Selectivity coefficients liquid-membrane electrodes

Liquid membrane electrodes are not capable of being specific for only one ion in solution. There is always some interference from other ions in solution with the given analyte. The selectivity coefficient provides an indication of the ability of an electrode to measure a particular ion in the presence of another ion. The response of an electrode to an interfering ion can be included in the Nernstian equation ... 

Liquid membrane electrodes are subject to interferences from ions other than that of prime selectivity. For example, the Ca-ISE is also responsive to Mg2+ and Ba2+, the selectivity coefficients being approximately 0.01 for each ion. This indicates that the electrode is only 100 times more sensitive to Ca than to these ions, and this is normally much more important with respect to Mg than to Ba where food analysis is concerned. There are techniques which can be used to minimise the interference of Mg2+. 

The drawbacks of the original Orion liquid membrane electrode model were successfully overcome by simply incorporating the neat exchanger within a thin matrix of PVC (9). Besides having more favourable mechanical and physical characteristics, these novel PVC electrodes had prolonged functional lifetimes and a performance, in terms of Nernstian calibration, response times and selectivity coefficients, on a par with the original Orion 92-20 model. In fact, the mean content of calcium determined with the respective electrodes in tap water on 10 consecutive days was 31.55 and 31.57 ppm (9). 

In contrast to the liquid membrane electrodes, the selectivity coefficients of the exclusively fluoride or silver ion-conducting solid membrane electrodes are clearly determined by the ratio of the corresponding solubility products, and are thus independent of the respective electrode manufacturers. 

Many of the ions listed in Table 5 are indicated by the corresponding coated wire electrodes exactly according to the Nernst equation in the concentration range 10" to 10" M. For sulfate and oxalate anions a slope of about 28 mV per power of ten in activity results at 25°C. Table 5 illustrates the good selectivity behavior of these electrodes. In all cases the selectivity coefficients are more favorable than those for the corresponding liquid membrane electrodes [150]. The response time is very short — only a few seconds. The lifetime of these microelectrodes is claimed to exceed 3 months. 

For practical purposes, it is preferable to use equation (1) of the foregoing, allowing Ky to represent the u and d ratios. Remember that, as indicated earlier, the sign must be -i- for cation-selective and - for anion-selective electrodes. Exchange sites and selectivity coefficients for various liquid membrane ion-selective electrodes can be found in Durst [11]. 

From the basic parameters initial concentration of ions, their standard transfer potential, distribution coefficients for neutral components, equilibrium constants of reactions taking place in the system, volume of phases, and temperature, a unique general problem for the Galvani potential difference and distribution concentration of all components was established. A numerical solution to the problem with the help of computer program EXTRA.FIFIl provided a good means for quantitative investigation of the liquid-liquid interface. It is also useful for the study of liquid-liquid extraction, electroextraction, voltammetry at interface of two immiscible electrolyte solutions (ITIES) [15,18], liquid-liquid membrane ion-selective electrodes, biomembrane transport, and other fields of science and engineering. 

Table 1 Membrane compositions and selectivity coefficients of selected liquid membrane ion-selective electrodes ...

Another component of the polymer membrane of ion-selective electrodes is the plasticizer. It lowers the glass transition temperature of the polymer, serves as the solvent for the ionophore and other membrane additives, increases the polarity of the membrane, which facilitates the distribution of ions between two phases. The plasticizer content in the membrane is approx. 65% of weight. Due to the plasticizer, the membrane can be considered a liquid phase, since the diffusion coefficient of the dissolved, low-molecular-weight ionophores is approx. 10" - - 10 cmV [38]. Such membranes are often called liquid/polymer or solvent polymeric in the literature. 

Because polyelectrolytes are nonvolatile, the most important thermodynamic property for vapor + liquid phase equilibrium considerations is the vapor pressure of water above the aqueous solution. Instead of the vapor pressure, some directly related other properties are used, e.g., the activity of water a, the osmotic pressure 71, and the osmotic coefficient < . These properties are defined and discussed in Sect. 4. Membrane osmometry, vapor pressure osmometry, and isopiestic experiments are common methods for measuring the osmotic pressure and/or the osmotic coefficient. A few authors also reported experimental results for the activity coefficient y i of the counterions (usually determined using ion-selective electrodes) and for the freezing-point depression of water AT p. The activity coefficient is the ratio of activity to COTicentration ... 

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