Potassium selective membrane electrode

Figure 4-2 Typical EMF response of potassium selective membrane electrode to changes in the activity of potassium in the sample solution. Bracketed interval represents the normal reference interval of potassium concentration in blood. (From D Orazio R in Clinical chemistry laboratory management and ctinicai correlations, iewendrowski K, ed. Philadelphia Uppincott,Wiliiams and Wilkins, 2002 455.)...

Before leaving the subject of the mechanism of metal-ion transport across membranes, a cautionary result and a by-product of these investigations might be reported. Hall has found that the antibiotic monamycin forms complexes in solution with K+, Rb+, and Cs+, but not with Na+ or Li+. However, it seems that their antibacterial action is due to their lytic effects on cell membranes rather than to any ion-transporting properties they may possess. The metal-ion selectivity of nonactin and valino-mycin has been put to use in the design of potassium-selective membrane electrodes. These contain the antibiotics as components of the membrane, and can be used to estimate K+ in the presence of Na+, for example in human serum. 

Grygolowicz-Pawlak E, Bakker E (2011) Thin layer coulometry irai statsing protocol with potassium-selective membrane electrodes. Electrochim Acta 56(28) 10359-10363... 

Potentiometric titration has been applied to the determination of potassium in seawater [532-534], Torbjoern and Jaguer [533-544] used a potassium selective valinomycin electrode and a computerised semiautomatic titrator. Samples were titrated with standard additions of aqueous potassium so that the potassium to sodium ion ratio increased on addition of the titrant, and the contribution from sodium ions to the membrane potential could be neglected. The initial concentration of potassium ions was then derived by the extrapolation procedure of Gran. 

Potassium ions are assayed with a selective membrane electrode employing a poly(vinyl-chloride)-membrane doped with a specific ionophore. The classical carrier is the antibiotic valinomycin. Active research is going on at many places to develop synthetic ionophores for both cations, sodium and potassium. 

The overall response of an ion-selective membrane electrode, such as the BME-44 ionophore -PVC-DOS system for potassium ions, described earlier is the result of several consecutive and parallel processes of which the following four main ones are most frequently considered/... 

Fig.6. Comparison of the response time curves of BME-44/DOS based potassium selective membranes recorded in concentrated KCl and KSCN solutions. Inset EMF-log aK calibration plot of the BME-44/DOS based electrode in KCl (o) and KSCN ( ) solutions. The response time curves were recorded in the concentration ranges marked with arrows. Activity step from 0.1 mol/1 KCl to 1.0 mol/1 KCl or to 0.1 mol/1 KSCN. KSCN flow rate 115 ml/min. 

For measurement of blood urea levels, typically in the range of 3 to 10 mM, variations in background levels of potassium (normally 2 to 6 mM) can influence the emf value, and therefore reliable measurements can only be made by measuring K+ levels separately with a K" "-selective membrane electrode (usually based on valinomycin see K+-1 in Fig. 4) and correcting the output voltage of the biosensor for these variations. 

Potentiometry is the measurement of the potential at an electrode or membrane electrode, so the detector response is in units of volts. The potentio-metric response tends to be slow, so potentiometry is used infrequently in analysis.47 One example is the use of a polymeric membrane impregnated with ionophores for the selective detection of potassium, sodium, ammonium, and calcium 48 In process chromatography, potentiometry may be used to monitor selected ions or pH as these values change over the course of the gradient. 

Electrodes based on solutions of cyclic polyethers in hydrocarbons show a selective response to alkali metal cations. The cyclic structure and physical dimensions of these compounds enable them to surround and replace the hydration shell of the cations and carry them into the membrane phase. Conduction occurs by diffusion of these charged complexes, which constitute a space charge within the membrane. Electrodes with a high selectivity for potassium over sodium (> 1000 1) have been produced. 

J. Pick, K. Toth, E. Pungor, M. Vasak, and W. Simon, A potassium-selective silicone-rubber membrane electrode based on neutral carrier, Anal Chim Acta 64, 477-480 (1973). 

Figure 4.20.A shows a more recent cell reported by Cobben et al. It consists of three Perspex blocks, of which two (A) are identical and the third (B) different. Part A is a Perspex block (1) furnished with two pairs of resilient hooks (3) for electrical contact. With the aid of a spring, the hooks press at the surface of the sensor contact pads (4), the back side of which rests on the Perspex siuface, so the sensor gate is positioned in the centre of the block, which is marked by an engraved cross as in the above-described wall-jet cell. Part B is a prismatic Perspex block (2) (85 x 24 x 10 mm ) into which a Z-shaped flow channel of 0.5 mm diameter is drilled. Each of the wedges of the Z reaches the outside of the block. The Z-shaped flow-cell thus built has a zero dead volume. As a result, the solution volume held between the two CHEMFETs is very small (3 pL). The cell is sealed by gently pushing block A to B with a lever. The inherent plasticity of the PVC membrane ensures water-tight closure of the cell. The closeness between the two electrodes enables differential measurements with no interference from the liquid junction potential. The differential signal provided by a potassium-selective and a sodium-selective CHEMFET exhibits a Nemstian behaviour and is selective towards potassium in the presence of a (fixed) excess concentration of sodium. The combined use of a highly lead-selective CHEMFET and a potassium-selective CHEMFET in this type of cell also provides excellent results.

Other potentiometric electrode systems are ion-selective electrodes such as fluoride, calcium, magnesium, sodium, potassium and chloride, selective gas electrodes based on membranes such as 02, C02, CO, NO, N02 and S02, and enzyme electrodes. These electrodes fall beyond the scope of this book and are not discussed further. 

In the case of a neutral non-ionic chelating agent we have neutral carrier-selective electrodes transport is achieved by selective complexa-tion of certain ions. The best-known electrode of this kind is the potassium-selective electrode, whose membrane consists of a valinomycin macrocycle immobilized in phenylether. The important criterion appears to be the size of the cavity in the centre of the macrocycle and interferences are from cations with similar hydrated ionic radius, such as Rb+ and Cs+. 

Conduction occurs by diffusion of these charged complexes, which constitute a space charge , within the membrane. Electrodes with a high selectivity for potassium over sodium (> 1000 1) have been produced. 

Most polymer membrane ISEs are prepared by dissolving an ionophore in a polyvinylchloride (PVC) membrane. A large variety of plasticisers are used to increase the dielectric constant of the PVC and improve its hydrophilicity. Some membranes have complexes of the ions to be sensed to increase membrane conductivity, such as potassium tetraphenylborate in K -selective membranes. There is an extensive literature on the arcane arts of polymer membranes for electrodes with dissolved ionophores and a good review of this is given by Professor Ronald Armstrong in Section 3.7 of Gabor Harsanyi s book. Polymer films in sensor applications [14]. 

P7. Pioda, L. A. R., Simon, W., Bosshard, H.-R., and Curtius, H. C., Determination of potassium ion concentration in serum using a highly selective liquid membrane electrode. Clin. Chim. Acta 29, 289-293 (1970). 

Other types of electrodes have been designed to measure the concentrations of ions other than H30. The simplest example of such an ion-selective electrode is a metal wire, which can be used to detect the concentration of the corresponding metal ion in solution. Silver and copper wires can be used reproducibly in this way to determine the concentrations of Ag and Cu, respectively. Still other electrodes have been developed to detect specific ions. For example, glasses of chemically modified composition are used to construct membrane electrodes to determine potassium and sodium ions or halogen ions. 

Calcium ISEs contain a calcium-selective membrane, which encloses an inner reference solution of calcium chloride often containing saturated silver chloride (AgCl) and physiological concentrations of sodium chloride and potassium chloride (KCl) and an internal reference electrode. The reference electrode, usually of Ag/AgCl, is immersed in this inner reference solution. 

Ion-selective electrodes with tip diameters in the range of 0.5 to 10 pm have been developed for ions such as potassium, calcium, and chloride, and these have been used to study the distribution of these ions in both the extra- and intra-cellular fluid. These electrodes are used in the potentiometric mode, and the specificity is established by using a selective membrane that is only permeable to the ion of interest. Voltammetric techniques have also been useful for in vivo measurements the most widely used is the oxygen electrode, which incorporates a polymer film that is only permeable to oxygen. 

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