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 for chemical analysis and physicochemical studies. The voltage of the glass electrode, relative to some reference value, is sensitive to changes in the activity of a certain type 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 the alkali-silicate GE are similar to hydrogen electrodes, reversible concerning H+.
In 1925, P.M. Tookey Kerridge developed the first glass electrode for 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 the miniature glass electrode, maximizing the surface area of the tool by heat treating the platinum with platinum chloride at red heat, thus enabling a much larger signal; her design was the predecessor of many of the glass electrodes used today.
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 have been utilized in a wide range of applications including pure research, control of industrial processes, analysis of foods and cosmetics, measurement of environmental indicators, and microelectrode measurements such as cell membrane electrical potential and soil acidity.
Types
Almost all commercial electrodes respond to single-charged ions, like H+, Na+, Ag+. The most common glass electrode is the pH-electrode. Only a few chalcogenide glass electrodes are sensitive to double-charged ions, like Pb2+, Cd2+ and some others.
There are two main glass-forming systems: silicate matrix based on a molecular network of silicon dioxide (SiO2) with additions of other metal oxides, such as Na, K, Li, Al, B, Ca, etc. and chalcogenide matrix based on a molecular network of AsS, AsSe, AsTe.
Interfering ions
Because of the ion-exchange nature of the glass membrane, it is possible for some other ions to concurrently interact with ion-exchange centres of the glass and distort 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 pPNAelectrodes can be changed to pAg function by soaking in a silver salt solution.
Interference effects are commonly described by the semiempirical Nicolsky-Shultz-Eisenman equation (also known as Nikolsky-Shultz-Eisenman equation), an extension to the Nernst equation. It is given by
where E is the emf, E0 the standard electrode potential, z the ionic valency including the sign, a the activity, i the ion of interest, j the interfering ions and kij is the selectivity coefficient. The smaller the selectivity coefficient, the less is the interference by j.
To see the interfering effect of Na+ to a pH-electrode:
Range of a pH glass electrode
The pH range at constant concentration can be divided into 3 parts:
Complete realization of general electrode function, where potential depends linearly on pH, realizing an ion-selective electrode for hydronium.