Anion-selective liquid-membrane electrodes

In order to improve the practical application of potentiometric sensors, much effort must be devoted to developing membrane materials whose selectivity deviates fi-om the Hofmeister series. Chloride- and nitrite-selective electrodes based on lipophylic vitamin B12 derivatives have been reported by Simon.xhese electrodes were the first anion-selective electrodes of which selectivity sequence did not obey the Hofmeister series. Since then, a number of anion-selective liquid-membrane electrodes have been developed using a variety of hosts metallocenes, diphosphonium dication salts, diquartemary ammonium dication salts, bisthiourea derivatives, metalloporphyrins, lipophylic macrocyclic polyamines, cytosine-dependent triamine and metallophtalocyamines. " " Among this large variety of compounds studied, the metalloporphyrins seem to be the most promising for the preparation of effective ion-selective potentiometric sensors. " - ... 

Male K, Luong J, Gibbs B and Konishi Y 1993 An improved FIA biosensor for the determination of aspartame Appl. Biochem. Biotechnol. 38 189-201 Martin G and Meyerhoff M 1986 Membrane-dialyzer injection loop for enhancing the selectivity of anion-responsive liquid-membrane electrode in flow systems Anal. Chim. Acta 186 71-80... 

G. B. Martin and M. E. Meyerhoff, Membrane-Dialyzer Injection Loop for Enhancing the Selectivity of Anion-Responsive Liquid-Membrane Electrodes in Flow Systems. Part I. A Sensing System for NOx and Nitrite. Anal. Chim. Acta, 186 (1986) 71. 

Guilbault and Montalvo were the first, in 1969, to detail a potentiometric enzyme electrode. They described a urea biosensor based on urease immobilized at an ammonium-selective liquid membrane electrode. Since then, over hundreds of different applications have appeared in the literature, due to the significant development of ion-selective electrodes (ISEs) observed during the last 30 years. The electrodes used to assemble a potentiometric biosensor include glass electrodes for the measurement of pH or monovalent ions, ISEs sensitive to anions or cations, gas electrodes such as the CO2 or the NH3 probes, and metal electrodes able to detect redox species some of these electrodes useful in the construction of potentiometric enzyme electrodes are listed in Table 1. 

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. 

Table 21-2 lists some liquid-membrane electrodes available from commercial sources. The anion-sensitive electrodes shown make use of a solution containing an anion-exchange resin in an organic solvent. Liquid-membrane electrodes in which the exchange liquid is held in a polyvinyl chloride gel have been developed for Ca-, K", NOj, and BF4. These have the appearance of crystalline electrodes, which are considered in the following section. A homemade liquid-membrane ion-selective electrode is described in Feature 21-1. 

Liquid membrane electrodes were known for some years, but it is likely that the introduction of the calcium ion-selective hquid membrane electrode by Ross [9] in 1967 stimulated the intensive developmental research that resulted in the present commercial availability of hquid membrane electrodes for a significant number of cations and anions. In general the cations determinable are double-charge, while the anions are frequently single-charge. 

Liquid membrane electrodes (1) classical ion-exchangers (with mobile positively and negatively charged sites as hydrophobic cations or hydrophobic anions), for example K+, Cl selective electrodes, (2) liquid ion-exchanger based electrodes (with positively or negatively charged carriers, ionophores), for example Ca2+, NOJ selective electrodes, (3) neutral ionophore based liquid membrane electrodes (with electrically neutral carriers, ionophores), for example Na+, K+, NI I), Ca2+, Cl selective electrodes. 

When the large tricaprylammonium cation (Aliquat 336 S, General Mill, USA) dissolved in 1-decanol (0.01 to 0.1 M) is used, the resulting liquid membrane electrode responds to many anions. In order to obtain stable potentials, the organic phase must be shaken ahead of time with an approximately 1 M solution of the sodium salt of the anion to be measured [150]. In this way, electrodes have been reported in the literature which are selective for HPO " [151], CrO ", ZnCl , PdCll" [152], MnO, Cr20 -, lOj [143],salicylate-[153],HgCl -, Hgir, FeCU [154] and COi" [147] anions. 

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. 

A supported liquid and a PVC-based membrane selective for dodecyl sulfate (DS ) ion was described by Arvand-Barmchia et al. The electroactive element was a membrane containing a dissolved ion associatirm complex of DS with cetylpyridinium (CP" cation dissolved in acetophenone. Nemstian response towards the DS anion was achieved over the craicentration range from 8.3 X to 1.0 X 10 M at 25 °C. The proposed electrode also showed good selectivity and precision (RSD about 2.0 %), and was usable within the pH range of 4.0-6.8. The liquid membrane electrode could find application in the direct determination of DS by the standard addition method at pH 5.0, and exhibited useful analytical characteristics for the determination of sodium dodecyl sulfate (SDS) in detergents and real samples. 

Before the appearance of analytically useful ISEs with liquid membranes, Sollner and Shean [200] obtained a liquid ion-exchange membrane with marked selectivity for anions. The first ISE with a liquid membrane was the calcium electrode described by Ross [179] with Ca " -dialkylphenylphosphate in dioctylphenylphosphonate. 

The potentiometric determination of bromide anions with ion-selective electrodes is possible with commercial electrodes that are commonly based on solid Ag2S—AgBr ion conductor membranes [148-150]. Recently, a novel liquid film sensor has been proposed by Ganjali etal. [151] Determination of bromide was reliable without significant interference from common ions such as chloride and iodide, and was reported down to a micromolar level. The electroactive species in the liquid membrane... 

Ion-Selective Electrodes based on Bis-Thiourea Receptors. Bis-thiourea derivatives 14, 15, and 17, which have a good membrane solubility, sufficient lipophilicity to prevent leaching into the aqueous sample solution, and a low tendency for self-aggregation in nonpolar solvents, were incorporated into PVC matrix liquid membranes for ISEs. While membrane electrodes based on the dibutyl derivative 14 gave a phosphate response almost identical to that of a conventional anion-exchanger electrode, a membrane electrode based on the phenyl-substituted bis-thiourea 15 exhibited a slightly improved phosphate response, which seems to be the result of improved complexation of phosphate in the sensor membrane. 

The sign will be + for cation-selective electrodes and - for those that are anion-selective. To be predominantly responsive to the concentration Ci, the factor Kij must be small. Where liquid membrane ion-selective electrodes responsive primarily to double-charge ions are concerned, and for interference by single-charge ions, equation (1) modifies to ... 

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