Matched potential method

The advantage of this approach is that it can be used when the response of the electrode is not Nernstian or even linear. However, the drawback is that the selectivity coefficient may change under different experimental conditions, particularly the concentrations at which the measurements were made. In most situations, the selectivity obtained from matched potential method cannot be directly compared to the values obtained from other methods. 

The selectivity of ISEs is evaluated by calculating selectivity coefficients. Over the years, several different methods have been used to obtain these coefficients, the most common being the separate solution method, the fixed interference method [15] and the matched potential method [16]. In the separate solution method, the potential response is determined in each of the two separate solutions, one containing only the primary ion at an activity ai and the other containing only the interfering ion at an activity Uj = a. The selectivity coefficient or the relative response of the electrode to the two ions I and J, K s is calculated from the following equation ... 

The selectivity coefficient in the matched potential method is defined as the activity ratio of primary and interfering ions which results in the same potential change as in a reference solution ... 

Matched Potential Method. This is a strictly empirical variation of the fixed interference method. Numerical values of selectivity coefficients may vary with solution conditions, for example, relative concentrations of the ions. The matched potential method allows the analyst to obtain an empirical value under the experimental conditions of the analysis. Suppose you wish to know the relative interference of sodium ion in blood in the measurement of lithium ion (ca. 1 mM) in the serum of a bipolar (manic depressive) patient taking Li2C03. A reference... 

The matched potential method has been recommended as the preferred method by the Intematiorial Union of Pure and Applied Chemistry (lUPAC Ref. 17). Horvai has examined the procedure in more detail for applicability to all sensors (Ref. 18). 

We mentioned that selectivity coefficients determined using the Nicolsky equation often vary, depending on relative concentrations and measurement conditions, and the equation does not dpply if the response is non-Nemstian (often the case with secondary ions). The matched potential method is an attempt to deal with this. Bakker and co-workers introduced a new formulism that provides a.clear interpretation of the matched potential method, elegantly deriving an equation based on ion exchange in which the last term in Equation 13.49 becomes where... 

V. P. Y. Gadzekpo and G. D. Christian, Determination of Selectivity Coefficients of Ion-Selective Electrodes by a Matched-Potential Method. Anal. Chim. Acta, 166 (1984) 279. 

G. Horvai, The Matched Potential Method, a Generic Approach to Characterize the Differential Selectivity of Chemical Sensors, Sensors and Actuators B, 43 (1997) 94. See also G. Horvai, Trends in Anal. Chem., 16 (1997) 260. 

Tohda, K. Pragoe. D. Hibata. M. Umezawa. Y. Studies on the matched potential method for determining the selectivity coefficients of ion-selective electrodes based on neutral ionophores Experimental and theoretical verification. Anal. Sci. 2001, 17, 733-743. 

The inner filling solution for the sensors is usually 0.01 M NaCl, which is also used to condition the potentiometric sensors. Electrochemical potential is measured with the following galvanic cell Ag/AgCl/bridge electrolyte/sam-ple solution/ion-selective membrane/inner filling solution/ AgQ/Ag. A high impedance pH-mV meter is used to measure the electrochemical potential. Selectivity coefficients are evaluated by the matched potential method (also known as method of mixed solutions), or via the separate solution method. 

In the matched potential method, the potentiometric selectivity coefficient K y is defined as the ratio of the concentrations of the primary (x) and interfering ions (y), which give the same potential change under the same condition, that is, under a fixed concentration of the primary ion as a background. In the separate solution method, selectivity coefficients K5,y are calculated using the cell EMF values obtained in 0.01 M solutions of both interfering anions (y) and the primary ion (x). 

Given the reliance of these methods on Eq. (20), it is important to note that deviations from this equation have been reported, particularly for mixtures of ions of different charge (31). Moreover, it has been pointed out that many biased values have been reported using these methods [28]. This has led to a resurgence of interest in the matched potential method [32], which provides a measure of the selectivity of an ISE. Details of the relative merits of each of these methods have been summarized in an excellent review ]28]. 

Which gives the parameters of the electrode potential, E, measured with respect to a reference electrode for Nernstian response. Thus, an ISE is taken to respond selectively to an ion. A, of activity a and charge z in the presence of an interfering ion, B, of activity ag and charge Zg, where k g is the selectivity coefficient. The various approaches to determining selectivity coefficients were summarized (, 62-63)> in the early heady days of ISE researches, but more recently some additional proposals have been made, such as, the matched potential method of Gadzekpo and Christian (6M). 

The selectivity coefficient, defines the ability of an ISE to distinguish a particular ion from others (5). According to lUPAC, can be evaluated in mixed in solutions of primary and interfering ion (Fixed Interference Method), or separate solutions (Separate Solution Method and Matched Potential Method). The smaller the value of the greater the electrode s preference for the principal ion. 

Gadzekpo, V.P.Y. and Christian, G.D. (1984) Determination of selectivity coefficients of ion-selective electrodes by a matched-potential method. Anal. Chim. Acta, 164, 279-282. 

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