Separation Solution Methods

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  

There are two ways for obtaining the value from the calibration curves  

In the equal potential method, we can pick in the calibration curves an appropriate E value (horizontal line) with E = E2, then reading the corresponding activity values since 

In the equal activity (concentration) method, a vertical line is drawn in the dynamic range of the electrode response of the calibration graph. We read the j and 2 1 ues and get the A = j - 2 difference. From the simplified Nicolsky-Eisenmann equation, we obtain 

Usually, the RT/F is substituted by the slope (S) of the experimentally obtained calibration curve. 

---

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.

Method 2 is the separate solution method [38,135], The EMF of a cell consisting of an ISE and a reference electrode is measured in a solution of determinand J alone (in the absence of interferent K) and then in a solution of interferent K alone (in the absence of determinand J), the activities of the determinand and the interferent in the two solutions being the same. If the respective values of EMF are Ei and E2 and E ise for both ions are given by the simple Nernst equation, then can be calculated from the relationship,... 

Calculated according to the separate solution method, using AEMF values obtained at 10 M of 5 -GMP orS -AMP. 

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... 

SOURCE Adapted from ref.2. a Separate solution method. [Mg2+J = 10"2M. 

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... 

Figure 1. Principal pathways to generate unsupported catalytic materials. The methods indicated in the centre of the scheme are discussed in other sections of this book. The methods of fusion and precipitation will be discussed more in detail to illustrate the consequences of the different reaction environments on the structural properties of the final products. The dashed line separates solution methods (bottom) from high-temperature reactions (top).

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 ... 

In the separate-solution method, the cell voltage is measured first in a solution of free determinant i, followed by ) measured in a solution of interferant j. By applying Eq. (17) for these two solutions, Eq. (21) is obtained ... 

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. 

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). 

The separate solution method The cell potential is measured for two solutions. 

Essentially, there are two separate-solution methods with the respective solutions producing e.m.fs., E and E2. The... 

---