Exchange ising

The potentiometric response of receptor 15 for H2PC>4 is not sufficient for a phosphate-selective electrode, but the strong response and high selectivity of this electrode for the dianion SO " is striking.62 The response down to 10 6 M is a very big improvement as compared to the response of the anion exchanger ISE, for which the linear range of the SO2- response extends only down to 10 3 M. The linear range of response of the former ISE from 1.0 x 10"6 to 1.0 x 10 2 M with a slope of -28.1 1.8 mV decade"1 shows that the present electrode responds to SO and not to HSO. The electrode based on 15 is the first neutral host-based electrode with an appreciable selectivity for sulfate. 

The selectivities of Cs+ ISEs based on dibenzo-18-crown-6 derivatives <1986FZA241> or 2,3-benzoquinone 15-crown-5 ethers <1996AN127> are of limited interest, because they are not dissimilar to those found with iono-phore-free ion-exchanger ISEs < 1977ANA399>. Nowadays, the best Cs+ selectivities are obtained by using calixarene-derived crown ethers . 

It was found that utilization of QAS with increased steric accessibility of exchange center, containing nitrogen atom with three methyl substituents and one super lipophilic substituent, - tris(2,3,4-dodecyloxy)benzyl, - leads to dramatic, up to 7 orders, increase of ISE s selectivity towards sulfate in the presence of single-charged anions. Incorporation of hexyl 4-trifluoro-acetylbenzoate into membrane leads to additional substantial increase - up to 4 orders - in sulfate selectivity. 

The sufficient selectivity to a principal component is the most important condition determining the possibility of ion-selective electrodes (ISEs) practical appliances. In this work, the relationship between the potentiometric selectivity of alkylammonium-selective electrodes and factors such as the nature of plasticizer, ion-exchanger and substitution degree of cationic nitrogen atoms of the principal and foreign ions, is discussed. 

It has been found that the influence of the membrane composition on the ISE selectivity is mainly governed by substitution degree of the nitrogen atom, but virtually does not depend upon the length of hydrocarbon substituents. The key factor in the selectivity of DBP-plasticized membranes with both kinds of ion-exchangers and of the o-NPOE-TNOBS-membrane is the ionic... 

The results obtained indicate that the ion-exchanger nature, generally not taken into account when developing ISEs for alkylammonium cations, actually influences strongly the selectivity of such ISEs and should be paid attention to when choosing optimal membrane composition. These data will be useful for finding ways to control the ISEs selectivity by rational choice of the membrane composition. 

Despite the fact that a great lot of ion-selective electrodes (ISEs) with liquid and film polymeric membranes for the determination of physiologically active amines (PhAA) has been described, the factors responsible for their selectivity have not yet been studied sufficiently. In this work, the influence of plasticizer and ion-exchanger nature on the selectivity of ISEs reversible to PhAA cations of various stmctures has been discussed. 

S-2.2.2 Neutral Carrier Electrodes hi addition to charged liquid ion exchangers, liquid-membrane electrodes often rely on the use of complex-forming neutral carriers. Much effort has been devoted to the isolation or synthesis of compounds containing cavities of molecular dimensions. Such use of chemical recognition principles has made an enormous impact upon widespread acceptance of ISEs. The resulting neutral carriers can be natural macrocyclic molecules or synthetic crown... 

In the above derivation we may assume that aA-(S) = aA- and aB (S) = aB, because by analogy with the build-up of an electrode potential (see pp. 26-27) the build-up of the ion-exchange potential will not significantly alter the original concentrations of A- and B in the solution under test. Hence in eqn. 2.80 the ratio aB-(n)/aA- n), which reflects the exchange competition of B versus A, still depicts the interference ratio of B in more straightforward manner than does the so-called selectivity constant (k), usually mentioned by ISE suppliers. 

For the halide ISEs on the basis of silver sulphide we may assume, e.g., in the case of chloride, the following exchange reaction ... 

For metal ISEs based on silver sulphide we can assume for other divalent metals analogously the exchange reaction... 

Solvent polymeric membranes conventionally consist of ionophore, ion exchanger, plasticizer, and polymer. The majority of modem polymeric ISEs are based on neutral carriers, making the ionophore the most important membrane component. Substantial research efforts have focused on the development of highly selective ionophores for a variety of analytes [3], Some of the most successful ionophores relevant to biomedical applications are depicted in Fig. 4.1. 

The liquid-membrane electrode is another important type of ion-selective electrode. The internal filling solution contains a source of the ion under investigation, i.e. one for which the ion exchanger is specific, while also containing a halide ion to allow the reference electrode to function. The physico-chemical behaviour of the ISE is very similar to that of the fluoride electrode, except that ise and the selectivity are dictated by the porosity of a membrane rather than by movement through a solid-state crystal. 

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