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.     

Ion-selective electrodes for gold and silver determination

Some new ion-selective electrodes for silver and gold are described. They are based on the ion-associate species formed by the cyanide, chloride or thiourea complexes of the metals, with hydrophobic anions or cations, as appropriate. The electrodes have been applied to the determination of gold and silver in various technological process solutions in industry.     

Potentiometric selectivity coefficients: Problems of experimental determination

The main problems arising in the determination of selectivity coefficients of ion-selective electrodes using the methods recommended by IUPAC are considered. A method for the determination of the limiting (thermodynamically justified) selectivity coefficients based on the analysis of the time dependence of the selectivity coefficients found experimentally using the separate solutions method is justified theoretically and experimentally. It has been demonstrated that the proposed method ensures the reliable determination of low values of selectivity coefficients (as low as n × 10^?6). The criteria of the applicability of the proposed method are formulated and a particular algorithm of its implementation is proposed.     

Ion-selective characteristics of heavy metal myristate liquid membrane electrodes

Liquid membrane electrodes of myristate soaps of heavy metals, namely copper, cadmium, zinc, nickel, cobalt, strontium and barium in a benzene-n-butanol mixture were prepared. Each electrode showed nernstian response in common metal ion test solution. The range of concentration of the test solution for the Nernstian behaviour lies between 10^?4 and 10^?1M. With a dissimilar metal ion in test solution the metal soap liquid membrane electrode indicates deviations from Nernst's equation depending on the nature of the metal ion, suggesting different selectivities for different ions. The selectivity ratios are therefore calculated.     

Anion selectivity studies on liquid membrane electrodes

Selectivity coefficients of liquid-membrane electrodes for common inorganic anions were measured in electrodes containing tris(l,10-phenanthroline)iron(II), tris(4,7-diphenyl-1,10-phenanthroline)iron(II) or tetraheptylammonium ion in nitrobenzene, and tris(4,7-diphenyl-1,10-phenanthroline)iron(II) ion in nitrobenzene, chloroform or n-amyl alcohol as the liquid membrane. With the exception of the amyl alcohol electrode, selectivity coefficients were relatively independent of membrane composition and followed a common sequence of decreasing selectivity: PF₆^?> ClO₄^?>SCN^?~I^?~BF₄^?>NO₃^?>Br^?>Cl^?. This sequence parallels the order of increasing anion hydration energy, suggesting that aqueous phase solvation energies play a predominant role in determining electrode selectivity for these ions. Time-dependent behavior of liquid-membrane electrodes on transfer between solutions containing different ions also is described. Instantaneous e.m.f. readings were used to determine selectivity coefficients.     

Ion-selective electrodes – istory and conclusions

 The working mechanism of ion-selective electrodes has been investigated. The contradictions of theories deduced from so-called analogous phenomena are discussed in detail. Based on experiments made by the author’s research group the conclusion was drawn that only the surface (the active groups located there) takes part in the potential-determining reaction and the bulk resistance of the membrane plays a role only in the selection of the instrument used for potential measurement. Received: 15 May 1995/Revised: 12 August 1995/Accepted: 11 August 1995     

Ion-selective electrodes based on metalloporphyrins for gibberellic acid determination in agricultural products

This work describes the construction, evaluation and analytical application of electrodes selective to the gibberellate anion for the determination of gibberellic acid in agricultural products. Several types of PVC membrane electrodes without internal reference solution were prepared using the manganese(III) complex of meso -tetraphenylporphyrin (TPP) as ionophore and dibutyl phthalate (DBP), as plasticizer. The incorporation of lipophilic chemical species as additives, was also carried out aiming the evaluation of the response characteristics of the electrodes. To accomplish the analysis of commercial agricultural products a selective membrane composed of 28.0% (w/w) of PVC, 66.0% (w/w) of plasticizer and 6% (w/w) of ionophore was used, with no additive. This potentiometric unit presented a linear response between 10^-4 and 10^-1 mol L^-1 in gibberellate, a slope of about ⫙ mV dec^-1 and a reproducibility of about ǃ mV day^-1. The potentiometric analysis of gibberellic acid in commercial products was carried out by direct potentiometry and the results obtained were compared with those provided by HPLC.     

Ion-selective electrodes – istory and conclusions

 The working mechanism of ion-selective electrodes has been investigated. The contradictions of theories deduced from so-called analogous phenomena are discussed in detail. Based on experiments made by the author’s research group the conclusion was drawn that only the surface (the active groups located there) takes part in the potential-determining reaction and the bulk resistance of the membrane plays a role only in the selection of the instrument used for potential measurement. Received: 15 May 1995/Revised: 12 August 1995/Accepted: 11 August 1995     

Limits of detection and selectivity coefficients of liquid membrane electrodes

An equation has been derived theoretically describing the emf of a liquid membrane electrode: E = E(0) - (RT/F) ln[(C + radicalC (2) + A (x))/2]. Experimental data are in good agreement with this equation. The parameter A(x) is related to the limit of detection. When the ion-exchanger dissociates completely in the membrane phase, A(x) is given by 4sigma(2)/b(x). Here sigma represents the concentration of the ion-exchanger in the liquid membrane and b(x) is a quantity related to the ion-selectivity.     

Ion-selective electrodes and enzyme electrodes in environmental and clinical studies.

Electrodes have been developed for the assay of glucose, urea, amino acids, uric acid, phosphate, nitrate and perchlorate. The electrodes for the organic compounds are enzyme electrodes which are prepared by chemically immobilizing an enzyme over the outside of a conventional ion-selective electrode. These electrodes will be discussed in depth. The progress and the development of the electrodes that show sensitivity and selectivity for phosphate, nitrate and perchlorate will be outlined. The basis of these sensors is a complex of a transition metal of either an analog of thiourea or an organic chelator, such as 1,10-phenanthraline. Such electrodes respond linearly to phosphate, nitrate or perchlorate, and show selectivity over sulphate, halides and acetate. The linear range of all these electrodes is approx. 10(-1)-10(-5) M with a near Nernstian slope and a reproducibility of 1%. The electrodes are stable and can be used continuously.     

Ion-selective electrodes for determination of phenylalanine based on uranyl complexes of phosphoryl-containing podands

The possibility of phenylalanine determination using membrane ion-selective electrodes based on uranyl complexes with phosphoryl-containing podands was shown. The variation of the procedure of membrane preparation (either entrapping the preliminarily synthesized uranyl-podand complex in the membrane phase or conditioning of the podand-containing membrane in the uranyl solution) was found to have practically no effect on the electrode properties. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1729–1732, October, 2000.     

Ion-selective electrodes for the determination of ionic liquids in water using direct potentiometry

Ion-selective electrodes (ISEs) with membranes based on ion exchangers plasticized with o-nitrophenyl octyl ether (NPOE), diethyl sebacate (DES), or dibutyl phthalate (DBP) are used for the determination of ionic liquids (ILs) in water. The membrane composition is optimized with respect to the ion-exchanger concentration and the plasticizer.     

Interpretation of the selectivity and detection limit of liquid ion-exchanger electrodes

An equation has been derived which describes the e.m.f. of a liquid ion-exchanger membrane electrode in conditions of low concentration levels of the primary and interfering ions. The equation is based on the assumption that if the external solution contains no excess of ions which may exchange with the organic phase, then the concentration of the exchanger at the interface decreases, and this is responsible for formation of a diffusion layer inside the membrane. Therefore the potential response depends on the initial concentration of the ion-exchanger in the membrane phase, on the thicknesses of the diffusion layer on both sides of the interface, and on the diffusion coefficients of the species in both phases. This equation explains the non-Nernstian behaviour of the electrode in the presence of interferents, as well as the variation of the conditional selectivity coefficients. The parameters mentioned also influence the detection limit of an electrode. The electrode behaviour has been tested in unstirred solutions and in solutions stirred at different rates. Through its influence on the diffusion layer thickness, the stirring also influences the electrode potential and the characteristics of the electrode.     

Ion-selective electrodes in organic analysis-determination of xanthate

A PVC membrane xanthate-selective electrode has been developed for the direct determination of xanthates of primary and secondary alcohols. The xanthate electrode is highly selective and exhibits a Nernstian response in the range 5.0 x 10(-2)-7.1 x 10(-5)M xanthate with a slope of 58.6 mV per concentration decade. The electrode has a wide working pH range (5.0-11.5), a fast response time (less than 30 sec) and is stable for at least two months.     

Ion-selective chalcogenide electrodes for a number of cations

Ion-selective chalcogenide disc electrodes have been developed which are responsive to cations such as silver, lead, chromium(III), nickel, cobalt(II), cadmium, zinc, copper(II) and manganese(II) ions. Each was prepared by using the corresponding metal chalcogenide with silver sulphide. An electrode was assembled with both a compacted and a sintered disc. The sintered electrodes were more sensitive and stable than the compacted ones. Response to silver ion was 59.5 mV pAg , to lead, nickel, cadmium, zinc and copper(II) 29.5 mV pM and to chromium(III) 20 mV pM . Cobalt(II) and manganese(II) electrodes had a non-Nernstian response of 25 mV pM . Both selenides and tellurides can be used for potentiometric determination, but the manganese(II) electrode serves as an analytical tool only when the disc consists of manganese(II) telluride and silver sulphide.     

Ion-selective membrane electrodes for the determination of the muscle relaxants pancuronium,tubocurarine, gallamine and succinylcholine

Ion-selective PVC membrane electrodes for the peripherial muscle relaxants pancuronium (PAN), tubocurarine (TUB), gallamine (GAL) and succinylcholine (SUC) are presented. The preparation of the electrode membranes contaimng ion pairs of the muscle relaxants with two different anionic counter ions, dipicrylaminate (DPA) and tetraphenylborate (TPB), are reported. The detection limits for all electrodes were ca. 10^?6 mol 1^?1 at physiological pH values and the measured slopes were dose to the values theoretically expected. Changes in the detection limits and the slopes of the electrodes in the pH range 2.5–11.0 were found to be due to reversible changes in the protonation state of the amine groups of the muscle relaxant (TUB) or the counter ion (DPA). The PAN-TPB and PAN-DPA ion-pair complexes were observed to have a bettet stability than those formed with TUB, GAL and SUC. The observed selectivity coefficients of the PAN-TPB electrodes were 10^?0.3 towards TUB and 10^?1.8 towards GAL.     

Ion-selective electrodes for potentiometric determination of ranitidine hydrochloride, applying batch and flow injection analysis techniques.

New ranitidine hydrochloride (RaCl)-selective electrodes of the conventional polymer membrane type are described. They are based on incorporation of ranitidine-tetraphenylborate (Ra-TPB) ion-pair or ranitidine-phosphotungstate (RaPT) ion-associate in a poly(vinyl chloride) (PVC) membrane plasticized with dioctylphthalate (DOP) or dibutylphthalate (DBP). The electrodes are fully characterized in terms of the membrane composition, solution temperature, and pH. The sensors showed fast and stable responses. Nernstian response was found over the concentration range of 2.0 x 10(-5) M to 1.0 x 10(-2) M and 1.0 x 10(-5) M to 1.0 x 10(-2) M in the case of Ra-TPB electrode and over the range of 1.03 x 10(-5) M to 1.00 x 10(-2) M and 1.0 x 10(-5) M to 1.0 x 10(-2) M in the case of Ra-PT electrode for batch and FIA systems, respectively. The electrodes exhibit good selectivity for RaCl with respect to a large number of common ions, sugars, amino acids, and components other than ranitidine hydrochloride of the investigated mixed drugs. The electrodes have been applied to the potentiometric determination of RaCl in pure solutions and in pharmaceutical preparations under batch and flow injection conditions with a lower detection limit of 1.26 x 10(-5) M and 5.62 x 10(-6) M at 25 +/- 1 degrees C. An average recovery of 100.91% and 100.42% with a relative standard deviation of 0.72% and 0.53% has been achieved.