Christopher M.A. Brett

Measurement of pH. Definition, standards, and procedures (IUPAC Recommendations 2002)

The definition of a “primary method of measurement ” has permitted a full consideration of the definition of primary standards for pH, determined by a primary method (cell without transference, Harned cell), of the definition of secondary standards by secondary methods, and of the question whether pH, as a conventional quantity, can be incorporated within the internationally accepted system of measurement, the International System of Units (SI, Système International d’ Unités). This approach has enabled resolution of the previous compromise IUPAC 1985 Recommendations [Pure Appl. Chem.57, 531 (1985)]. Furthermore, incorporation of the uncertainties for the primary method, and for all subsequent measurements, permits the uncertainties for all procedures to be linked to the primary standards by an unbroken chain of comparisons. Thus, a rational choice can be made by the analyst of the appropriate procedure to achieve the target uncertainty of sample pH. Accordingly, this document explains IUPAC recommended definitions, procedures, and terminology relating to pH measurements in dilute aqueous solutions in the temperature range 5-50 °C. Details are given of the primary and secondary methods for measuring pH and the rationale for the assignment of pH values with appropriate uncertainties to selected primary and secondary substances.

On the electrochemical behaviour of aluminium in acidic chloride solution

Abstract The electrochemical behaviour of aluminium in 0.1 M hydrochloric acid solution positive of the open circuit potential (OCP) has been studied using open circuit potential measurements, linear sweep voltammetry and electrochemical impedance spectroscopy at stationary and rotating high purity aluminium disc electrodes. Freshly polished electrodes show an induction time before the OCP changes negatively towards its steady-state value. Steady-state current-voltage profiles show two distinct almost linear, non-Tafelian regions positive of the OCP. Evidence for pitting corrosion through a salt film and dissolution via ionic migration through the thin oxide film as parallel corrosion paths is discussed.

On the adsorption and electrochemical oxidation of DNA at glassy carbon electrodes

Abstract New experimental results concerning the adsorption and anodic oxidation of DNA denatured in acid at glassy carbon electrodes using differential pulse, square wave and cyclic voltammetry, and impedance have been obtained, in the pH range 0–12 and as a function of adsorption potential and time of adsorption. The assumption up until now of a completely irreversible oxidation of guanine and adenine residues is shown not to be true. An explanation for these and previous results, based on desorption of the oxidation products and their diffusion away from the electrode surface, physically impeded for multilayer adsorption of DNA residues, is proposed. This situation corresponds to long adsorption times, as is the case here, and is corroborated by scanning electron microscopy.

The application of electrochemical impedance techniques to aluminium corrosion in acidic chloride solution

Electrochemical impedance techniques are applied to the study of aluminium corrosion in hydrochloric acid solution in the passive region. Impedance spectra are obtained in stationary solution and in solution moving due to electrode rotation. The form of the spectra shows the importance of multistep dissolution, ionic migration through the oxide layer, relaxation effects and the influence of chloride ion.

Electrochemical sensors and biosensors based on redox polymer/carbon nanotube modified electrodes: a review.

The aim of this review is to present the contributions to the development of electrochemical sensors and biosensors based on polyphenazine or polytriphenylmethane redox polymers together with carbon nanotubes (CNT) during recent years. Phenazine polymers have been widely used in analytical applications due to their inherent charge transport properties and electrocatalytic effects. At the same time, since the first report on a CNT-based sensor, their application in the electroanalytical chemistry field has demonstrated that the unique structure and properties of CNT are ideal for the design of electrochemical (bio)sensors. We describe here that the specific combination of phenazine/triphenylmethane polymers with CNT leads to an improved performance of the resulting sensing devices, because of their complementary electrical, electrochemical and mechanical properties, and also due to synergistic effects. The preparation of polymer/CNT modified electrodes will be presented together with their electrochemical and surface characterization, with emphasis on the contribution of each component on the overall properties of the modified electrodes. Their importance in analytical chemistry is demonstrated by the numerous applications based on polymer/CNT-driven electrocatalytic effects, and their analytical performance as (bio) sensors is discussed.

Characterisation of passive films formed on mild steels in bicarbonate solution by EIS

Passive films formed anodically on carbon steel, chromium steel and high speed steel (HSS) in 0.5 M NaHCO3 and in 0.5 M NaHCO3+0.01 M KCl have been characterised by electrochemical impedance spectroscopy (EIS). Carbon steel presents a wide passive anodic region (0.0–0.9 V vs. saturated calomel electrode (SCE)), while those of chromium steel and HSS, either as-received or heat-treated, are narrower because of transpassive anodic dissolution of Cr above ∼0.4 V. This is clearly seen through changes in the impedance spectra. An equivalent circuit is proposed to fit the experimental data, leading to determination of oxide film capacitance and film resistance. The semiconductor properties of the passive films were investigated by Mott–Schottky plots. All steel samples behave like n-type semiconductors, showing that the passive film properties are dominated by iron. However, same differences are found concerning the concentration of donors, i.e. oxygen vacancies, and thickness of the space charge region, which are correlated with the different pitting corrosion resistances of the passivated steels.

Poly(methylene blue) modified electrode sensor for haemoglobin

Poly(methylene blue) modified electrodes have been prepared by potential cycling from aqueous electrolyte solution containing methylene blue on glassy carbon substrates. They have been used successfully for the determination of haemoglobin via its electrooxidation at fixed potential using the batch injection analysis technique in which whole blood samples diluted with phosphate or acetate buffer are injected directly over the modified electrode immersed in inert electrolyte. Electrode response is fast, stable, and reproducible. For blood from healthy donors, a good linear correlation was found between the results from the electrochemical injection method and the conventional clinical spectrophotometric measurement after cyanidation. The blood from potentially ill donors showed deviations from the linear correlation suggesting that this may be a good method for detecting haematological problems.

Direct electrochemical determination of carbaryl using a multi-walled carbon nanotube/cobalt phthalocyanine modified electrode.

The electrochemical detection of carbaryl at low potentials, in order to avoid matrix interferences, is an important challenge. This study describes the development, electrochemical characterization and utilization of a glassy carbon (GC) electrode modified with multi-wall carbon nanotubes (MWCNT) plus cobalt phthalocyanine (CoPc) for the quantitative determination of carbaryl in natural waters. The surface morphology was examined by scanning electron microscopy, enhanced sensitivity was observed with respect to bare glassy carbon and electrocatalytic effects reduced the oxidation potential to +0.80 V vs. SCE in acetate buffer solution at pH 4.0. Electrochemical impedance spectroscopy was used to estimate the rate constant of the oxidation process and square-wave voltammetry to investigate the effect of electrolyte pH. Square-wave voltammetry in acetate buffer solution at pH 4.0, allowed the development of a method to determine carbaryl, without any previous step of extraction, clean-up, or derivatization, in the range of 0.33-6.61 micromol L(-1), with a detection limit of 5.46+/-0.02 nmol L(-1) (1.09+/-0.02 microg L(-1)) in water. Natural water samples spiked with carbaryl and without any purification step were successfully analyzed by the standard addition method using the GC/MWCNT/CoPc film electrode.

The wall-jet ring-disc electrode: Part I. Theory

Abstract A theoretical treatment is presented for the wall-jet ring-disc electrode. Velocity profiles for the radial and axial flow in the wall-jet system are derived. A general treatment for transforming the convective diffusion equation into the same form as that for a rotating ring-disc electrode is presented. This transformation means that the results of the rotating ring-disc electrode for the limiting currents, the collection efficiency, and diffusion layer titration curves can be applied directly to the wall-jet ring-disc electrode.

An alcohol oxidase biosensor using PNR redox mediator at carbon film electrodes

A new amperometric biosensor for ethanol monitoring has been developed and optimised. The biosensor uses poly(neutral red) (PNR), as redox mediator, which is electropolymerised on carbon film electrodes and alcohol oxidase (AlcOx) from Hansenula polymorpha as recognition element, immobilised by cross-linking with glutaraldehyde (GA) in the presence of bovine serum albumin (BSA) as carrier protein. Optimisation of variables affecting the system was performed and, for chronoamperometric measurements, a potential of -0.300 V versus saturated calomel electrode was chosen in 0.1M sodium phosphate buffer saline at pH 7.5. The optimised biosensor showed a good sensitivity of 171.8+/-14.8nAmM(-1) and the corresponding detection limit (signal-to-noise-ratio=3) of 29.7+/-1.5 microM. Stability studies showed a good preservation of the bioanalytical properties of the sensor, 57.6% of its initial sensitivity remaining after 3 weeks (the sensor was used two to three times per week). No significant interferences were found from compounds usually present in wine. The biosensor was used for the determination of ethanol in Portuguese red and white wines.