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.     

Potentiometric selectivity coefficients of liquid ion-exchange membranes for univalent and divalent ions

Digital simulation has been used to calculate potential—activity responses and concentration-dependent selectivity coefficients for membranes bathed in mixtures of univalent and divalent ions. For an ideal, dissociated, univalent-site liquid ion-exchange membrane, more accurate computations, based on Nernst—Planck equations, were possible than have been reported previously. Simulation results were used with four suggested equations for the membrane potential (including the IUPAC recommendation) to determine which equation gives best fit and most nearly constant selectivity coefficients. When concentration profiles found by digital simulation are employed, improved response equations are given which closely describe the dependence of surface concentrations on the mobility ratio for the bi-ionic case. From these equations a closed form solution to the diffusion equation yields an explicit expression relating the bi-ionic selectivity coefficient to membrane loading, single ion partition coefficients, and ion mobilities.     

Potentiometric determination of cephalothin

The composition of a pseudo-liquid potential-determining phase for the cephalothin-selective electrode has been determined and the following basic electrode parameters were examined: measurement range, slope, limit of detection, selectivity and lifetime. This paper discusses the effect of side-chain substituents on the electrode properties, the electrode having been used for cephalothin determination in the range 43-436.5 mg l-1 (standard deviation, 0.64-4.5 mg l-1).     

Potentiometric determination of potassium ferricyanide

A rapid and accurate electrometric method for the determination of potassium ferricyanide solutions has been investigated. It consists in titrating ferricyanide solutions potentiometrically against standard AgNO₃, using a silver indicator electrode in conjunction with a saturated calomel electrode connected by a KNO₃ bridge. Either of the reagents may be used as the titrant. A marked change in e.m.f is observed at thc end-point corresponding to the formation and precipitation of Ag³[Fe(CN)₆]. The curves have a regular form, a pronounced maximum in dE/dV occurs at the end-point and the results are very accurate and reproducible. The reaction, though simple, affords a quantitative method for the determination of ferricyanide or silver. The effect of neutral salts and ethanol on the accuracy of the end-point has been studied.     

Potentiometric determination of 8-chlorotheophylline

Two methods are described for the potentiometric determination of 8-chlorotheophylline in aqueous solution. Both methods are based on the reaction of 8-chlorotheophylline with silver ions. In the first, the nitric acid produced in the reaction is titrated with sodium hydroxide. In the second, the 8-chlorotheophylline is titrated directly with silver nitrate in sodium carbonate medium. Both methods give reasonably accurate and reproducible results, and are suitable for the assay of 8-chlorotheophylline in pharmaceutical preparations.     

Potentiometric determination of aminal stability constants

Potentiometric titration was used to determine the logarithms of the stepwise equilibrium constants for the species formed between morpholine and formaldehyde in aqueous solution, ionic strength 0.5 and 2.5M (KCl) at 25 degrees C. The instrumental and computational techniques developed for metal-ligand stability constant determination were applied. Formaldehyde is equivalent to the metal-ion and is represented by M while neutral morpholine is equivalent to the ligand and is represented by L. The stability constants of the following equilibria were determined by non-linear regression (figures in parentheses are at ionic strength 2.5 M KCl): M + L left arrow over right arrow ML (hemi-aminal) logK(1) = 2.90 +/- 0.02 (2.980 +/- 0.004); ML + L left arrow over right arrow ML(2) (bis-aminal); log K(2) = 1.3 +/- 0.2 (1.41 +/- 0.07); MLH left arrow over right arrow ML + H(+) (protonated hemi-aminal) pK(a) = 5.87 +/- 0.01 (6.411 +/- 0.005); ML(2)H left arrow over right arrow ML(2) + H(+) (protonated bis-aminal) pK(a) = (7.6 +/- 0.2). the pK(a) of the protonated bis-aminal could only be determined at the higher ionic strength. The results are in good agreement with reported values determined using the classic formol titration. The automated titration system acquired the full time course of the pH change upon each titrant addition allowing a kinetic analysis to be performed as well as an equilibrium analysis. The forward and reverse rate constants for M + L left arrow over right arrow ML were 0.77M(-1) sec(-1) and 8.1 x 10(-4) sec(-1). respectively.     

Potentiometric and thermometric determination of mercaptopyrimidines

Summary Potentiometric and Thermometric Determination of Mercaptopyrimidines Potentiometric and thermometric methods for the determination of several 2- and 6-mercaptopyrimidines are described. Mercapto groups can be titrated as acids in water or in dimethylformamide media. The use of acetic acid as solvent allows titration of the basic pyrimidine nucleus. Thermometric titrimetry is also a valuable procedure for the titration of mercapto groups, either directly with sodium hydroxide or by back-titration with hydrochloric acid after addition of excess of alkali. About 10–25 mg of sample can be determined with an error better than 1%.     

Potentiometric determination of silver with dithiooxamide

Potentiometric titration with dithiooxamide solution can be used to determine silver in the 0.0100–50.0 p.p.m range with an average accuracy of ca. 0.5% and a relative standard deviation ranging from 1.26% to 0.03% The metals commonly associated with silver can be masked with fluoride and EDTA The advantages of the method over other potentiometric and common spectrophotometric methods are outlined.     

Potentiometric determination of dialysate urea nitrogen

An enzymatically modified ammonium ion-selective electrode has been applied for the determination of urea in spent dialysate. The biosensor has been used in a simple flow-injection analysis (FIA) system. The system enables one to perform over 25 dialysate urea nitrogen (DUN) determinations per hour. The interferences from other components of posthemodialysis fluid were eliminated by simultaneous measurements with non-modified enzymatically ion-selective electrode. It is possible to use both the sensors in a simplified differential potentiometric system. The results of DUN determination using the biosensor/FIA system and a conventional method of urea determination were comparable. The presented analytical system can potentially find wider biomedical application in the monitoring of hemodialysis progress.     

Potentiometric determination of autoprotolysis constants of solvents

A new method is proposed for determination of autoprotolysis constants by titration of a weak acid with strong base or strong acid with weak base, and applied to water and ethylene glycol (EG). The constants found were pK(w) = 13.90-13.97, and pK(EG) = 15.4 +/- 0.2.     

Computational prediction and experimental selectivity coefficients for hydroxyzine and cetirizine molecularly imprinted polymer based potentiometric sensors

In spite of the increasing usages number of molecularly imprinted polymers (MIPs) in many scientific applications, the theoretical aspects of participating intra molecular forces are not fully understood. This work investigates effects of the electrostatic force, the Mulliken charge and the role of cavity's backbone atoms on the selectivity of MIPs. Moreover, charge distribution, which is a computational parameter, was proposed for the prediction of the selectivity coefficients of MIP-based sensors. In the computational approaches and experimental study, methacrylic acid (MAA) was chosen as the functional monomer and ethylene glycol dimethacrylate (EGDMA) as the cross linker for hydroxyzine and cetirizine imprinted polymers. Ab initio, DFT B3LYP method was carried out on molecular optimization. With regard to results obtained from molecules optimization and hydrogen bonding properties, possible configurations of 1:n (n ≤ 5) template/monomer complexes were designed and optimized. The binding energy for each complex in gas phase was calculated. Depending on the most stable configuration, hydroxyzine and cetirizine imprinted polymer models were designed. The calculations including the porogen were also investigated. The theoretical charge distributions for the template and some potential interfering molecules were calculated. The results showed a correlation between the selectivity coefficients and the theoretical charge distributions. The results surprisingly show that charge distribution based model was able to predict the selectivity coefficients of MIP based potentiometric sensors.     

Variability of selectivity coefficients of solid-state ion-selective electrodes

The generalized model for the selectivity mechanism of solid-state ion-selective electrodes has been experimentally verified. The experimental parameters investigated were the concentration of interfering ion, temperature and stirring. Among the systems studied were electrodes sensitive to chloride (bromide, iodide), bromide (chloride, iodide), iodide (chloride, bromide), silver (copper, lead), copper (silver, lead) and lead (silver, copper), the species given in brackets being considered as the interferents. The model has been confirmed except for cases where the concentration of ions formed at the electrode surface by metathesis is too small to be the factor that dictates the electrode potential.     

Automated determination of detection limits and selectivity coefficients of ion-selective electrodes by using a microcomputer-controlled potentiometric system

The detection limit and the potentiometric selectivity coefficients of ion-selective electrodes are determined automatically with a microcomputer-controlled potentiometric system. Measurements of these parameters for three commercially available electrodes of the liquid membrane type (chloride, nitrate and calcium electrodes) gave results in good agreement with those reported in the literature. The non-linear least-squares fit evaluation of data (potential activities) and the selection of the appropriate transfer functions are described. The reproducibility of the results is discussed.     

Potentiometric determination of tetraphenylborate with silver nitrate

A simple potentiometric method for determining the tctraphenylborate ion (C₆H₅)₄B- contents of organic amine tetraphcnylborate salts has been developed. The compounds arc dissolved in 1 : 1 aqueous acetone; and the solution is buffered at a pH of 5 with 3M acetic acid and 3M sodium acetate. The resulting solution is then titrated with 0.06N aqueous silver nitrate, using a silver indicating electrode, a glass reference electrode and a Beckman pH meter to indicate EMF changes. Platinum electrodes in conjunction with the Malmstadt automatic titrator can also be used. Good end-points were obtained for all structure types except the primary aromatic amine borates. Values for (C₆H₅)₄B- found were within 2% of theory for 22 compounds so analyzed.     

Potentiometric and volumetric determination of 6-mercaptopurine

Three methods for the determination of 6-mercaptopurine in the pure state and in tablets are described. One method is based on the reaction of this compound with silver ions, whereby an equivalent amount of nitric acid is liberated, the latter being determined potentiometrically with sodium hydroxide. The other two methods involve titration with chloramine-T, with potentiometric and visual end-point-detection. All methods afford accurate and reproducible results and can be used for routine assay of 6-mercaptopurine in tablets.     

Potentiometric determination of penicillins with ion-selective electrodes

Quaternary ammonium, phosphonium and arsonium membrane electrodes sensitive to benzylpenicillin, ampicillin and oxacillin have been investigated. The order of merit of electrode performance is cetyltrioctylammonium> cetyltrioctylphosphonium> cetyltrioctylarsonium. The electrodes are suggested for use in rapid determination of penicillin drugs by direct potentiometry.     

Potentiometric flow-injection determination of iodide and iodine

A potentiometric flow-injection system is described, in which iodide in the concentration range 10(-6)-10(-1)M may be determined at rates of up to 360 samples/hr at a flow-rate of 17.8 ml/min. Iodine, after on-line reduction to iodide with 0.1M sodium metabisulphite, can also be determined, with a throughput of 60 samples/hr for 10(-5)-10(-3)M iodine. Analyses of two pharmaceutical preparations for iodide and iodine are reported, and the results are in reasonable agreement with titrimetric values.     

Potentiometric determination of sulphur in secondary alumina

Summary A procedure for determination of sulphur in adsorbates based on combustion of samples in a high-frequency induction furnace, and subsequent registration by potentiometric titration in an alcoholic medium is described. Optimum conditions for the titration are discussed. A new type of indicator electrode with integrated impedance transformation electronics has been used. The technique was applied with good results to samples of secondary alumina (standard deviation 0.01%).

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Potentiometrische Bestimmung von Schwefel in sekundärem Aluminiumoxid

Zusammenfassung Das beschriebene Verfahren zur Schwefelbestimmung in Adsorbaten beruht auf der Verbrennung der Probe in einem Hochfrequenz-Induktionsofen und nachfolgender potentiometrischen Titration in alkoholischem Medium. Die optimalen Titrationsbedingungen werden diskutiert. Es wurde eine neuartige Indicatorelektrode mit integrierter Impedanztransformation benutzt. Bei der Anwendung auf sekundäres Aluminiumoxid wurden gute Ergebnisse erzielt (Standardabweichung 0,01%).