Esma Kılıç

Hydrogen ion-selective poly(vinyl chloride) membrane electrode based on a p-tert-butylcalix[4]arene-oxacrown-4.

A hydrogen ion-selective poly(vinyl chloride) (PVC) membrane electrode was developed using p-tert-butylcalix[4]arene-oxacrown-4 as ionophore. Apart from the ionophore, plasticisers and lipophilic anions were blended, in various proportions, with PVC in tetrahydrofurane and mixtures so prepared were poured onto glass surfaces to form the membrane. 2-Nitrophenylpentylether has proved to be the best alternative as plasticiser and the lipophilic anions tried have turned out to have an adverse effect on the pH response. The electrode of the optimum characteristics had a composition of 2% p-tert-butylcalix[4]arene-oxacrown-4, 68.3% o-NPPE and 29.7% PVC. The electrode showed an apparent Nernstian response in the pH range 2-11 with a slope of 54.2+/-0.4mVpH(-1) at 20+/-1 degrees C. It has a rapid potential-response to changes of pH, high ion selectivity towards lithium, sodium and potassium, and other characteristics comparable to those reported for the conventional pH glass membrane electrode. It appears to be a suitable potentiometric indicator electrode specifically for hydrofluoric acid solutions.

A review of enzymatic uric acid biosensors based on amperometric detection.

This review summarizes the studies carried on the development of amperometric uric acid biosensors over the past twenty years. Sensing principles, enzyme immobilization techniques, the electrode types, different approaches and various matrices used for biosensor fabrication are presented along with their benefits and limitations. Uric acid biosensors based on different modes of transducing devices such as optical, potentiometric, conductometric are also referred.

The non-aqueous titrimetric assay of selected antibiotics using tetra-N-butylammonium hydroxide as titrant

This study was carried out to develop a potentiometric titration method in non-aqueous media for the determination of some commonly used antibiotics. For this purpose, five antibiotics, namely ampicillin, amoxycillin trihydrate, rifampin, netilmicin sulphate and ciproflaxacin hydrochloride, were potentiometrically titrated using pyridine as solvent and tetrabutylammonium hydroxide as titrant, at 25 degrees C and under a nitrogen atmosphere. The method was found to be highly accurate and precise having a relative standard deviation of less than 1.0%. Also it was shown that the method could be successfully applied to assay of commercial pharmaceuticals containing the above-mentioned antibiotics. The results of recovery studies for standard additions in pharmaceutical preparations were satisfactory. The proposed method is simple, rapid and sufficiently precise for quality control purposes.

An Amperometric Biosensor for Xanthine Determination Prepared from Xanthine Oxidase Immobilized in Polypyrrole Film

In order to prepare a biosensor for the determination of xanthine, electropolymerization of pyrrole on Pt surface was carried out with an electrochemical cell containing pyrrole, ferrocene (as a electron mediator) and tetrabutylamonium tetrafluoroborat in acetonitrile by cyclic voltammetry between 0.0 and 0.9 V (vs SCE) at a scan rate of 50 mV/s upon Pt electrode. Xanthine oxidase was immobilized by a glutaraldehyde/bovine serum albumin (BSA) crosslinking procedure on to polypyrrole film after the electropolymerization processes. The response of the biosensor against xanthine was measured after 3–4 min following the application of a constant potential of +0.7 V (vs SCE). The resulting biosensor exhibits excellent electrocatalysis for the xanthine. The amperometric determination is based on the electrochemical detection of H2O2, which is generated in enzymatic reaction of xanthine. The effect of various experimental conditions was examined for the determination of the analytical performance. The sensor responds to xanthine with a detection limit of 1.0 × 10−6 M. The response current increases linearly with xanthine concentration up to 4.0 × 10−4 M. The sensor remains relatively stable for 45 days.

The non-aqueous titrimetric assay of the selected anti-inflammatory agents using tetra-n-butylammonium hydroxide as titrant.

A potentiometric titration method in non-aqueous media is proposed for the determination of some commonly used anti-inflammatory agents. The direct potentiometric titration of three anti-inflammatory agents, namely mefenamic acid, fenbufen and ibuprofen; and the indirect potentiometric titration of diclofenac sodium was carried out in acetonitrile solvent using tetra-n-butylammonium hydroxide as titrant, at 25 degrees C and under a nitrogen atmosphere. The method was found to be highly accurate and precise, having a relative standard deviation of <1.0% for all anti-inflammatory agents studied. Also, it was shown that the method could be successfully applied to the assay of commercial pharmaceuticals containing the above-mentioned anti-inflammatory agents. The validity of the method was tested by the recovery studies of standard addition to pharmaceuticals and the results were found to be satisfactory. The proposed method is simple, rapid and sufficiently precise for quality control purposes.

Titrations in non-aqueous media. Part X. Potentiometric and conductimetric titrations of amino acids with tetrabutylammonium hydroxide in pyridine and acetonitrile solvents

Of the 22 α-amino acids studied, all but three (tyrosine, cystine and cysteine) have been titrated potentiometrically using a standard solution of tetrabutylammonium hydroxide in two solvent systems, 90% pyridine-water and 90% acetonitrile-water. The half-neutralisation potentials and the corresponding pK′a1 and pK′a2 values of the amino acids in these solvent systems have been calculated. Potentiometric titrations of binary mixtures of the two dicarboxylic amino acids, glutamic and aspartic acid, with the other monocarboxylic amino acids have also been carried out. The end-points of the titrations performed in the 90% acetonitrile-water solvent system were found to be much sharper than those of the titrations carried out in the 90% pyridine-water solvent system. All the determinations of amino acids, either in single or in binary mixtures, have been shown to have an error of less than ±2%.The α-amino acids and their binary mixtures with glutamic and aspartic acids have also been titrated conductimetrically in the same two solvent systems. Almost all the titrations gave either no end-points or non-stoicheiometric end-points. It has been shown that the solvent systems used in this work are suitable for potentiometric titrations of amino acids, but are not suitable for the conductimetric titrations.

Urea Biosensors Based on PVC Membrane Containing Palmitic Acid

A new urea biosensor was prepared by immobilizing urease with four different procedures on poly(vinylchloride) (PVC) ammonium membrane electrode containing palmitic acid by using nonactine as an ammonium-ionophore. The analytical characteristics were investigated and were compared those of the biosensor prepared by using carboxylated PVC. The effect of pH, buffer concentration, temperature, urease concentration, stirring rate and enzyme immobilization procedures on the response to urea of the enzyme electrode were investigated. The linear working range and sensitivity of the biosensor were also determined. The urea biosensor prepared by using the PVC membranes containing palmitic acid showed more effective performance than those of the carboxylated PVC based biosensors. Additionally, urea assay in serum was successfully carried out by using the standard addition method.

Protonation constants of some substituted 2-hydroxy-1-naphthalideneanilines in dioxan-water mixtures

The protonation constants of ten Schiffs bases synthesized by the condensation of 2-hydroxy-1-naphthaldehyde with aniline and o-, m-, and p-substituted methylanilines, ethylanilines and methoxyanilines in various dioxan-water mixtures have been determined potentiometrically using an electrode system calibrated in concentration units of the hydrogen ion. The influence of substituents in the molecular structure on the protonation constants is discussed. The log K(2) values relating to the protonation of imine nitrogen atoms are in accordance with the Hammett relationship for all solvent compositions. A reverse relationship is observed between log K(2) for all Schiffs bases and the mole fraction of dioxan. However, the log K(1) values corresponding to the protonation of naphtholate O-ions have been found to increase with the increase in concentration of dioxan. Both electrostatic and non-electrostatic contributions influencing these constants are briefly discussed.

Protonation constants of some pyridine derivatives in ethanol-water mixtures

Abstract Protonation constants of a number of mono- and disubstituted pyridines were determined potentiometrically in 0, 10, 20, 30, 40, 50, 60, 70, 80% (v/v) ethanol-water mixtures at 25°C with an ionic strength of 0.1 M. Data were calculated by a computer programme. The logarithm of the protonation constants of pyridines linearly decreased with increasde of ethanol contents but the values determined in water and 80% ethanol did not follow this linear trend. Furthermore, the effects of the substituents on the basicity of pyridine, the additives of these effects and the applicability of the Hammett equation to the behaviour of substituents are discussed.

Preparation of Pt/polypyrrole-ferrocene hydrogen peroxide sensitive electrode for the use as a biosensor

A polypyrrole electrode with ferrocene mediator is prepared and its sensitivity to hydrogen peroxide is investigated. The polypyrrole is deposited upon a 0.5 cm2 Pt plate by the polymerization of pyrrole by scanning the electrode potential between 0.0 and 0.9 V at a scan rate of 50 mV/s. The platinum/polypyrrole-ferrocene (Pt/PPy-Fc) electrode is prepared by adding ferrocene to the coverage medium. The electrode’s sensitivity to hydrogen peroxide is investigated at room temperature using 0.025 M phosphate buffer at pH 7. The working potential is 0.7 V, the concentrations of pyrrole and ferrocene are 0.2 M and 10 mM. Polypyrrole was coated on the electrode surface within 26 cycles.