Cation-selective electrodes development

Soon after the initial development of the heparin sensor, an electrode for the detection of the polycation protamine was proposed [38] based on a polymeric membrane doped with the cation exchanger tetrakis-(4-chlorophenyl)borate. Protamine is a polypeptide and usually administered as a heparin antidote. Protamine is a polycation with an average charge of +20 and is rich in arginine (Fig. 4.8). The response function of protamine-selective electrodes is similar to the heparin response function (Fig. 4.9b). 

The membrane of the glass electrode used for pH measurements is selectively permeable to hydrogen ions and from this basic concept a whole range of ion-selective electrodes have been developed. Varying the composition of the glass membrane can change the permeability of the glass and several cation-sensitive... 

Development of lithium selective electrodes (LiSE) and their application in clinical chemistry have been amply reviewed Several models of lithium ion specific electrodes are commercially available. The central problems in developing such sensing devices are their dynamic range, the accuracy and precision by which the signals are correlated to the concentration of the analyte and the selectivity towards that species, especially in relation to other alkali metal cations. Additional problems of practical importance are the times of response and recovery and the durability of the electrode in the intended service. 

The earliest ion sensors were the ion-selective electrodes or ISEs. These initially were used for the measurement of protons, i.e. as a pH-sensitive electrode. Later ISEs were developed for the detection of other cations and then anions. The history of the ISE has been reviewed elsewhere [12], however, a brief introduction will be given here. 

The lithium and thallium (I) salts of i satin-3-oxime (isatin oximates) were employed in the development of ion-selective electrodes for these cations. Transition metal complexes of isatin derivatives can also be employed as catalysts for the oxidative self-coupling of alkylphenol s639,640. 

Low-symmetry crown ethers, possessing (3m + )-crown- z scaffolds, show a less pronounced binding ability than symmetrical crown ethers, but display much higher selectivities for specific cations . For instance, polymer membrane thallium(l)-selective electrodes based on dibenzo-crown ether derivatives 67 have been developed <1996CCR171>, because of the environmental and biological implications associated with the poisoning of Tl+ ions. 

The potential of liquid-membrane electrodes develops across the interface between the solution containing the analyte and a liquid-ion exchanger that selectively bonds with the analyte ion. These electrodes have been developed for the direct potentiometric measurement of numerous polyvalent cations as well as certain anions. 

Ion selective electrodes provide a simple and accurate method for the determination of many ions in solution. These have been developed using the same electrochemical principles as the pH electrode, which is basically an ion selective electrode specific for hydrogen ion. Thus, concentrations of Na+, K+, Ca +, and Pb + as examples of cations, and F , Cl , Br , I", and CN as anions may all be measured using this method. With the appropriate measuring precautions and attention to possible interferences, most cations can be determined at concentrations as low as 10 to 10" M, and lead for example down to 10 M. 

Electrodes selective for H (i.e. pH meters) have been known for many years. Electrodes selective for other ions, however, are a more recent arrival, their entrance due in large part to the work of Simon and co workers. Because of the medical importance of such sensors, particularly for blood analysis, the alkali and alkaline earth cations have been given major attention in developing these electrodes, the first calixarene-based ion selective electrode being designed for Na". McKervey and Diamond and their coworkers have been especially active in this field and have devised ion selective electrodes for Na using compounds... 

Anion selective electrodes are much less well-developed than their cation counterparts, the first calixarene-based entry into this field being a calix[4]arene carrying two cobalticinium groups on the upper rim which shows some ability to recognize adipate anions (see ref. 1, p. 135)." ... 

An ion-selective electrode consists of a semipermeable membrane in contact with a reference solution on one side and the sample solution on the other. The membrane has to be selectively permeable to either a cation or an anion, but the penetration of the related counter ion must be restricted. Thus, charge sepeiration occurs at the interface leading to a potential difference (Donnan potential) which contains the analytically useful information. Within the membrane the diffusion of an ion is promoted by a concentration gradient, and when the mobilities of the cations and anions vary greatly, a diffusion potential is additionally developed by charge separation. The change in the membrane potential predominates, under well-deHned conditions (pH, ionic force, temperature), over changes in the overall cell potential due to concentration differences in the substance in question in the analyte. Hence, the cell potential is proportional to the potential drop over the ion-selective membrane. 

Some of these compounds were incorporated into ion-selective electrodes. An especially successful application is their use in the detection of divalent calcium cation. Several of the structures developed in the Simon laboratory are shown in Fig. 4. 

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