Nernst equation glass electrode

The construction of these electrodes is exactly similar to that already described for the pH responsive glass electrode. They must of course be used in conjunction with a reference electrode and for this purpose a silver-silver chloride electrode is usually preferred. A double junction reference electrode is often used. The electrode response to the activity of the appropriate cation is given by the usual Nernst equation ... 

The Nernst equation shows that the glass electrode potential for a given pH value will be dependent upon the temperature of the solution. A pH meter, therefore, includes a biasing control so that the scale of the meter can be adjusted to correspond to the temperature of the solution under test. This may take the form of a manual control, calibrated in 0 C, and which is set to the temperature of the solution as determined with an ordinary mercury thermometer. In some instruments, arrangements are made for automatic temperature compensation by inserting a temperature probe (a resistance thermometer) into the solution, and the output from this is fed into the pH meter circuit. 

The purpose of the silver-silver chloride combination is to prevent the potential that develops from changing due to possible changes in the interior of the electrode. The potential that develops is a membrane potential. Since the glass membrane at the tip is thin, a potential develops due to the fact that the chemical composition inside is different from the chemical composition outside. Specifically, it is the difference in the concentration of the hydrogen ions on opposite sides of the membrane that causes the potential—the membrane potential—to develop. There is no half-cell reaction involved. The Nernst equation is... 

To leam how a pH electrode can be constmcted and why the potential developed across a thin layer of glass can be related to pH by a variant on the Nernst equation. 

The number of Gb sites on the outer membrane increases with decreasing [H ] and thus its potential becomes increasingly negative with respect to inner surface with increasing pH. The Nernst equation can be simplified and written in the following form for the glass electrode when the temperature is 20°C ... 

The response of real glass electrodes is described by the Nernst-like equation Response of E = constant + 0(0.059 16) log JZlH+(outside)... 

The potential of a glass electrode is given by a form of the Nernst equation very similar to that of an ordinary hydrogen electrode, but of course without the H2 ... 

The measurement of pH using a glass electrode obeys the Nernst equation. The typical response of a pH meter at 25.00°C is given by the equation... 

Fortunately, glass electrodes have wide applicability in solvents other than water. Even in a solvent as different from water as acetonitrile, glass electrodes respond reversibly to changes in hydrogen ion activity, in agreement with the Nernst equation. In setting up pH values for reference buffers, it is not always possible to use internal reference electrodes without liquid junction. For example, Ag-AgX electrodes are unstable in acetonitrile owing to slow formation of species of the type Ag X +j. When external reference electrodes must be substituted, the reliability of the measurements is reduced because uncertainties in the liquid junction are always present. 

Most pH determinations are made by electrometric methods, the pH of the unkown solution (X) being calculated from that of a known standard (S) and the emf ( x and s) of a cell composed of a hydrogen ion-responsive electrode (for example, a glass electrode or a hydrogen gas electrode) coupled with a reference electrode (calomel, silver-silver chloride). This cell is filled successively with the standard solution S and with the unknown solution X. A liquid junction potential j exists where these solutions make contact with the concentrated KCl solution of the reference electrode. From the Nernst equation for the cell reactions and assuming an ideal hydrogen ion response ... 

The first and third half-reactions have half-cell potentials (AgCl CH Ag) and (Hg2Cl2 cr Hg)that can be combined and called A ef because they make a constant contribution to the cell voltage. The second reaction is the source of a variable potential in the cell, corresponding to the free energy of dilution of H30 from a concentration of 1.0 M to an unknown and variable concentration, and its potential exists across the thin glass membrane of the glass electrode. The Nernst equation for the cell can therefore be written as... 

The theoretical approach to this problem was first limited to the application of the Nernst equation which predicts potential charge equal to 2.3 — Volts for a pH unit. This equation is satisfied by a glass electrode as the most popular device for pH measurements in electrochemistry. In turn, in the case of ISFET s device, the potential change value was much below the Nernst value. Therefore, the Nernst equation cannot be applied for the insulator such as a metal oxide which is neither an electric nor ionic conductor [118]. 

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 ... 

The resistance of the whole electrode, depending on its composition and dimensions, is in a broad range of 1-1000 MO. Due to the high resistance the cable connecting it to the measuring device (voltmeter, pH meter) must be protected from electrostatic interferences. The potential difference between the two sides of the glass membrane is proportional to the difference between the pH values on both sides, according to the Nernst equation... 

The Great Nernst Hiatus." In direct potentiometry using membrane electrodes, a similar situation has occurred in the past 8 decades. Every quantitative and instrumental analysis textbook has treated the glass electrode as a battery obeying the principles of reversible thermodynamics and the Nernst equation. 

The acid and alkaline errors of the pH measurements by the glass electrode have baffled chemists ever since the glass electrode was created. After many studies, the conclusion has been that it is too complicated to be understood (30). The Nernst equation cannot explain the nonlinearity at the very low and very high hydrogen ion concentrations. If we accept the following concepts, the acid and alkaline errors are easily understood. 

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 ... 

Considering the Nernst equation as it applies to the measurement of pH, calculate the voltage change at a glass electrode nsed to measure pH for each unit change in pH. 

It is very difficult to measure pH values of less than two in aqueous solution with a glass electrode, because the Nernst equation breaks down at such low pH values. To determine p values of less than about 2 or more than about 11 spectrophotometric or NMR measurements may be used instead of, or combined with, pH measurements. 

As stated previously, the observed potential from the glass electrode varies in a known manner, the Nernst equation. This equation shows the relationship between any potentiometric sensing electrode and the ion to which the electrode is sensitive. When applied to the pH glass electrode and the hydrogen ion activity, it... 

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