Potential boundary, glass electrode

Due to the disadvantage of working with cells without liquid junctions, in practice the operational scale uses buffer solutions with known conventional pH for the calibration of cells with liquid junction [e.g., the convenient glass electrode (with the calomel or silver electrode, respectively, as reference)]. After calibration of the measuring cell (with a buffer of known conventional pH), the acidities of unknown samples can be measured in the same solvent. It is clear that for the standard buffers used, the conventional and the operational pH are identical. However, we cannot assume such an identity for the unknown samples. This is because the activities and the mobilities of the different ionic species might change the potential on the boundary with all liquid junctions (even without taking effect of the nonelectrolytes into account). 

Figure 21-10 Diagram of glass/calomel cell for the measurement of pH. Esce is the potential of the reference electrode, j is the junction potential, fl is the activity of hydronium ions in the analyte solution, ] and 2 the potentials on either side of the glass membrane, is the boundary potential, and fl2 is the activity of hydronium ion in the internal reference solution.

The lower part of Figure 21-10 shows four potentials that develop in a cell when pH is being determined with a glass electrode. Two of these, F Ag.Agci and sce> are reference electrode potentials that are constant. A third potential is the junction potential Ej across the salt bridge that separates the calomel electrode from the analyte solution. The fourth and most important potential shown in Figure 21-10 is the boundary potential, E which varies with the pH of the analyte solution. The two... 

The potential of a glass indicator electrode has three components (1) the boundary potential, given by Equation 21-9 (2) the potential of the internal... 

Boundary potential, E, The resultant of two potentials that develop at the surfaces of a glass membrane electrode. Bronsted-Lowry acids and bases An acid of this type is defined as a proton donor and a base as a proton acceptor the loss of a proton by an acid results in the formation of a species that is a potential proton acceptor, or conjugate base of the parent acid. Buffer capacity The number of moles of strong acid (or strong base) needed to alter the pH of 1.00 L of a buffer solution by 1.00 unit. 

Thus, the boundary potential depends only on Ihe hydrogen ion activiiics of the solutions on cither side of the membrane. For a glass pH electrode, the hydrogen ion activity of the internal solution a. is held constant so that Equation 2.5-11 simplifies to... 

The glass electrode has a spherical glass membrane which is immersed in the solution of unknown pH to be examined. The membrane is filled with a solution of known pH. If a different concentration and/or activity of hydrogen ions to that of the interior solution exists at the exterior surface of the glass membrane, a corresponding phase-boundary potential E in contact with the outside solution is produced at the thin-walled glass membrane. This phase-boundary potential obeys the Nernst equation ... 

In all cases the test gave confirmation of a theoretical law whose main characteristic is that the boundary potential changes, as does the potential at a reversible hydrogen or oxygen electrode, on both sides of the neutral point, with the logarithym of the acidity, respectively alkalinity. In the case of soft glass, the observation of the interface potentials is so easy that one can base a titrimetric procedure on this voltage. 

Figure 23-4 reveals that the potential of a glass electrode has two components the fixed potential of a silver-silver chloride electrode n and the pH-dependent boundary potential , . 

The phase boundary potential E ) is controlled by the ion-exchange equilibrium between the ions in solution and those immediately inside the glass surface. The diffusion potential E ) is normally constant and is the result of interdiffusion of ions in the glass. It does affect the selectivity and pH range of the electrode. 

The examples of unified approach to diffusion potential modeling for solution/ solution (immiscible liquids) and solutirai/membrane boundaries can be found in refs. [25-27] and [91, 92]. There are already no doubts that diffusion potential contributes to the apparent membrane potential and affects strongly the calibration curves of ion-selective electrodes [26, 93, 94]. This can take place for glass electrode as well (see refs, related to pH measurements in Sect. 3.3.3). 

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