Glass electrode

A glass electrode is a type of ion-selective electrode made of a doped glass membrane that is sensitive to a specific ion. The most common application of ion-selective glass electrodes is for the measurement of pH. The pH electrode is an example of a glass electrode that is sensitive to hydrogen ions. Glass electrodes play an important part in the instrumentation in analytical chemistry and in physicochemical studies. The voltage of the glass electrode, relative to some reference value, is sensitive to changes in the activity of certain types of ions.

History The first studies of glass electrodes (GE) found different sensitivities of different glasses to change the medium's acidity (pH), due to the effects of the alkali metal ions. In 1906, M. Cremer, the father of Erika Cremer, determined that the electric potential that arises between parts of the fluid, located on opposite sides of the glass membrane, is proportional to the concentration of acid (hydrogen ion concentration). In 1909, S. P. L. Sørensen introduced the concept of pH, and in the same year F. Haber and Z. Klemensiewicz reported results of their research on the glass electrode in The Society of Chemistry in Karlsruhe. In 1922, W. S. Hughes showed that alkali-silicate glass electrodes (containing silanols) are similar to hydrogen electrodes, as both are reversible concerning H+. In 1925, P. M. Tookey Kerridge developed the first glass electrode for the analysis of blood samples and highlighted some of the practical problems with the equipment such as the high resistance of glass (50–150 MΩ). During her PhD, Kerridge developed a glass electrode aimed to measure small volume of solution. Her clever and careful design was a pioneering work in the making of glass electrodes.

Applications Glass electrodes are commonly used for pH measurements. There are also specialized ion-sensitive glass electrodes used for the determination of the concentration of lithium, sodium, ammonium, and other ions. Glass electrodes find a wide diversity of uses in a large range of applications including research labs, control of industrial processes, analysis of foods and cosmetics, monitoring of environmental pollution, or soil acidity measurements. Micro-electrodes are specifically designed for pH measurements on very small volumes of fluid, direct measurements in geochemical micro-environments, or determining the electrical potential of the cell membrane in biochemical studies. Heavy duty electrodes capable of withstanding several tens of bar of hydraulic pressure also allow for measurements in water wells in deep aquifers. Additionally, they can be used to directly determine in situ the pH of pore water in deep clay formations. For long-term in situ measurements, it is critical to minimize KCl leakage from the reference electrode compartment (Ag / AgCl / KCl 3 M), use glycerol-free electrodes to avoid fuelling microbial growth, and prevent unexpected but severe perturbations related to bacterial activity (pH decrease due to sulfate-reducing bacteria, or methanogen bacteria).

Types All commercial electrodes somewhat selectively respond to single-charged ions, such as H+, Na+, Ag+. The most common glass electrode is the pH-electrode. Only a few chalcogenide glass electrodes are presently known to be sensitive to double-charged ions, such as Pb2+, Cd2+, and some other divalent cations. There are two main types of glass-forming systems:

The most common one: a silicate matrix based on an amorphous molecular network of silicon dioxide (SiO2, the network former) with additions of other metal oxides (network modifiers), such as Na, K, Li, Al, B, Ca..., and; A less used one: a chalcogenide matrix based on a molecular network of AsS, AsSe, or AsTe.

Interfering ions

Because of the ion-exchanging nature of the glass membrane, it is possible for some other ions to concurrently interact with ion-exchange sites of the glass, distorting the linear dependence of the measured electrode potential on pH or other electrode functions. In some cases, it is possible to change the electrode function from one ion to another. For example, some silicate pPNA electrodes can be changed to pAg function by soaking in a silver salt solution. Interference effects are commonly described by the semi-empirical Nikolsky-Shultz-Eisenman equation, an extension to the Nernst equation. It is given by:

E = E 0 + R T z i F ln ⁡ [ a i + ∑ j ( k i j a j z i / z j ) ] {\displaystyle E=E^{0}+{\frac {RT}{z_{i}F}}\ln \left[a_{i}+\sum _{j}\left(k_{ij}a_{j}^{z_{i}/z_{j}}\right)\right]}

where E is the electromotive force (emf), E0 the standard electrode potential, z the ionic valency including its sign, a the activity, i the ion of interest, j the interfering ions and kij is the selectivity coefficient quantifying the ion-exchange equilibrium between the ions i and j. The smaller the selectivity coefficient, the less is the interference by j. To see the interfering effect of Na+ to a pH-electrode:

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