Studies on the cationic selectivity of the potentiometric membrane containing thiamine metatungstate as ionophore

An unconventional structure was obtained by extraction of K⁺ ions with nitrobenzene solution of thiamine metatungstate. This shows that K⁺ penetration into the organic phase is not an exchange process. The solution of thiamine metatungstate in nitrobenzene acts as a spacial structure that entraps K⁺ ions in a molar ratio of 1 : 2. The absence of a chemical reaction between K⁺ and the metatungstic anion (proved experimentally) and the specificity of the interaction with K⁺, suggests that the formation of the structure potassium – thiamine metatungstate (KTM) could be controlled by dimensional criteria. The nitrobenzene solution of KTM shows a remarkable potentiometric selectivity for K⁺ compared with any other alkaline and alkaline earth cations. The potentiometric K⁺-selective electrode, based on KTM as ionophore, responds linearly in the range 3.8 × 10^–5–1.0 × 10^–1 mol/L, with a slope of 56 mV/ decade and a detection limit of 1.2 × 10^–5 mol/L. The electrode maintained these response characteristics over a period of more than two months. Received: 12 May 1998 / Revised: 28 July 1998 / Accepted: 30 July 1998     

Cetyldimethylethylammonium-Selective PVC Electrode Basedon its Ion-Associate with Phosphotungstic Acid

 A PVC membrane electrode selective for cetyldimethylethylammonium (CDEA) ion was prepared. The active element is a plasticized poly vinyl chloride membrane containing a dissolved ion associate complex of CDEA with phosphotungstic acid (PTA). The electrode showed a near-Nernstian response within the CDEA⁺ concentration range 2.34 × 10⁻⁶ to 1.96 × 10⁻⁴ M at 25 ± 1^{^}C, good selectivity, and is usable within the pH range 3.5–9.0. The isothermal temperature coefficient of the electrode amounted to −0.00012 V/°C. The electrode has been used for the direct determination of cetyldimethylethylammonium bromide either by the standard addition method or by potentiometric titration against PTA. Received January 1, 1998. Revision June 13, 2000.     

Arylboronic compounds – novel carriers for catecholamine selective membrane electrodes

Arylboronic acids are used as novel carriers for membrane electrodes suitable for direct potentiometric determination of the catecholamine drug dobutamine. The carriers are capable of binding diol groups of catecholamines; solvent extraction data confirm the formation of an 1 : 1 complex. For the electrode based on octyloxyphenylboronic acid, the slope of electrode function is S = 58 mV decade^–1; the detection limit is 1.7 · 10^–5 mol/L, the linear range 5 · 10^–4– 1 · 10^–2 mol/L, the response time 10–20 s. The results suggest the potential use of boronic carriers for the detection of biogenic catecholamines. Received: 25 November 1998 / Revised: 2 March 1999 / Accepted: 11 March 1999     

A PVC-based cryptand C2B22 membrane potentiometric sensor for zinc(II)

A PVC membrane electrode for zinc ions based on cryptand C2_B22 as membrane carrier was prepared. The electrode exhibits a linear stable response over a wide concentration range (5.0 × 10^–2– 5.0 × 10^–5 mol/L) with a slope of 24 mV/ decade and a limit of detection of 3.98 × 10^–5 mol/L (2.6 μg/g). It has a fast response time of about 30 s and can be used for at least 4 months without any divergence in potential. The proposed sensor revealed good selectivities for Zn²⁺ over a wide variety of other metal ions and could be used in a pH range of 4–7. It was used as an indicator electrode in potentiometric titration of zinc ion. Received: 26 February 1998 / Revised: 25 May 1998 / Accepted: 28 May 1998     

Amperometric method for the determination of myoglobin based on the electrochemical reduction on the glassy carbon electrode

An irreversible reduction peak of oxymyoglobin (MbO₂) was observed on the bare glassy carbon electrode (GCE) in acetate buffer solution under atmospheric conditions. It is the reduction of bonded oxygen in Mb, but not the heme Fe(III)/Fe(II) redox couple that underwent electrochemical reaction on the electrode. The peak current achieved a maximum value in acetate buffer solution of pH 4.0. The peak potential was pH dependent, suggesting that the proton was involved in the electrochemical reaction. Furthermore, the peak current was linearly related to the concentration of myoglobin in the range of 2.5 × 10^–8∼ 1.0 × 10^–6 mol · L^–1 with a detection limit of 5 × 10^–9 mol · L^–1. Received: 20 March 1998 / Revised: 24 June 1998 / Accepted: 1 July 1998     

End-point detection of the potentiometric titration of anionic polyelectrolytes using an anionic surfactant-selective plasticized poly (vinyl chloride) membrane electrode and an anionic surfactant as a marker ion

A plasticized poly (vinyl chloride) (PVC) membrane electrode sensitive to dodecylbenzenesulfonate (DBS) ion is applied to the determination of anionic polyelectrolytes such as potassium poly (vinyl sulfate) (PVSK) by potentiometric titration, using a poly (diallyldimethylammonium chloride) (Cat-floc) solution as a titrant. The end-point of the titration is detected as the potential jump of the plasticized PVC membrane electrode caused by decrease in the concentration of DBS ion added to the sample solution as a marker ion, due to the ion association reaction between the DBS ion and Cat-floc. The effects of the concentration of DBS ion, coexisting surfactants and electrolytes in the sample solution and pH of the sample on the degree of the potential jump at the end-point were examined. A linear relationship between the concentration of anionic polyelectrolyte and the end-point volume of the titrant exists in the concentration range from 2 × 10^–5 to 4 × 10^–4 N for PVSK, alginate, and carrageenan. Received: 30 April 1998 / Revised: 6 July 1998 / Accepted: 11 August 1998     

Lead selective membrane electrode using cryptand(222) neutral carrier

The construction, performance characteristics and applications of a polymeric membrance electrode for lead(II) ion are reported. The electrode was prepared by incorporating cryptand(222) as the neutral carrier into a plasticized poly(vinyl chloride) membrane. The influence of membrane composition, pH and concentration of internal reference solution were investigated. The electrode exhibits a potentiometric response for Pb²⁺ ion over the concentration range 10^–1–10^–5 M with a detection limit of 5 × 10^–6 M Pb²⁺. It shows a relatively fast response time of about 30 s and can be used for about two months without any considerable divergence in potential. The proposed sensor revealed good selectivities for Pb²⁺ in the presence of several metal ions and could be used in the pH range of 2.5 to 7.5. It was used as an indicator electrode in the potentiometric titration of Pb²⁺ with EDTA. Received: 20 November 1998 / Revised: 26 March 1999 / Accepted: 31 March 1999     

Lead selective membrane electrode using cryptand(222) neutral carrier

The construction, performance characteristics and applications of a polymeric membrance electrode for lead(II) ion are reported. The electrode was prepared by incorporating cryptand(222) as the neutral carrier into a plasticized poly(vinyl chloride) membrane. The influence of membrane composition, pH and concentration of internal reference solution were investigated. The electrode exhibits a potentiometric response for Pb²⁺ ion over the concentration range 10^–1–10^–5 M with a detection limit of 5 × 10^–6 M Pb²⁺. It shows a relatively fast response time of about 30 s and can be used for about two months without any considerable divergence in potential. The proposed sensor revealed good selectivities for Pb²⁺ in the presence of several metal ions and could be used in the pH range of 2.5 to 7.5. It was used as an indicator electrode in the potentiometric titration of Pb²⁺ with EDTA. Received: 20 November 1998 / Revised: 26 March 1999 / Accepted: 31 March 1999     

Copper(II)-selective membrane electrode based on a recently synthesized naphthol-derivative Schiff’s base

A PVC membrane electrode for copper(II) ions based on a recently synthesized naphthol-derivative Schiff’s base as membrane carrier was prepared. The sensor exhibits a Nernstian response for Cu²⁺ ions over a wide concentration range (5.0 × 10^–6–5.0 × 10^–2 mol/L) with a detection limit of 3.1 × 10^–6 mol/L (0.2 μg/mL). It has a very short response time of about 5 s and can be used for ?3 months without any divergence in potential. The proposed electrode revealed good selectivities over a wide variety of other cations including alkali, alkaline earth, transition and heavy metal ions and could be used in a pH range of 4.0–7.0. It was successfully applied to the direct determination and potentiometric titration of copper ion. Received: 23 February 1999 / Revised: 4 May 1999 / Accepted: 5 May 1999     

Potentiometric and conductometric analysis of the binding of Barium ions with alkali polyacrylate

 The interaction between Poly(acrylic acid) and barium ions is analyzed using potentiometric (barium specific electrode) and conductometric titrations. At full ionization of the polyelectrolyte, in the presence of M⁺ counterions (M⁺=Li⁺, Na⁺ and K⁺), the binding ratio [Ba²⁺]_bound/C _p on the chain is determined, showing no significant difference between the three alkali ions. When the added Ba²⁺ concentration does not exceed 0.2×C _p, all barium ions bind with the polymer, i.e. none can be detected in the solution with the barium selective electrode. Assuming that monovalent counterions divide the electrostatically condensed and “atmospheric” ions and using Eisenberg plots of the conductivity excess, the experimental data allows to calculate the distribution of the different acrylic species on the fully deprotonated chain (free carboxylate groups, bound groups with M⁺ and with Ba²⁺ ions). Assuming the formation of a bidentate Ba(COO)₂ species and taking into account that part of the remaining groups bind with M⁺ ions, the calculated complexation constant (log K _c=6.5) is satisfactorily independent of the complexation ratio. The displacement ratio of M⁺ ions by attaching Ba²⁺ ions is also calculated, showing interestingly a continuous decrease between 1.4 and 0.9 as r increases. The latter result is attributed to the change of the averaged electrostatic potential of the chain, in relation with the binding of barium ions. Received: 14 April 1998 Accepted: 23 April 1998     

Preparation of an ion-selective electrode by chemical treatment of copper wire for the measurement of copper(II) and iodide by batch and flow-injection potentiometry

The preparation of an ion-selective electrode by chemical treatment of copper wire and its application for the measurements of copper (II) and iodide ions is described. The proposed reaction mechanism at the sensing surface, which explains the response of the electrode to Cu²⁺ and iodide ions, is discussed. The prepared electrode was suitable for direct potentiometric measurements of iodide and copper (II) in batch experiments down to concentrations of 1 × 10^–5 mol L^–1. A tubular electrode, prepared in the same way, may be used as a potentiometric sensor in a flow-injection analysis for Cu (II) and/or iodide determinations. Received: 4 December 1998 / Revised: 31 March 1999 / Accepted: 6 April 1999     

Lead-selective electrode based on the misfit compound (PbS)1.18TiS2

A lead-selective solid-contact electrode was prepared on the basis of the misfit compound (PbS)_1.18TiS₂. The electrode exhibits the slope of the electrode function is -(26 ±1) mV/pc and provides the determination of 1 × 10^-5-5 × 10^-2 M lead in the pH range 2.75–5.0. High selectivity of the electrode for Ni²⁺, Co²⁺, Zn²⁺, Cu²⁺, Ag⁺, K⁺, Ba²⁺, and Sr²⁺ ions was demonstrated. The addition of graphite to the active phase of the membrane impaired the selectivity of the electrode. The electrode was used for the potentiometric indication of the titration end point in the determination of lead in copper alloys. Presented at the V All-Russian Conference with the Participation of CIS Countries on Electrochemical Methods of Analysis (EMA-99), Moscow, December 6–8, 1999.     

An Ni(chitin)2 modified nitric oxide microsensor

The development of a simple, sensitive, selective, and stable amperometric nitric oxide microsensor is described. It is based on Ni(chitin)₂ mediators immobilized on a platinum, Nafion modified electrode. The detection of NO is based on the Ni(chitin)₂ catalysis of NO oxidation at an anodic potential of +0.74 V (vs. SCE). The catalytic peak current is linear for a NO concentration in the range of 8.5 × 10^–8 mol/L to 1.5 × 10^–5 mol/L, with a correlation coefficient of 0.9992. The detection limit of the microsensor is 5.0 × 10^–8 mol/L. It is suitable for the direct measurement of NO in biological systems. Received: 5 February 1999 / Revised: 15 June 1999 / Accepted: 17 June 1999     

Construction and Evaluation of a Promazinium Cation-Selective Electrode

 The construction of a plasticised PVC matrix-type promazinium cation-selective membrane electrode and its use in the potentiometric determination of promazine hydrochloride in pharmaceutical preparations are described. It is based on the use of the ion-associate species, formed by promazinium cation and tetraphenylborate (TPB) counter ion. The basic electrode performance characteristics are evaluated according to IUPAC recommendations. It exhibited a linear response for 1 × 10⁻²−1 × 10⁻⁵ M of promazine hydrochloride solutions with a cationic Nernstian slope over the pH range 2–6. Common organic and inorganic cations showed negligible interference. Direct potentiometric determination of 1 × 10⁻²−1 × 10⁻⁵ M aqueous promazine hydrochloride using this membrane electrode system showed an average recovery of 99.5% with a mean standard deviation of 1.5%. This electrode was successfully used for monitoring the titration of promazine hydrochloride with sodium tetraphenyl borate and for determining promazine hydrochloride in ampoules. Received June 15, 2001 Revision November 6, 2001     

A novel chemically modified carbon paste electrode based on a new mercury(II) complex for selective potentiometric determination of bromide ion

A new modified carbon paste electrode based on a recently synthesized mercury (II) complex of a pyridine containing proton transfer compound as a suitable carrier for Br⁻ ion is described. The electrode has a linear dynamic range between 3.00×10⁻² and 1.0×10⁻⁵ M with a near-Nernastian slope of 61.0±0.9 mV per decade and a detection limit of 4.0×10⁻⁶ M (0.32 ppm). The potentiometric response is independent of the pH of the solution in the pH range 4.0–8.3. The electrode possesses the advantages of low resistance, fast response and good over a variety of other anions. It was applied as an indicator electrode in potentiometric titration of bromide ions and for the recovery of Br⁻ from tap water.     

Voltammetric determination of cadmium(II) using a chemically modified electrode

An 1-(pyridylazo)-2-naphthol modified glassy carbon electrode has been investigated as sensor for the measurement of trace levels of Cd²⁺. Cd²⁺ is deposited on the surface of a PAN modified glassy carbon electrode at –1.10 V (vs. SCE) via forming Cd²⁺–PAN and subsequent reduction at the electrode. In the following step, Cd-PAN is oxidized, and voltammograms are recorded by scanning the potential in a positive direction. Calibration plots were found to be linear in the range 2 × 10^–8 mol/L to 8 × 10^–7 mol/L. The detection limit was 5 × 10^–10 mol/L, and the coefficient of variation, determined on one single electrode at a concentration of 5 × 10^–7 mol/L, was calculated to be 3.2% (n = 5). Using this new kind of modified electrode, trace levels of Cd(II) in water samples were determined; the average recovery was calculated to be 98.78%. Received: 17 August 2000 / Revised: 19 December 2000 / Accepted: 27 December 2000     

Silver(I)-selective electrode based on [Bz2Oxo4(18)dieneS4] tetrathia macrocyclic carrier

A polystyrene-based membrane of 7,8:16,17-dibenzo-6,9,15,18-tetraoxo-1,5,10,14-tetrathiacyclooctadeca-7,16-diene [Bz₂Oxo₄(18)dieneS₄] was fabricated using sodium tetraphenylborate (NaTPB) and dioctyl phthalate (DOP) as anion excluder and plasticizing agent. The best performance was obtained from the membrane with the composition ionophore [Bz₂Oxo₄(18)dieneS₄]:polystyrene:DOP:NaTPB, 5:100:150:10 (w/w). The response of the electrode was linear over a wide range of concentration, 1.26×10^–6–1.00×10^–1 mol L⁻¹ for silver ion with a Nernstian slope of 58.4±0.1 mV per decade and a detection limit of 1.0×10⁻⁶ mol L⁻¹. The electrode was found to be chemically inert and of adequate stability with a response time of 10 s and could be used for a period of 3 months without change of potential. It worked satisfactorily in mixtures containing up to 35% (v/v) non-aqueous content. The proposed membrane sensor had good selectivity for Ag⁺ over a wide variety of metal ions in the pH range 2.2–8.5. It was successfully used as an indicator electrode in potentiometric titration of silver ion. The electrode was also useful for determination of Ag⁺ in waste from photographic films.     

Determination of citric acid in soft drinks using flow injection with potentiometric detection

A flow injection analysis (FIA) system with potentiometric detection has been developed for the determination of citric acid in commercial fruit juices using a copper-selective tubular electrode. It consists of the complexation of citrate ion with copper (II) ion, monitoring the free copper (II) concentration. Linear relationships between the potential and log concentrations of citric acid in the range of 1 × 10^–3 mol/L – 1 × 10^–2 mol/L were attained with samples injected in the system without prior treatment. Repeatability was determined by assessing the relative standard deviation for ten replicate injections of different juice samples which varied from 1.2% to 3.0%. The results provided by the automated system were compared with those of an enzymatic conventional method used for the analysis of citric acid in food and a relative deviation of less than 4.0% was found. Recovery trials resulted in values from 97.0% to 101.0%. The system allows a sampling frequency of 60 samples/h. Received: 22 October 1998 / Revised: 14 December 1998 / Accepted: 16 December 1998     

Ion-selective electrode for measuring low Ca2+ concentrations in the presence of high K+, Na+ and Mg2+ background

In this work, ion-selective electrodes for calcium ion were investigated. Two ionophores were used in the membranes: ETH 1001 and ETH 129. An internal filling solution buffered for primary ion was used that allowed the lower detection limit to be decreased down to 10^−8.8 M. Theoretical and experimental electrode characteristics pertaining to both primary and interfering ions are discussed. Better behavior was obtained with the electrode prepared with ETH 129 in the membrane. This electrode would be the most likely candidate for obtaining a low Ca²⁺ detection limit in measurements performed with high K⁺, Na⁺, Mg²⁺ background, which is found inside the cells of living organisms, for example. The potentiometric response of the electrode in solutions containing main and interfering ions is in good agreement with simulated curves obtained using the Nernst–Planck–Poisson (NPP) model.     

A simple spectrophotometric procedure for the determination of antimony (III) and (V) in antileishmanial drugs

A spectrophotometric method for the selective determination of antimony (III) and (V) in antileishmanial drugs is described. The procedure is based on the reaction of Sb(III) with bromopyrogallol red (BPR) in neutral solution. As a consequence of the Sb-BPR complex formed, the absorbance of BPR, at 560 nm, decreases proportionally to the amount of Sb(III) in the analyte solution. The calculated apparent molar absorptivity and determination limits are 3.67 × 10⁴ L · cm^–1 · mol^–1 and 1.65 × 10^–6 mol/L, respectively. Sb(V) is determined after reduction to Sb(III) by iodide. The Sb(V) content determined in ten samples of Glucantime varied from 75.40 ± 0.97 to 94.47 ± 1.0 mg/mL. Sb(III) was detected in all samples analyzed, and mean values ranged from 5.19 ± 0.16 to 10.52 ± 0.15 mg/mL. The method is suitable for the routine quality control of pharmaceutical formulations. Received: 26 July 1996 / Revised: 17 October 1996 / Accepted: 11 December 1996