V. V. Egorov

H+-selective electrodes based on neutral carriers : specific features in behaviour and quantitative description of the electrode response

The influence has been studied of the membrane and solution composition on the response of H(+)-ISEs with plasticized polymer and liquid membranes based on the neutral carriers N,N-dioctylaniline and tridecylamine in association with trioctyloxybenzene sulphonic acid. It is shown that the extraction processes at the membrane/solution interface exert the main effect on the response limits by inducing essential changes in the activity of potential-determining ions in the membrane. At low pH, the amine extraction of acids followed by neutralization (free amines binding in ion-pairs) is the relevant process, while at high Ph it is the extraction of metal cations with amine salts of a lipophilic acid, with the consequent displacement of amine from the salts. Equations are suggested to represent the interphase potential of the H(+)-ISE membranes with allowance for these extraction processes. The experimental electrode responses of both liquid and polymer membranes are shown to be well described by the equations for the interphase potential, thus indicating its dominant contribution to the membrane potential.

Ion-selective electrodes for determination of organic ammonium ions: Ways for selectivity control

The influence of the ISE membrane composition on the selectivity for primary, secondary, tertiary, and quaternary alkylammonium cations, as well as for cations of physiologically active amines, has been investigated. Factors studied include the effect of plasticizer (2-nitrophenyl octyl ether, o-NPOE; dibutyl phthalate, DBP; dinonyl adipate, DNA; tris(2-ethylhexyl) phosphate, TEHP) and ion exchanger (potassium tetrakis(4-chlorophenyl)borate, K(TpClPB); potassium tris(nonyloxy)benzenesulfonate, K(TNOBS)), as well as that of the lipophilic cationic additive (tetradecylammonium nitrate, (TDA)NO(3)) and neutral carrier (dibenzo-18-crown-6) presence in membrane. It has been established that plasticizer nature affects K(i,j)(pot) values both when the target and/or foreign ions have non-ionic polar groups capable of specific interaction with plasticizer, and when the only difference consists in the substitution degree of their ionic groups. K(i,j)(pot) values for quaternary alkylammonium cations over primary-tertiary ones change in the following order: TEHP>DBP approximately DNA>o-NPOE. The highest K(i,j)(pot) value change is achieved for the primary-quaternary alkylammonium cation pair, amounting to 3 and 4.7 orders for membranes containing K(TNOBS) and K(TpClPB) as ion exchangers, respectively. In its turn, the ion exchanger influence on the selectivity depends on plasticizer nature, it being maximal for o-NPOE (about 2 orders) and practically non-existent for TEHP. On the whole, as compared to K(TpClPB)-based ISEs, those based on K(TNOBS) show higher selectivity for primary-tertiary alkylammonium cations over quaternary ones. Incorporation of (TDA)NO(3) into membrane causes further improvement of selectivity for primary-tertiary alkylammonium cations in the case of K(TNOBS) only. The maximal total effect of the ion exchanger and lipophilic ionic additive is observed for ISEs with DNA-plasticized membranes and is over 3 orders. The influence of crown ether on the selectivity also depends significantly upon ion exchanger and plasticizer nature. For ISEs with o-NPOE-plasticized membranes the K(i,j)(pot) value changes can be as great as 3 (ion exchanger K(TNOBS)) and even 4.5 (ion exchanger K(TpClPB)) orders. On the contrary, for ISEs with TEHP-plasticized membranes the crown ether effect on the selectivity is unessential. The results obtained are explained by peculiarities of organic ammonium cations solvating by plasticizer and association of cations with ion exchangers.

A new sulfate-selective electrode and its use in analysis

The improvement of the steric accessibility of exchange centers of higher quaternary ammonium salts (QAS) by replacing several long-chain alkyl substituents at the nitrogen atom with methyl groups enhanced the selectivity of ion-selective electrodes (ISEs) based on the specified ion exchangers for the SO42− ion up to six orders of magnitude in the presence of singly charged anions. This can be qualitatively explained by the specific features of ion pair formation between quaternary ammonium cations and singly and doubly charged anions. The effect of the steric accessibility of the QAS exchange center on the selectivity of ISEs is partially retained in the presence of a neutral anion carrier, hexyl p-trifluoroacetylbenzoate (HTFAB), which is used for enhancing the selectivity for the sulfate ion in the presence of singly charged anions. A sulfate-selective electrode with a reasonable selectivity for practical purposes was proposed. It is based on the HTFAB-higher QAS ion pair bearing three methyl substituents at the nitrogen atom. The ISE was used in the analysis of natural water.

pH-ISEs with an expanded measuring range based on calix[4]arenes: specific features of the behaviour and description of the electrode response.

This article contains a description of pH-ISEs with plasticised PVC membranes that contain calix[4]arene or p-tertbutylcalix[4]arene as an ionophore and tetradecylammonum nitrate taken in a subequivalent amount relative to calyxarene as an ionic additive. It has been discovered that real ISEs have an extraordinarily wide measuring range which is about 1.5-2 pH units wider then the measuring range of tridecylamine-based ISEs under the same conditions. Just as in the case of amine-type neutral carriers-based pH-ISEs, the upper detection limit depends on the concentration and lipophilicity of anions and the lower limit, on the concentration and lipophilicity of cations present in the sample solution. A simple quantitative description of the electrode response and the detection limits of such ISEs is suggested which is based on inclusion of extraction equilibria at the membrane/solution interface for two limiting cases: complete dissociation and strong association of ions in the membrane. Interface equilibria constants that occur in the acid and alkali media are estimated experimentally in model extraction systems. Characteristics of the ISEs calculated with use of the estimated constants agree satisfactorily with those measured experimentally. Probable reasons of the expansion of the measuring range of the ISEs considered in comparison with ISEs based on amine-type neutral carriers are discussed.

Metalloporphyrin-Based Anion-Selective Electrodes with Unusual Selectivity

The selectivity of ISEs based on metalloporphyrins studied significantly depends on both the nature of the central atom of the metal–porphyrin complex and the nature and concentration of lipophilic ionic additives. A strong dependence of the ISE potential on the pH of a test solution is a distinctive feature of all studied metalloporphyrin-based ISEs. In this context, the use of metalloporphyrins functioning as neutral carriers of anions is preferable, because membranes based on them can be optimized using lipophilic cationic additives that simultaneously suppress the pH response. Electrodes based on metalloporphyrins that exhibit their own ion-exchange properties demonstrate a strong pH response in a wide range of pH; therefore, it is feasible to use them only in strongly buffered solutions.

The effect of the ion exchanger site-counterion complex formation on the selectivities of ISEs.

Using the model of ideally associated solution, the effect of ion association of the ion exchanger sites with main and foreign counterions on the selectivity of ISEs based on liquid ion exchangers has been considered. Equations which describe the potentiometric selectivity coefficient as a function of ion association constants in the membrane phase and of standard free energies of transfer of the determined and foreign ions from water to the membrane are obtained for the following main cases: (a) the determined and foreign ions are single-charged; (b) the determined ion is double-charged and the foreign ion is single-charged. It is shown that in the case of single-charged main and foreign ions, the ratio of the ion association constants has a great effect on the potentiometric selectivity of membranes, only if the ion exchanger sites produce less strong associates with the determined counterion as compared with the foreign one. Otherwise, this effect is insignificant. The selectivity for double-charged ions should increase, other things being equal, as the first constant of association of these ions with the ion exchanger sites increases. The effect of producing ion triplets of the type I(2)R((+/-)) on the selectivity of ISEs is also considered. Experimental data are presented which illustrate the effect of the nature of the ion exchanger on the potentiometric selectivity. Some procedures employing the factor of ion association for increasing the potentiometric selectivity of liquid ion exchange membranes are considered.

Improved Separate Solution Method for Determination of Low Selectivity Coefficients

Simple, fast, and theoretically substantiated experimental method for determination of improved selectivity coefficients is proposed. The method is based on the well-known fact that low selectivity coefficients determined by the separate solution method (SSM) are time-dependent and, upon our finding, this dependence is a well-defined linear function of time raised to the certain negative power. In particular, the selectivity coefficients obtained for equally charged primary and foreign ions by SSM linearly depend on time to the minus one-fourth. It was found that extrapolation of experimental data using this function to the intersection with Y axes gives reliable values of rather low selectivity coefficients (down to n × 10(-7)), which strongly differ from those measured using SSM and correspond well with the values obtained using the modified separate solution method (MSSM) proposed by Bakker. At the same time, the new method is free of one very essential limitation inherent to MSSM, namely, it is applicable after the conditioning of electrodes in the primary ion solution and can be repeated many times.

Metal Complexes as Promising Ionophores for the Production of Anion-Selective Electrodes with Improved Selectivity

The possibility of producing liquid anion-selective electrodes with improved selectivity based on the capability of the metal atom to coordinate the analyzed anions is illustrated using numerous examples. The subjects under consideration were electrodes based on high-stable lipophilic metal complexes with polydentate organic ligands, electrodes based on lipophilic organometallic compounds capable of coordinating anionic ligands, and electrodes based on higher quarternary ammonium salts that respond to lipophilic anionic metal complexes, in particular, the electrodes with the ligand function. The theoretical principles underlying the above electrode operation are discussed. Methods for optimizing the composition of the membrane and of the studied solution are suggested with the aim to improve the selectivity of the electrode.

Ibuprofen-selective electrode on the basis of a neutral carrier, N-trifluoroacetylbenzoic acid heptyl ester

An ibuprofen-selective electrode on the basis of a composition of an anion exchanger, 2,3,4-tris(dodecyloxy)benzyltrimethylammonium with a neutral carrier, N-trifluoroacetylbenzoic acid heptyl ester, is proposed. It exhibits good selectivity, a wide pH-range, and a low limit of detection. A procedure for the determination of ibuprofen in tablets is developed.

Ion-selective liquid electrodes: Problems of description and experimental determination of selectivity

Current state of selectivity theory for liquid-membrane ion-selective electrodes was considered. Analytical expressions for the selectivity coefficients of electrodes based on liquid ion exchangers, neutral carriers, and charged carriers were obtained using phase boundary potential model. The reasons responsible for the dependence of experimentally determined selectivity coefficients on the determination conditions are discussed. The main ways for the electrode design optimization providing dramatic improvement of the selectivity are considered.