Crystal Membrane Electrode

Fig. 2.10. Membrane electrode types. 1 Glass electrode 2, 3 and 4 crystal membrane electrodes. 

By running a potentiometric precipitation titration, we can determine both the compositions of the precipitate and its solubility product. Various cation- and anion-selective electrodes as well as metal (or metal amalgam) electrodes work as indicator electrodes. For example, Coetzee and Martin [23] determined the solubility products of metal fluorides in AN, using a fluoride ion-selective LaF3 single-crystal membrane electrode. Nakamura et al. [2] also determined the solubility product of sodium fluoride in AN and PC, using a fluoride ion-sensitive polymer membrane electrode, which was prepared by chemically bonding the phthalocyanin cobalt complex to polyacrylamide (PAA). The polymer membrane electrode was durable and responded in Nernstian ways to F and CN in solvents like AN and PC. 

Crystal-membrane electrodes, such as the fluoride electrode. Here a crystal is responsible for the exchange of ions in the case of the fluoride electrode, it is a LaF3 crystal. The advantage of this type of electrode is their high selectivity towards one single species because of the unique affinity of the ion in solution (e.g. fluoride) for the crystal (e.g. LaF3)2. 

D3. D Orazio, P., and Rechnitz, C. A., Protein response of silver sulfide crystal membrane electrodes. Anal. Chem. 49, 41-44 (1977). 

The membrane is a conducting solid. Both single crystal and peUet-pressed crystalhne substance mixtures can be used in membrane construction. Table 2, accumulated from data available from several electrode manufactmers data sheets, shows information relative to crystal membrane electrodes and then-application capabilities. As can be seen, crystal membrane electrodes, with the exception of that selective to F, involve Ag2S or crystal mixtures where one component is Ag2S and the other the sulphide of the selective ion of interest. The membranes are generally produced by pressing the polycrystalline substance in a pellet press. 

With metal sulphide/sUver sulphide crystal membranes, the silver ions are again the mobile species. The cadmium ion-selective crystal membrane electrode (CdS/Ag2S membrane) can be used as an example of the mechanism involved. CdS wUl be more soluble than Ag2S. The activity of the sulphide ion wiU be controUed at the interface by that of the cadmium ion ... 

FiO. 13.12. Crystal membrane electrode. (Reproduced by permission of Orion Research, Inc.)... 

The designation membrane is used here in a very general sense, as many materials are used. Commercially available electrodes include liquid membrane units, solid-state electrodes, glass membrane electrodes, and plastic membrane electrodes. General classes of ion specific electrodes in addition to those cited are immobilized-liquid membrane electrodes, mixed-crystal membrane electrodes, enzyme electrodes, and antibiotic electrodes (Rechnitz, 1973). Certain of these membrane electrodes are now discussed in some detail. A generalized membrane electrode is illustrated in Figure 6.1a. 

With fluoride-selective single crystal membrane electrodes only interferences of the second kind are observed which exert an influence on the solution equilibria of bound and free ions. It is known that ions combine with F" ions to form undissociated species ... 

Fig. 27. pH dependence of the EMF between a fluoride-selective single crystal membrane electrode and a reference electrode... 

LaFs crystals developed by J. W. Ross and M. S. Frant as the first non-glass membrane electrode... 

The glass membrane of the electrodes discussed above may be replaced by other materials such as a single crystal or a disc pressed from finely divided crystalline material it may be advantageous to incorporate the crystalline material into an inert carrier such as a suitable polymer thus producing a heterogeneous-membrane electrode. 

In solid-state electrodes the membrane is a solid disc of a relatively insoluble, crystalline material which shows a high specificity for a particular ion. The membrane permits movement of ions within the lattice structure of the crystal and those ions which disrupt the lattice structure the least are the most mobile. These usually have the smallest charge and diameter. Hence, only those ions that are very similar to the internal mobile ions can gain access to the membrane from the outside, a feature that gives crystal membranes their high specificity. When the electrode is immersed in the sample solution, an equilibrium is established between the mobile ions in the crystal and similar ions in the solution and the resulting potential created across the membrane can be measured in the usual manner. 

The liquid-membrane electrode is another important type of ion-selective electrode. The internal filling solution contains a source of the ion under investigation, i.e. one for which the ion exchanger is specific, while also containing a halide ion to allow the reference electrode to function. The physico-chemical behaviour of the ISE is very similar to that of the fluoride electrode, except that ise and the selectivity are dictated by the porosity of a membrane rather than by movement through a solid-state crystal. 

Several classical ion-selective electrodes (some of which are commercially available) have been incorporated into continuous systems via suitable flow-cells. In fact, Lima et al. [112] used a tubular homogeneous crystal-membrane (AgjS or AgCl) sensor for the determination of sulphide and chloride in natural and waste waters. However, the search for new active materials providing higher selectivity and/or lower detection limits continues. Thus, Smyth et al [113] tested the suitability of a potentiometric sensor based on calix[4]arene compounds for use in flow injection systems. They found two neutral carriers, viz. methyl-j3-rerr-butylcalix[4]aryl acetate and... 

An example is described here for the measurement of fluoride ions in solution. The fluoride electrode uses a LaF3 single crystal membrane and an internal reference, bonded into an epoxy body. The crystal is an ionic conductor in which only fluoride ions are mobile. When the membrane is in contact with a fluoride solution, an electrode potential develops across the membrane. This potential, which depends on the level of free fluoride ions in solution, is measured against an external constant reference potential with a digital pH/mv meter or specific ion meter. The measured potential corresponding to the level of fluoride ions in solution is described by the Nernst equation ... 

Table 5.10 summarizes the presently available electrodes categorized as glass, ion-exchange membrane, crystal membrane, and liquid membrane. These electrodes can be used either for direct potentiometric measurements of ionic activity after calibration of the Nemst expression for the particular electrode or to monitor a potentiometric titration when a selected reaction that involves the monitored ion is available. Table 5.10 also indicates the common interfering ions. Several instrument companies are endeavoring to develop potentiometric-membrane electrodes to monitor directly ions in body fluids. 

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. 

Other types of electrodes are listed in Table 8.9. The glass membrane is replaced by a synthetic single-crystal membrane (solid-state electrodes), by a matrix (e.g., inert silicone rubber) in which precipitated particles are imbedded (precipitate electrodes), or by a liquid ion-exchange layer (liquid-liquid membrane electrodes). The selectivity of these electrodes is determined by the composition of the membrane. All these electrodes show a response in their electrode potentials according to the Nemst equation. 

Sohd state electrodes may he constructed as crystal membrane or precipitate-impregnated membrane type. The latter appeared on the scene first ca. 1950, but because of technical difficulties in their construction httle advance in use was apparent until 1966. Around this time, both forms of sohd state electrode started a climb to prominence as indicated by Buck [12] 1968, Eisenmann [13] 1969 and Ross [14] 1969. 

These electrodes have the silver ion as the mobile ion. Immersion in a solution of the selective ion results in a change in the silver ion activity, resulting in the development of a potential over the interfaces of the membrane. The theory will be discussed in detail for the crystal membrane solid-state electrode section to follow. 

Figure 4 Crystal membrane ion-selective electrodes (courtesy of ATI Orion, Boston, MA).

Where the crystal membrane contains both Ag2S and a silver hahde, the electrode is then selective to the hahde involved. The silver hahde wiU have higher solubhity than the silver sulphide. It wdl, however, have an equihbrium solubihty such that the hahde activity in the membrane wiU be significantly less than its activity in the test solution. The extent of the difference in activities on either side of the interface generates a potential difference resulting in the movement of the conducting silver ions. The extent of the difference in potential relates through this movement to the activity (concentration) of the hahde in the test solution. 

Single crystal and solid-state membrane electrodes... 

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