Solid-state membranes, selective electrodes

Unlike ion-selective electrodes using glass membranes, crystalline solid-state ion-selective electrodes do not need to be conditioned before use and may be stored dry. The surface of the electrode is subject to poisoning, as described earlier for a Ck ISE in contact with an excessive concentration of Br. When this happens, the electrode can be returned to its original condition by sanding and polishing the crystalline membrane. 

Walters [24] examined the effect of chloride on the use of bromide and iodide solid state membrane electrodes, and he calculated selectivity constants. Multiple linear regression analysis was used to determine the concentrations of bromide, fluorine, and iodide in geothermal brines, and indicated high interferences at high salt concentrations. The standard curve method was preferred to the multiple standard addition method because of ... 

The simplest solid-state membranes are designed to measure test ions, which are also the mobile ions of the crystal (first-order response) and are usually single-substance crystals (Figure 4.11). Alternatively, the test substance may be involved in one or two chemical reactions on the surface of the electrode which alter the activity of the mobile ion in the membrane (Figures 4.12 and 4.13). Such membranes, which are often mixtures of substances, are said to show second- and third-order responses. While only a limited number of ions can gain access to a particular membrane, a greater number of substances will be able to react at the surface of the membrane. As a result, the selectivity of electrodes showing second- and third-order responses is reduced. 

Apart from interference the greatest problem in the use of ion-selective electrodes is that of contamination. Any insoluble material deposited on the surface of the electrode will significantly reduce its sensitivity and oil films or protein deposits must be removed by frequent and thorough washing. It is possible to wipe membranes with soft tissue but they can be easily damaged. Solid-state membranes are more robust but they must not be used in any solution which might react with the membrane material. 

Traditionally, potentiometric sensors are distinguished by the membrane material. Glass electrodes are very well established especially in the detection of H+. However, fine-tuning of the potentiometric response of this type of membrane is chemically difficult. Solid-state membranes such as silver halides or metal sulphides are also well established for a number of cations and anions [25,26]. Their LOD is ideally a direct function of the solubility product of the materials [27], but it is often limited by dissolution of impurities [28-30]. Polymeric membrane-based ISEs are a group of the most versatile and widespread potentiometric sensors. Their versatility is based on the possibility of chemical tuning because the selectivity is based on the extraction of an ion into a polymer and its complexation with a receptor that can be chemically designed. Most research has been done on polymer-based ISEs and the remainder of this work will focus on this sensor type. 

Considerable work has been devoted to the development of solid membranes that are selective primarily to anions. The solid-state membrane can be made of single crystals, polycrystalline pellets, or mixed crystals. The resulting solid-state membrane electrodes have found use in a great number of analytical applications. 

Fig. 13.7. Forms of ion-selective electrodes with solid state membranes (a) with internal reference electrode (b) with ohmic contact (c) with ohmic contact and combined reference electrode.

In these sensors the technology developed for ISFET construction is used in conventional electrodes. Links between the membrane and internal reference are metallic (ohmic contact), by deposition of the metal on the membrane (solid state membranes), or by deposition of an ion-selective membrane on a metal. This latter is an integrated sensor. 

The sensitivity of ion-selective electrodes can be excellent in metal-ion-buffered solutions, that is, at fairly high total metal-ion concentrations. Under these conditions, commercial solid-state membrane electrodes were shown to follow the Nemst equation in fresh waters down to pCu 11-12 and after preconditioning in a metal-ion buffer to pCu = 13 (Sigg and Xue, 1994 Sunda and Hanson, 1979 Xue and Sigg, 1990). Some ion-selective electrodes, especially the Cu electrode, show interference in high chloride media (seawater). 

In the present chapter, the relationship between the electrode potential and the activity of the solution components in the cell is examined in detail. The connection between the Galvani potential difference at the electrode solution interface and the electrode potential on the standard redox scale is discussed. This leads to an examination of the extrathermodynamic assumption which allows one to define an absolute electrode potential. Ion transfer processes at the membrane solution interface are then examined. Diffusion potentials within the membrane and the Donnan potentials at the interface are illustrated for both liquid and solid state membranes. Specific ion electrodes are described, and their various modes of sensing ion activities in an analyte solution discussed. The structure and type of membrane used are considered with respect to its selectivity to a particular ion over other ions. At the end of the chapter, emphasis is placed on the definition of pH and its measurement using the glass electrode. 

The application of potentiometric detection in ion-chromatography is favoured by the progress in the field of membrane ion-selective electrodes (ISE). The electrodes with solid-state membranes were mostly employed for determination of halides, pseudohalides and some other anions binding silver ions. The use of fluoride electrode in multidetector, chromatographic system offers very low detection limit of 1.2 ng fluoride in injected samples. Application of bromide electrode in the same system provides even five-fold better detectability. The same level of detectability was reported by Butler and Gershey for iodide with iodide ISE. In the system with preconcentration step the detectability can be lowered by an order... 

The solid-contact reference systems used in all-solid-state membrane based ion selective electrodes are discussed in Sect. II.9.3.4.3. 

Another solid-state membrane of extremely high selectivity and appKcability is the single crystal LaFs membrane electrode [26]. The crystal is doped with europium to lower its electrical resistance... 

Glass pH electrodes and most other practical ion-selective electrodes (ISEs) are membrane devices. The term electrode is, strictly, a misnomer because the active ion-selective membrane has on one side an analyte (the test solution) and on the other an internal (reference) solution containing an internal reference electrode the internal solution and electrode together provide the electrical contact to the membrane. A solid-state ion-selective electrode is one in which the internal components are replaced by a direct back contact to a metal or semiconductor, i.e., an electronic conductor. This is not a trivial matter, because most ion-selective materials are ionic conductors, and... 

At low analyte concentrations, the activity of interfering ions must be considered. Ion-selective electrodes can be divided into categories based on the nature of the membrane material. These categories include glass membrane electrodes, liquid membrane electrodes, and solid-state membrane electrodes. 

Solid-state membrane electrodes include H+, F , CN , Cl , 1, Br, H2S, CN , thiourea, Pb, Cu, and Cd selective electrodes. In this category, silver halide and silver sulphide electrodes are found. The macroscopic ion-selective electrodes made with these systems have been applied in many fields, such as medicine, food industry, and environmental studies (56, pp. 93,96,103,104) but their full potential has not yet been exploited in terms of SECM studies. 

Several quite different types of membranes have been used in the construction of lon-selective electrodes, namely (1) glasses (2) solid-state membranes (3) heterogeii us membranes and (4) liquid membranes. 

The fundamental problem of a reference potential when solid contact is applied has been discussed in seminal papers. Initially, thermodynamic criteria of stability for solid-state sparingly soluble sulfide membranes were presented [5, 6], while other authors showed the application of mixed conductor AgF as a solid contact for an ionically conducting LaFs solid-state ISE membrane [7, 8]. Different contacts for ion-selective electrodes with glass membrane were reviewed by Nikolskii and Materova [9]. 

There have been many attempts to develop selective precipitate membrane electrodes for the important sulfate and phosphate anions. Unfortunately they have all failed to date. A sulfate-sensitive solid-state membrane electrode with a pellet of 31.7% Ag2S, 31.7% PbS, 31.7% PbS04 and 5% CU2S has been described [91], but its selectivity over I, HPO4" and SO3 is low. 

Instead of using pellets mixed with the respective silver ion powder, these materials can simply be suspended in the solution to achieve the same selective behavior (see Fig. 26). A simple Ag2S solid-state membrane electrode can then serve as the measuring electrode. Above all one must eliminate the presence of O2 and be sure of the optimum pH value, since both of these factors affect the sulfide equilibria. 

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