M. Antonietta Baldo

Steady state voltammetry at microelectrodes for the hydrogen evolution from strong and weak acids under pseudo-first and second order kinetic conditions

Abstract The reaction of hydrogen evolution involving discharge of protons arising from strong and weak acids, namely perchloric, nitric, acetic, ethylenediaminetetra-acetic and dihydrogen phosphoric acids, was investigated in the concentration range 0.1–10 mM by steady state voltammetry at inlaid platinum micro-disc electrodes (of 10–12.5 μm radius). The second-order CE mechanism for the weak acids was rationalised by digital simulation, demonstrating the effects of the various kinetic, thermodynamic and diffusion parameters on the reduction process. In particular, it was found that proton reduction from acetic acid is associated with a completely mobile equilibrium over the whole concentration range studied, but in buffer solutions containing a large excess of the conjugate base the CE process becomes affected kinetically. For the other two weak acids the chemical step controls the kinetics at any concentration employed.

Development of a voltammetric electronic tongue for discrimination of edible oils

AbstractIn this paper, we propose a novel strategy to perform cyclic voltammetric measurements with a platinum microelectrode directly in edible oil samples. The microelectrode was employed as an electronic tongue that, along with the application of chemometrics to the current–potential responses, proved useful for discriminating oils on the basis of their quality and geographical origin. The method proposed here is based on the use of suitable room temperature ionic liquids, added to oils as supporting electrolytes to provide conductivity to the low-polarity samples. The entire voltammograms, recorded directly on the oil/RTIL mixtures, were processed via principal component analysis and a classification technique (K nearest neighbors), to extract information on samples characteristics. Data processing showed that oils having different nature (i.e. maize and olive) or geographical origin (i.e. olive oils coming from different regions) can be distinguished. FigureA novel strategy to perform voltammetric measurements with a platinum microelectrode directly in edible oil samples is presented. The microelectrode is employed as an electronic tongue that, along with the application of chemometrics to the voltammetric responses, allows oil discrimination according to their quality and geographical origin.

The use of a remote stripping sensor for the determination of copper and mercury in the Lagoon of Venice

The determination of the labile fraction and total concentration of copper and mercury in different sites of the Lagoon of Venice, using a remote electrochemical sensor based on a gold fiber microelectrode, is presented. The remote sensor was used in conjunction with either potentiometric stripping analysis (PSA) or square-wave anodic stripping voltammetry (SWASV). The results obtained show that using PSA, in situ detection of the labile fraction of both metals can be achieved down to 0.22 and 0.04 ppb for copper and mercury, respectively. Under these conditions the relative standard deviation (RSD) was within 8 and 10% for copper and mercury, respectively. In contrast, no reliable in situ data could be obtained with SWASV. This is attributed to the lack of suitable software for baseline treatment with this technique. Laboratory measurements were employed for total concentration determination of the metal ions. The total concentration was obtained after acidification of the sample to pH 1 with HCl. Under these conditions, the concentration of both copper and mercury increased by about one order of magnitude. This allowed the determination of the two ions by SWASV, whereas the overlap of the stripping peaks of copper and mercury prevented their determination by PSA. The precision for the total concentration was also satisfactory, the RSD being 5 and 7% for copper and mercury, respectively. The effect of the matrix on the PSA and SWASV responses is discussed. Total concentrations determined by the anodic stripping technique are compared with those found by atomic absorption spectrometry.

A Study on the Suitability of Carbon Disk Microelectrodes for Trace Analysis of Lead and Copper by ASV

The performance of carbon microdisk electrodes for the simultaneous trace analysis of lead and copper by anodic stripping voltammetry is investigated. The evaluation of the two metal ions in a single step, i.e., by employing a plating potential at which both cations are reduced simultaneously, gives unpredictable results due to the interaction between lead and copper deposited onto the electrode surface. In particular, the calibration plots of lead are not linear, while the copper stripping peak is quite broad and irreproducible. A two-steps deposition procedure, based on a preelectrolysis of copper performed at a potential at which lead ion is not reduced, followed by a simultaneous reduction of the two cations at large enough negative potentials, provides, instead, linear calibration graphs for lead. Copper can be determined without interference by plating the metal at a negative potential where lead ion is not reduced. The reliability of the two-steps procedure for the determination of lead and copper is verified in rain and wine samples.

Assessment of linearity between steady-state limiting current and analytical concentration of weak acids in the reaction of hydrogen evolution

Abstract The reaction of hydrogen evolution from the acids: hydrochloric, hydrogen sulfate ion, monochloroacetic and lactic have been studied by steady-state voltammetry at platinum microdisc electrodes. From the experimental voltammetric parameters, such as half-wave potential, ( E 1 4 – E 3 4 ) values and steady-state limiting current, it has been ascertained that the second order CE mechanism, involved in the discharge of protons arising from the weak acids investigated, is characterized by a chemical reaction which is fast over the whole concentration range explored. Theoretical calibration plots have been examined in the concentration range 1 × 10−6–1 × 10−1M and the zone over which linearity exists has been verified both from a theoretical and experimental point of view.

Detection of hydroxide ions in aqueous solutions by steady-state voltammetry

The electrochemical oxidation of hydroxide ions on microdisk electrodes was studied to find the conditions where the steady-state current can be used for analytical applications. Results are presented for the dependence of the voltammetric response on the microdisk radius, the electrode material and the hydroxide concentration. Well-defined waves for hydroxide oxidation were obtained under steady-state conditions with all microdisks employed. However, the steady-state wave splits for some hydroxide concentrations and radii, but the overall current is still proportional to the concentration. The usefulness of the wave for analytical applications depends on the electrode material. Although the wave is seen on nickel, platinum and gold the latter is the most promising for the amperometric detection of basicity of aqueous solutions. On platinum, the wave can be used but the plateau is not so well defined. On nickel, oxide formation makes measurements difficult. Under steady-state conditions a well-defined wave attributed to the direct oxidation of hydroxide ions was also observed on a gold rotating disk electrode. The wave height was found to be proportional to the square root of the rotation speed, thus indicating that the process was under diffusion control. The limiting current was also found to be proportional to the concentration of hydroxide ions over the range 0.05-10 mM.

Comparison between classical and innovative class-modelling techniques for the characterisation of a PDO olive oil

An authentication study of the Italian PDO (protected designation of origin) olive oil Chianti Classico, based on near-infrared and UV–Visible spectroscopy, an artificial nose and an artificial tongue, with a set of samples representative of the whole Chianti Classico production and a considerable number of samples from a close production area (Maremma) was performed. The non-specific signals provided by the four fingerprinting analytical techniques, after a proper pre-processing, were used for building class models for Chianti Classico oils. The outcomes of classical class-modelling techniques like soft independent modelling of class analogy and quadratic discriminant analysis—unequal dispersed classes were compared with those of two techniques recently introduced into Chemometrics: multivariate range modelling and CAIMAN analogues modelling methods.

Voltammetry of hydroxide ion in aqueous solutions on gold microelectrodes

Abstract A well-defined wave for the oxidation of hydroxide ions was recorded with gold microdisc electrodes either in sodium sulphate or sodium perchlorate solutions made basic with sodium hydroxide. The wave appeared at about 1.3 V versus SCE, just before the background discharge. The plot of the limiting current corresponding to this wave against the radius of a series of microdisc electrodes gave a straight line which indicated that the process is diffusion controlled. Similarly, the limiting current was found to depend linearly on pH over the range 9–12.5. To our knowledge, the results presented in this article provide the first experimental evidence for the diffusion-controlled electrochemical oxidation of hydroxide ions in aqueous solutions.

An approach to the calibrationless determination of copper and lead by anodic stripping voltammetry at thin mercury film microelectrodes. Application to well water and rain

A method, not demanding calibration with standard solutions of the analysed ions, is proposed for the determination of lead and copper in synthetic and real samples, by employing thin mercury film electrodes formed onto carbon disc microelectrodes. The method is based on an equation derived by exploiting the characteristics of the in situ thin mercury film formation and steady state currents achievable at microelectrodes. The conditions under which this equation is applicable are examined. In particular the effects of the nature of the electrolyte, with particular concern towards chloride ions, ionic strength and pH of the medium are investigated. The validity of the proposed method is verified on synthetic aqueous solutions. It is shown that the method can be applied to samples characterised either by a low or high ionic strength, while the solutions must be acidic (pH < 6 is recommended) and should not contain more than 1 mM of chloride ions. Real samples, as well water and rain, under conditions fulfilling the above prescriptions, are also investigated. The comparison of data obtained employing the proposed method with those taken, or evaluated by using classical calibration procedures with standard solutions, shows that agreement within 11% is found for 2 and 0.6 μg l−1 levels for lead and copper, respectively, with a relative standard deviation of about 6%. For 10 and 3 μg l−1 levels of lead and copper, respectively, which are the average values in the real samples investigated, both accuracy and RSD are around 5%.

Voltammetry with microelectrodes in wine : determination of the total acidity

Abstract A voltammetric study of wine was performed in order to ascertain its suitability as a medium for analysis. The electrochemical processes of the probe molecule Ru(NH3)6Cl3 and of some electroactive species present in the medium were investigated with a platinum microelectrode and the relevant responses were compared with those obtained with an electrode of the same material of conventional size. The results indicated that with the latter electrode ohmic losses affect the voltammetric processes, whereas no problem arises with the microelectrode. It was also found that the acids present in wine are responsible for a large cathodic wave and its measurement, by voltammetric and chronoamerometric titrations, permitted the determination of the total acidity of wine with a relative standard deviation within 2.5%. The results obtained with this approach were compared with those obtained by the classical method commonly employed for the determination of the total titratable acidity of wine.