Glass Electrode

A glass electrode is a type of ion-selective electrode which is made up of a doped glass membrane which is sensitive to a specific ion. The electric potential of the electrode system in solution is sensitive to changes in the content of a some type of ions, which is reflected in the dependence of the electromotive force of galvanic element concentrations of these ions. Glass electrodes are usually used for pH measurements. Specialized ion sensitive glass electrodes are also used for determination of concentration of sodium, lithium, ammonium, and other ions. Glass electrodes have been utilized in a broad range of applications from pure research, control of industrial processes, to analyze cosmetics, foods, and comparison of indicators of the environment and environmental regulations: microelectrode measurements of membrane electrical potential of a biological cell, analysis of the acidity of soil, etc. Almost all the commercial electrodes respond to single charged ions, like Na⁺, H⁺, Ag⁺. The one of the most common glass electrode is the pH-electrode. Only a few chalcogenide glass electrodes are sensitive to double-charged ions, like Cd²⁺ , Pb²⁺, and some others.

There are 2 main glass-forming systems. They are silicate matrix based on the molecular network of silicon dioxide (SiO₂) with additions of the other metal oxides, like Na, K, Al, B, Li, Ca, etc. and chalcogenide matrix based on the molecular network of AsSe, AsS, AsTe.

Due to the ion-exchange nature of the glass membrane, it is possible for other ions to concurrently interact with ion-exchange centers of glass and to distort the linear dependence of the measured electrode potential on pH or other electrode function. In some of the cases it is possible to change the electrode function from one ion to another. Alkali error range is, at low concentration of hydrogen ions contributions of interfering alkali metals can be compared with the one of hydrogen ions. In this particular situation dependence of the potential on pH become non-linear. The effect is noticeable usually at pH > 12, and concentrations of sodium or lithium ions of 0.1 moles per litre or more. Potassium ions cause usually less error than the sodium ions. Acidic error range - at very high concentration of hydrogen ions the dependence of electrode on pH becomes non-linear and the influence of anions in solution becomes noticeable. These effects become noticeable usually at pH  < 1.