Separate solution method, selectivity

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). 

A commonly adopted approach to measure the selectivity of an ISE is the separate solution method, which is illustrated in Fig. 18a. 13. Here, two separate solutions, which contain the primary ion and interfering ion individually, are measured separately. The selectivity is calculated from... 

Fig. 18a. 13. Schematic representation of ion-selective electrode selectivity as determined by the separation solution method (SSM). E I) is the potential of the electrode in primary-ion solution and E(J) the electrode potential in the interfering ion solution. Both primaiy and interfering ions show Nernstian response.

The Mg-ISE is equipped with a liquid membrane containing a neutral carrier, N,N -octamethylene bis(A, A -dioctyl-malonamide) (ETH 5220), dispersed in a PVC matrix together with additives and plasticizers. The slope for the Mg-ISE was found to be stable and equal to 30 +1 mV/decade of Mg2"1" (at 37°C), and the selectivity coefficient for Ca2+ measured by the separate solution method was 0.35. 

Selectivity coefficients were obtained by the modified separate solution method (MSSM), which allows for the determination of unbiased selectivity coefficients [3]. The selectivity coefficients for Cs I were first... 

It is important for the analytical chemist to realize the selectivity coefficient of a particular electrode. Various methods have been suggested for determining the selectivity coefficient, including the fixed-interference method, separate solution method, and the fixed primary ion method (10,11). The most popular fixed interference method involves two solutions, one containing a... 

Selectivity coefficient — Kvf], is a measure of the contribution of an interfering ion B to the potential of an -> ion-selective electrode in a mixed solution containing the primary ion A and an interfering ion B. It is defined by the modified - Nikolskij-Eisenman equation. The smaller the value of JC °g, the better the selectivity of the electrode with respect to the primary ion A. Selectivity coefficients can be evaluated by measuring the response of an ion-selective electrode in mixed solutions of A and B (fixed interference method) or in separate solutions of A and B (separate solution method). 

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 ... 

There are various approaches for determining the selectivity of an ISE for a primary ion over an interfering ion. A straightforward approach is the separate solution method, where the potential of an ISE is determined in solutions of the primary and interfering ions separately but at equal ionic activities. The selectivity coefficient is then calculated as ... 

The separate solution method (SSM). The potential of a cell comprising an ISE and a reference electrode is measured in two separate solutions one containing only the primary ion i ( )), the other containing the interfering ion j (Ej), at the same activity (a, = aj). The value of the selectivity coefficient can be calculated on the basis of the Nikolsky-Eisenman equation ... 

To be validated, the selectivity of the method must be measured using a mixed solutions method because this method reflects real-world conditions for sample measurement. The separate solutions method gives only the... 

Separate Solution Method. In this method, calibration curves are prepared for each ion being tested. Parallel curves should result, with the potentials for the primary ion (if a cation) being more positive. The selectivity ratio can be related to the difference in potential between the two curves (see Problem 21). For two monovalent ions ... 

A sodium glass ion-selective electrode is calibrated using the separate solution method, for sodium response and potassium response. The two calibration... 

Selectivity coefficients measured with matched-potential, separate-solution method in 10 mM phosphate buffer, pH 10.0. 

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. 

Sensor Mediator Slope (mV decade" ) Selectivity coefficients, /cyo. e (separate solution method) ... 

Figure 31 Potentiometric response curves to the primary and interfering ions as measured with the separate solution method (SSM). As illustrated for ion K, the detection limit of the ISE may be significantly worse than for the primary ion and may make it difficult to determine unbiased selectivity coefficients.

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. 

The majority of ionophores for nitrate that are commercially available are based on quaternary ammonium salts, such as tridodecylmethylammonium and tetraoctadecylammonium salts. Electrodes based on these salts, however, lack selectivity and, most often, the selectivity pattern of the electrodes follows the classical Hofmeister selectivity series. The response of these electrodes, then, is primarily toward C104 with a log of 3.0 relative to nitrate, determined by the separate solution method (3, 24). In fact, in many cases, it would be necessary to pretreat sample solutions to remove some of the more highly interfering anions (EPA method 9210). 

Therefore, the selectivity coefficient can be determined by potential measurements in separate solutions containing only the analyte or interfering ion (Figure 7.2A the separate solution method, SSM). With potentials Ej and j at sample activities of the respective ions, Cj (SSM) and Aj (SSM), or with the intercepts of the E versus log a plots, the selectivity coefficient is given as... 

Several methods have been proposed for the experimental determination of ion-selectivity coefficient. They can be listed in the two groups separate solution methods and mixed solution methods. Applying both methods, the cell voltages (emf) are measured in a cell when the reference electrodes and the ISEs are dipped in solutions of known composition. When the separate solution method is used, pure primary ion solutions or pure solutions of the tested interfering ion are introduced into the measuring cell, while in mixed solution methods, both the primary and the selected interfering ions are introduced. 

In separation solution methods, a calibration curve is prepared for the primary ion and the other one for the selected interfering one. The simplified equation will give the - log dependence. Since for the calibration curve of the primary ion flg = 0 and for the interfering ion = 0, we can get two equations as follows ... 

Indexes just indicate that the AE is measured in mV and S in mV/decade units. In reality, however, the two calibration curves are not parallel. The Nicolsky-Eisenmann equation is not rigorously followed. Therefore, depending on where the horizontal or vertical lines are drawn, different selectivity coefficient values can be obtained with the separate solution methods. Therefore, it is recommended that the selectivity coefficient be measured in the range of the ionic concentration of the samples using mixed solution methods. 

Experimental protocols for the determination of selectivity coefficients have been laid out by lUPAC. One distinguishes two main methods, the separate solutions method (SSM) and the fixed interference method (ElM). ... 

As the name implies, the separate solutions method involves the measurement of the cell potential in a solution containing only the salt of the analyte (primary ion 7), followed by that in a solution containing only the salt of the interfering ion, J. One then obtains the selectivity coefficient for any activity as follows ... 

With respect to the comparability of selectivity coefficients obtained by the various methods, Liteanu et al. [457] recently proved with the aid of a strictly statistical treatment that in general the separate solution methods yield meaningless results ... 

---