted on the abscissa (horizontal axis) with increasing activity to the right. Comment: The cell emf is composed of identifiable interfacial and bulk potential differences arising from the membrane, reference electrodes, and junctions if present (see below). When referring to the potential difference at an ISE membrane interface, or across the ISE membrane including bulk and two interfaces, the terms interfacial potential difference, and ISE or membrane potential difference should be used to distinguish a portion of the cell emf under study. 3 . Combination Electrode. An ion-selective electrode and an external reference electrode combined in a single unit, thereby avoiding the need for a separate external reference electrode. (See also terms #17a and 17b Reference electrode). 4. Detection Limit. A calibration plot ordinarily has the shape shown in Fig. 1. By analogy with definitions adopted in other fields, a limit of detection could be defined as the concentration for which, under the specified conditions, the cell emf, E, deviates from the average emf in region I by a multiple of the standard error of a single measurement of the emf in this region I. The multiple selected depends on a statistical significance level selected. The fhdamental difference in the nature of ISE potentiometry (their logarithmic response) as compared to linear methods, justifies another type of definition of detection limit. For the sake of practical convenience, a simpler (and more convenient) definition is recommended at this time. The practical lower limit of detection may be taken as the activity (or concentration) of substance A at the point of intersection of the extrapolated linear midrange and final low concentration level segments of the calibration plot, as shown in Fig. 2. For electrodes that become insensitive to high ionic activities, or the slope of the curve changes its sign, an upper limit of determination may be taken as the activity (or concentration) of A at the point of intersection of the extrapolated midrange linear segment and the limiting high activity response. Comment: The reliability of the detection limit data, determined by the cross point method, is illustrated in Fig. 2. The reliability depends on: 1. The standard deviation of a single potential measurement in both linear regions (it need not be the 2. The number of data points taken into account to determine the parameters of the linear sections; 3. The concentration range used to determine the parameters of the linear sections. same in the Nernstian and concentration-independent regions); Based on the above, beside the experimental conditions (i.e., composition of the solution, the