Glass electrode theories

The concentration of the solution within the glass bulb is fixed, and hence on the inner side of the bulb an equilibrium condition leading to a constant potential is established. On the outside of the bulb, the potential developed will be dependent upon the hydrogen ion concentration of the solution in which the bulb is immersed. Within the layer of dry glass which exists between the inner and outer hydrated layers, the conductivity is due to the interstitial migration of sodium ions within the silicate lattice. For a detailed account of the theory of the glass electrode a textbook of electrochemistry should be consulted. 

Bates, R. G. Determination of pH Theory and Practice, New York, Wiley 1964 Eisenman, G., ed. Glass Electrodes for Hydrogen and Other Cations-Principles and Practice, New York, Dekker 1967... 

The search for models of biological membranes led to the formation of a separate branch of electrochemistry, i.e. membrane electrochemistry. The most important results obtained in this field include the theory and application of ion-exchanger membranes and the discovery of ion-selective electrodes (including glass electrodes) and bilayer lipid membranes. 

This type of membrane consists of a water-insoluble solid or glassy electrolyte. One ionic sort in this electrolyte is bound in the membrane structure, while the other, usually but not always the determinand ion, is mobile in the membrane (see Section 2.6). The theory of these ion-selective electrodes will be explained using the glass electrode as an example this is the oldest and best known sensor in the whole field of ion-selective electrodes. 

The glass electrode for pH measurements has long been a standard laboratory sensor and the subject of several monographs [74, 118, 349]. Consequently, only the basic theory will be given here, though electrodes sensitive to the alkah metal ions and silver will be considered in somewhat greater detail [95]. 

If there is a strong acid that completely dissociates, we prepare a solution of the acid of known concentration and use it to calibrate the glass electrode. The pH of the solution is calculated by estimating ym(H+) using the Debye-Hiickel theory. 

Thermodynamic properties in dilute aqueous solutions are taken to be functions of ionic strength so that concentrations of reactants, rather than their activities can be used. This also means that pHc = — log[H+] has to be used in calculations, rather than pHa = — log a(H + ). When the ionic strength is different from zero, this means that pH values obtained in the laboratory using a glass electrode need to be adjusted for the ionic strength and temperature to obtain the pH that is used to discuss the thermodynamics of dilute aqueous solutions. Since pHa = — log-/(H + ) + pHc, the use of the extended Debye-Hiickel theory yields... 

The theory and application of selective-ion electrodes have been extensively reviewed.143-151 One of the interesting sidelights is the fact that the internal reference electrode may be replaced by an apparent ohmic contact in many instances, as illustrated by Figure 5 Ale for the solid membrane electrode. Thus the glass electrode can be filled with mercury in place of the internal reference electrode,152 or a gold contact that is plated over with copper can be used.153 Likewise, a selective-ion electrode for calcium ion has been described that is coated on a platinum electrode 154 the contact appears to be mainly ohmic. 

In many calculations the hydrogen ion concentration is more accessible than the activity. For example, the electroneutrality condition is written in terms of concentrations rather than activities. Also, from stoichiometric considerations, the concentrations of solution components are often directly available. Therefore, the hydrogen ion concentration is most readily calculated from equilibrium constants written in terms of concentration. When a comparison of hydrogen ion concentrations with measured pH values is required (in calculation of equilibrium constants, for example), an estimate of the hydrogen ion activity coeflScient can be made by application of the Debye-Huckel theory if necessary, an estimate of liquid-junction potentials also can be made. Alternatively, the glass electrode can be calibrated with solutions of known hydrogen ion concentration and constant ionic strength. " ... 

The Glass Electrode, Methods, Applications and Theory. Erschie-nen bei J, Wiley Sons, New York 1941. 

K. L. Cheng has proposed a theory of glass electrodes based on capacitor theory in which the electrode senses the hydroxide ion in alkaline solution (where an is very small), rather than sensing protons. [K. L. Cheng, Capacitor Theory... 

There is a large literature on the design, performance, and theory of glass electrodes (37, 46-55). The interested reader is referred to it for more advanced discussions. 

Metal electrodes (and the hydrogen electrode) have been utilized for concentration measurement almost since the beginning of electrochemistry late in 19th century. Since early in this century chemists have been fascinated by the knowledge that a selectively permeable membrane would allow concentration measurement in a similar fashion. Before 1910 the remarkable selectivity of the glass electrode for hydrogen ion had been discovered and by the 30 s it was commercially available and the ion exchange theory of its operation had been presented. 

Figure 15-9 Acid and alkaline errors of some glass electrodes. A Corning 015, HjSO. B Corning 015, HCi. C Corning 015, IMNa. D Beckman-GP, 1 M Na. E L N Black Dot, 1 M Na. F Beckman Type E, 1 M Na. G Ross electrode. [From R. G. Bates, Determination of pH Theory and Practice, 2nd ed. (New York Wiley, 1973). Ross electrode data is from Orion, Ross pH Electrode Instruction Manual.]...

Michaelis (XS) mentioned that the glass membranes acted like his membranes of dried collodion. This observation points toward the current concept of membrane electrode theory. 

---