Anion-selective electrode

Many anion-selective ISEs are based on classic exchangers, complexes of 2-phenanthroline, and quaternary ammonium, phosphonium and arsonium salts. Not all of these possess the required selectivity, e.g. the electrodes based on dialkyltin(IV) compounds for HPO4 and H2PO4.  

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FIGURE 5-14 Structures of some chemical species useful for designing anion-selective electrodes (a) Mn(III) porphyrin (b) vitamin Bi2 derivative (c) tri-n-octyltin chloride (d) lipophilic polyamine macrocyclic compound. 

Anion selective electrodes, 156, 158 Anodic shipping voltammetry, 76 Antibody, 183 Antimony, 85... 

A. Malon, A. Radu, W. Qin, Y. Qin, A. Ceresa, M. Maj-Zurawska, E. Bakker, and E. Pretsch, Improving the detection limit of anion-selective electrodes an iodide-selective membrane with a nanomolar detection limit. Anal. Chem. 75, 3865-3871 (2003). 

In a related study (78CC934,79HCA2763) Lehn and coworkers incorporated guanidinium groups into macrocycles (e.g. 29). The resulting polyguanidium salts form stable complexes in water with phosphate and carboxylate anions. Unlike polyammonium anion receptors, these species remain protonated over a wide pH range and hold considerable potential in the development of anion-selective 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. 

Table 5), and several are now being used, or are potentially useful, for measuring key ocean elements. The most common use of direct potentiometry (as compared with potentiometric titrations) is for measurement of pH (Culberson, 1981). Most other cation electrodes are subject to some degree of interference from other major ions. Electrodes for sodium, potassium, calcium, and magnesium have been used successfully. Copper, cadmium, and lead electrodes in seawater have been tested, with variable success. Anion-selective electrodes for chloride, bromide, fluoride, sulfate, sulfide, and silver ions have also been tested but have not yet found wide application. 

Studies have been made towards connecting low-selectivity anion-selective electrodes to chromatographic columns, using the column to separate the ions and the ISE to detect them. For instance [30], a CWE has been reported for the detection of species such as carboxylic acids, sugar phosphates and nucleotides from mixtures which had been passed through an anion-exchange column. 

Development of Polymer Membrane Anion-Selective Electrodes Based on Molecular Recognition Principles... 

Several of the polymer membrane anion-selective electrodes described in the literature use quaternary ammonium salts as ion carriers (ionophores) (7). These electrodes respond according to the Hofmeister series (CIO4 > SCN > I > NO3 > Br - N3 > NC>2 > Cl > HCO3 acetate) (2, 5), which is the order of relative lipophilicity of the anions. Therefore, in strict terms, electrodes that respond according to this series could be considered "nonselective". 

In this paper, we report the development of ISEs that have been designed by using molecular recognition principles. Specific examples include the development of polymer membrane anion-selective electrodes based on hydrophobic vitamin B12 derivatives and a cobalt porphyrin. The selectivity patterns observed with these electrodes can be related to differences in the structure of the various ionophores, and to properties of the polymer film. 

In summary, it has been demonstrated that ISEs can be designed by employing molecular recognition principles. In particular, the feasibility of using hydrophobic vitamin B12 derivatives and electropolymerized porphyrin films in the development of polymer membrane anion-selective electrodes has been demonstrated. The studies indicated that the changes in the selectivity of these ISEs can be explained by the difference in structure of the ionophores. In addition, it was shown that by electropolymerization of a cobalt porphyrin, anion-selective electrodes can be prepared that have extended lifetimes compared with PVC-based ISEs, which use a similar compound as the ionophore. 

Porphyrins are often employed in sensors on account of their ability to act as cation hosts and, with a suitable metal ion coordinated, as redox catalysts. Electropolymerised poly(metalloporphyrin)s have been used as potentiometric anion-selective electrodes [131] and as amperometric electrocatalytic sensors for many species including phenols [132], nitrous oxide [133] and oxygen [134]. Panasyuk et al. [135] have electropolymerised [nickel-(protoporphyrin IX)dimethylester] (Fig. 18.10) on glassy carbon in the presence of nitrobenzene in an attempt to prepare a nitrobenzene-selective amperometric sensor. Following extraction of the nitrobenzene the electrode was exposed to different species and cyclic voltammetric measurements made. A response was observed at the reduction potential of nitrobenzene (the polyporphyrin film acts only to accumulate the analyte and not in a catalytic fashion). Selectivity for nitrobenzene compared with w-nitroaniline and o-nitroto-luene was enhanced compared with an untreated electrode, while a glassy carbon-... 

Lewis acidic hosts (Section V.A) illustrated important theoretical concepts such as the chelate effect and binding cooperativity, which have now been shown to exist for anion as well as cation binding. This work has also resulted in the crystallographic determination of eye-catching solid state receptor-anion complexes, while heteroelement NMR has allowed an accurate means of probing the solution phase structure of these complexes. Already, multinuclear tin systems are being built into functioning anion selective electrodes. 

Complexes of porphyrins with metals as perspective ionophores for anion-selective electrodes with improved selectivity 02MI63. 

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]. 

Kolthoff and Sanders (6) almost made the first anion-selective electrode with their study of silver halide disks. Close to Nemstian response was... 

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