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. where F is Faraday's constant (see Nernst equation). Alkali error range - at low concentration of hydrogen ions (high values of pH) contributions of interfering alkali metals (like Li, Na, K) are comparable with one of the hydrogen ions. In this situation dependence of the potential on pH become non-linear. The effect is usually noticeable at pH > 12, and at concentrations of lithium or sodium ions of 0.1 moles per litre or more. Potassium ions usually cause less error than sodium ions. Acidic error range – at a very high concentration of hydrogen ions (low values of pH) the dependence of the electrode on pH becomes non-linear and the influence of the anions in the solution also becomes noticeable. These effects usually become noticeable at pH < -1. Specialized electrodes exist for working in extreme pH ranges. Construction A typical modern pH probe is a combination electrode, which combines both the glass and reference electrodes into one body.