Glass asymmetry potential

As already explained, a glass electrode has an asymmetry potential which... 

It has become fairly common to adopt the manufacture of combinations of internal reference electrode and its inner electrolyte such that the (inner) potential at the glass electrode lead matches the (outer) potential at the external reference electrode if the glass electrode has been placed in an aqueous solution of pH 7. In fact, each pH glass electrode (single or combined) has its own iso-pH value or isotherm intersection point ideally it equals 0 mV at pH 7 0.5 according to a DIN standard, as is shown in Fig. 2.11 the asymmetry potential can be easily eliminated by calibration with a pH 7.00 0.02 (at 25° C) buffer solution. 

The const in (6.44) points to the importance of the construction aspects of ion sensors. Even if the glass membrane were placed between two identical solutions, the Em would not be zero. This is due to the fact that the membrane develops an asymmetry potential, which arises from the different degrees of mechanical stress at the interior and exterior interfaces of the glass. This affects the exchange current densities. We return later to this point, in the discussion of ion sensors with asymmetric membrane. 

Asymmetry potential — In case of any membrane it happens that the potential drop between the solution and either inner side of the - membrane is not completely identical so that a nonzero net potential drop arises across the entire membrane. This is best known for - glass electrodes and other - ion-selective electrodes. The reasons of asymmetry potentials are chemical or physical differences between each side of a membrane, in particular an inhomogeneous membrane structure resulting from fabrication conditions and/or curvature. Asymmetry p. can change in the course of membrane ageing. To measure asymmetry p. one should use a symmetrical cell with identical solutions and -> reference electrodes on each side of the membrane. 

The E.M.F. of this cell should be zero, but in practice the value is found to be of the order of rt 2 millivolts, for a good electrode. This small difference is called the asymmetry potential of the glass electrode it is probably due to differences in the strain of the inner and outer surfaces of the glass membrane. It is necessary, therefore, to standardize each glass electrode by means of a series of buffer solutions of known pH in this way the value of in equation (10) for the particular electrode is found. 

This cell closely resembles the original Sorensen cell, with a glass electrode substituted for the hydrogen electrode. Since glass electrodes are subject to an unpredictable asymmetry potential, pH measurements are made in practice by substituting a standard buffer for the sample solution and then comparing the pH of the sample with the pH of the standard, pH, according to the equation... 

Asymmetry potential A small potential that results from slight differences between the two surfaces of a glass membrane electrode. 

The asymmetry potential is a small potential across the membrane that is present even when the solutions on both sides of the membrane are identical. It is associated with factors such as nonuniform composition of the membrane, strains within the membrane, mechanical and chemical attack of the external surface, and the degree of hydration of the membrane. It slowly changes with time, especially if the membrane is allowed to dry out, and is unknown. For this reason, a glass pH electrode should be calibrated from day to day. The asymmetry potential will vary from one electrode to another, owing to differences in construction of the membrane. 

The diffusion potential results from a tendency of the protons in the inner part of the gel layer to diffuse Toward the dry membrane, which contains —SiO Na, and a tendency of the sodium ions in the dry membrane to diffuse to the hydrated layer. The ions migrate at a different rate, creating a type of liquid-junction potential. But a similar phenomenon occurs on the other side of the membrane, only in the opposite direction. These in effect cancel each other, and so the potential of the membrane is determined largely by the boundary potential. (Small differences in boundary potentials may occur due to differences in the glass across the membrane—these represent a part of the asymmetry potential.)... 

The pH meter cell must be standardised with buffers of known pH. Aqueous buffers are sometimes used when the solvent under investigation is a methanol-water mixture. One must then assume that liquid junction potentials thereby introduced are approximately constant from one test solution to another. 2 There is also a risk that transfer of a glass electrode from aqueous to methanolic solution may cause ir-reproducible changes in its asymmetry potential. A better procedure is to prepare standard buffers in methanol. Three such buffers were assigned pH values by the same procedure used for standard aqueous buffers. The assigned values, designated pH (5), should closely approximate pC %). If an unknown solution X is now measured in the standardised cell, the operational definition of its pH is... 

Normally, the assumption that inner and outer surfaces of the glass membrane have the same composition does not hold true, especially because the inner buffer solution is kept inside the electrode right from the production of the eleetrode and always stays there, whereas the outer surface is influenced by different solutions over the time of use. This gives rise to the so-called asymmetry potential. Further, one will usually find a sub-Nernstian slope, which is less than 0.059 V (at 25°C). (For the thermodynamic explanation, see [19]). Therefore an empirical equation of the following kind has to be used ... 

The measured potential may also be influenced by the so-called asymmetry potential. This is a residual potential between the two glass membrane surfaces when the solutions on both sides are identical. The asymmetry potential is due to differences in structural stresses at the two surfaces, differences in the surface history (ageing), and interaction with different solutions. The asymmetry potential may change with time. 

Precision measurements of the in situ pH below a depth of about 400 m require separate glass and reference electrodes with electrolyte junction and pressure compensation. A pH electrode withstanding IS 000 dbar has been described by Disteche (1959, 1962). Ihe first practicable deep-sea pH electrode was introduced by Ben-Yaakow and Kaplan (1968) and improved by Ben-Yaakow and Ruth (1974). The electrode can stand 6700 dbar hydrostatic pressure and has been tested to a depth of 5000m. The pressure is compensated for by replacing a part of the glass electrode housing by Tygon tube. The accuracy is 0.02 pH units. A correction for a shift of the asymmetry potential (2 mV for 6700 dbar) was included. 

As noted earlier, the potential of a glass indicator electrode has three components (1) the boundary potential, given by Equation 23-12, (2) the potential of the internal Ag-AgCl reference electrode re,2, and (3) the small asymmetry potential E sy. In equation form. 

The Nernst factor, 0.059 V for room temperature, does not always reach its theoretical value. In experimental work, it is better to use the expression Nernstian slope S of the function E = f(a). The slope must be determined empirically. The constant const in Eq. (7.8) is a combination of all terms not dependent on concentration. In practical work, the name asymmetry potential ( as) is preferred. This expression is derived from the expectation that the constant should be zero for a completely symmetric cell, i.e. if inner and outer solutions are of equal pH and if inner and outer reference electrodes are of identical types. In practice, Eas is not always zero but must be calibrated empirically by means of buffer solutions with known pH. By setting - log alHsO" ) = pH, the common form of the Nernst equation for the glass electrode results ... 

When electrodes are manufactured, every practical attempt is made to minimize this area. Electrodes that do not meet an asymmetry potential specification are eliminated. The asymmetry potential, in this case, is any difference in potential between glass and SCE reference electrodes, when immersed in pH 7 buffer (see Section 3.1.4 for an exact definition). Since it is not possible to obtain an isopotential point with electrodes, the pH meter isopotential point is set at the most likely point, pH 7. Since this point is only an estimation of the electrodes isopotential point, a slight error is observed if the measuring temperature is different than the buffering temperature. This is because a change in the slope made by changing the temperature compensator of the meter may not revolve around the same point as the slope of the electrode pair. 

Asymmetry potential The potential developed across the glass membrane with identical solutions on both sides. Also a term used when comparing glass electrode potential in pH 7 buffer. (See Chapter 3.)... 

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