Jose H. Santos

Correlations between gastropod shell dissolution and water chemical properties in a tropical estuary

Although poorly reported in the scientific literature, acidic waters characterize many South East Asian estuaries. The observation of shell dissolution in a typically marine gastropod whelk (Thais) prompted investigation into determining relationships between shell properties of this whelk and the water chemistry (including pH) of the Sungai Brunei estuary (Borneo) in which it occurs. Shell weight, shell length and topographical shell features were determined for populations of Thais gradata distributed along a gradient of pH and salinity ranging between 5.78 and 8.3 pH units, and 3.58 and 31.2psu. Shell weight varied independently of the co-varying acidity, salinity and calcium levels experienced. In contrast, shell length and a semi-quantitative variable based on shell sculpturing (shell erosion rank, SER) were significantly correlated with these water chemistry variables. This study brings attention to the potential use of estuarine organisms and systems in investigating current marine acidification questions.

Preparation of microelectrodes: comparison of polishing procedures by statistical analysis of voltammetric data

Manual and instrumental procedures for polishing 1 and 6 µm radius platinum microdisc electrodes were compared by performing a statistical evaluation of three basic electrochemical signal characteristics measured from steady-state voltammograms for oxidation of ferrocene in acetonitrile. The three signal characteristics measured were the limiting current Ilim, the half-wave potential E1/2 and the (E3/4–E1/4) potential difference, where E3/4 and E1/4 are the potentials at three quarters Ilim and one quarter Ilim, respectively. Fisher–Snedeckor F-test and Students t-test statistical data are presented together with corresponding critical values for the three electrochemical parameters obtained in the manual and instrumental polishing modes. The reproducibility of all measured data is significantly improved when the electrode is polished after each experiment. The mean values of the limiting currents were significantly better using the instrumental rather than the manual polishing method. However, with respect to uncertainty of the experimental values, instrumental polishing gave the best estimates of the measured parameters; confidence intervals were very small for E1/2 and (E3/4–E1/4) potential measurements, being ±0.20 and ±0.36 mV for E1/2 values and 0.11 and 0.19 mV for (E3/4–E1/4) at 1 and 6 µm electrodes, respectively. The instrument designed for polishing microelectrodes is also described.

Spectroelectrochemical studies on surface immobilized cytochrome c on ITO electrode by slab optical waveguide spectroscopy

Abstract Direct electron transfer between surface-immobilized cytochrome c and an indium–tin-oxide (ITO) electrode was investigated using ITO slab optical waveguide (SOWG) spectroscopy. The peak position of the absorption band of cytochrome c immobilized on the ITO electrode changed repeatedly between 409 and 414 nm with the electrode potential sweep, which corresponded to the oxidized and reduced forms of cytochrome c , respectively. It took a few seconds to complete the electron transfer reaction for surface-immobilized cytochrome c of less than monolayer coverage. The redox potential of surface-immobilized cytochrome c was estimated to be approximately −0.2 V vs. Ag/AgCl from the absorbance intensity change with respect to potential applied to the ITO electrode. These results suggest that, after adsorption on the ITO electrode, cytochrome c was still electrochemically active.

Prism-coupled multimode waveguide refractometer

A new refractometer has been developed based on changes in the effective refractive index (RI) of the highest-order TE (or TM) mode in a prism-coupled multimode planar waveguide induced by interaction between an evanescent field and a liquid sample. The waveguide was a 100-mu;m -thick quartz plate fixed on a poly(methyl methacrylate) support containing a flow cell. A pair of prism couplers contacted the quartz plate in the flow-cell region. Such an optical sensor can detect the RI of liquid in a wide range by monitoring the resonant angle of the highest-order mode that changes order number with changes in the samples RI. When a highest-order mode corresponding to a given RI range is used as the sensor probe, a slight RI change in this range can be detected by measurement of the output light intensity. With this method the sensor was demonstrated to have a resolution of 3x10(-5) for the RI of an aqueous solution. Combining this result with theoretical calculation indicates that the sensor can detect a 0.5-nm-thick monolayer adsorbed from an aqueous solution. Therefore, the sensor is suitable for real-time detection of biomolecular interactions.

Mercury-free anodic stripping voltammetry of lead ions using a PVS-doped polyaniline modified glassy carbon electrode

An anodic stripping voltammetric determination of lead ions in laboratory prepared solutions is described utilizing a poly(vinylsulfonate)-doped polyaniline modified glassy carbon electrode. The experimental signals observed were similar to those of typical stripping methods and showed excellent linearity within the micromolar concentration range studied.

In situ observation of absorption spectra and adsorbed species of methylene blue on indium-tin-oxide electrode by slab optical waveguide spectroscopy

In situ absorption spectra of methylene blue (MB) adsorbed on an indium-tin-oxide electrode were observed by slab optical waveguide (SOWG) spectroscopy. The absorption spectrum of the MB obtained by the SOWG spectroscopy coincided with that by conventional spectroscopy for the solution phase, and the MB existed as dimer at 0 V vs. Ag/AgCl. Upon the electrode potential of cathodic condition, the absorption spectra gradually changed showing the appearance of the monomeric species as well as the highly aggregated species of MB.

Effect of ion pairing on steady-state voltammetric limiting currents at microelectrodes. Part I. Theoretical principles

Voltammetric studies in solutions of high resistivity are facilitated by the use of microelectrodes under steady-state conditions. Such solutions are encountered with solvents of low permittivity because of the very sparing solubility of electrolytes. Moreover, in such media the supporting electrolyte, as well as the electroactive ionic species, is usually extensively ion paired. Here we predict the limiting current that will flow in these circumstances, when a monovalent ion undergoes a one-electron transfer at a hemispherical microelectrode to form a neutral product. The ion pairing equilibria are assumed to be fast but all diffusion coefficients are treated as distinct. An analytical solution is elusive in the general case, but a simple numerical procedure allows the limiting current to be predicted for any combination of the system parameters. Several special cases are also discussed, some of which yield explicit formulae for the limiting current. In a companion paper, experimental data are compared with the theoretical predictions.

Effect of ion pairing on steady state voltammetric limiting currents at microelectrodes Part II. Experimental studies on charged (Br−, Ag+) and uncharged (copper diethyldithiocarbamate) species in toluene

Steady state voltammetric studies on the reduction and oxidation of uncharged copper diethyldithiocarbamate, the reduction of silver cations and the oxidation of bromide anions have been performed in toluene at various concentrations of electroactive species and supporting electrolyte (tetrahexylammonium hexafluorophosphate), utilising platinum inlaid microdisk working electrodes. In cases where the supporting electrolyte is in excess, the voltammetric waves are without too much distortion from IR drop, and the experimental results for the limiting current are consistent with extensive pairing of the electroactive ion with the counter ion of the supporting electrolyte, as well as between the supporting ions themselves. The range of support ratios that can be successfully studied is restricted by the solubility of the supporting electrolyte and by the severe ohmic polarization that is encountered when the ionic content is low. Nevertheless, it has been possible to measure limiting currents over sufficient ranges of electroactive and supporting electrolyte concentrations to demonstrate that the theory in Part I of this two-part sequence of articles is adequate to interpret the experimental findings.

Flow injection analysis of copper diethyldithiocarbamate in high resistance toluene media using a microelectrode detector

Abstract Flow injection (FI) analysis of copper as the copper diethylthiocarbamate complex (Cu(Et2dtc)2) has been performed in a high resistance toluene carrier stream. The method is based on the oxidation of the copper complex at a platinum microelectrode fabricated to fit a Metrohm wall-jet flow cell. Electrodes of sizes 5 to 25 μm radii were tested in toluene-based carriers containing either 0.03 M tetrahexylammonium perchlorate (Hex4NClO4) or 35% acetonitrile. Detection limits (3 : 1 signal to noise ratio) in the range 1 × 10−7 M (5 μm radius electrode) to 5 × 10−8 M (25 μm radius electrode) were achieved, which are comparable to detection limits reported in lower resistance media under similar conditions. The system was applied to the analysis of copper in water samples after extraction of the metal as its diethyldithiocarbamate into toluene and good agreement was found with data obtained by other voltammetric methods.

Chemically modified carbon paste electrode for the detection of lead, cadmium and zinc ions

Purpose This paper aims to develop an inexpensive, portable, sensitive and environmentally friendly electrochemical sensor to quantify trace metals. Design/methodology/approach A sensor was constructed by modifying carbon paste electrode for the determination of lead, cadmium and zinc ions using square wave anodic stripping voltammetry (SWASV). The modified electrode was prepared by inserting homogeneous mixture of 2-hydroxy-acetophenonethiosemicarbazone, graphite powder and mineral oil. Various important parameters controlling the performance of the sensor were investigated and optimized. Electrochemical behavior of modified electrode was characterized by cyclic voltammetry. Findings Modified carbon pastes electrodes showed three distinct peaks at −0.50, −0.76 and −1.02 V vs silver/silver chloride corresponding to the oxidation of lead, cadmium and zinc ions at the electrode surface, respectively. The highest peak currents for all the metal ions under study were observed in the phosphate buffer solution at pH 1 with a deposition time of 70 s. The sensor exhibited linear behavior in the range of 0.25-12.5 μg mL-1 for lead and cadmium and 0.25-10.0 μg mL−1 for zinc. The limit of detection was calculated as 78.81, 96.17 and 91.88 ng mL−1 for Pb2+, Cd2+and Zn2+, respectively. The modified electrode exhibited good stability and repeatability. Originality/value A chemically modified electrode with Schiff base was applied to determine the content of cadmium, lead and zinc ions in aqueous solutions using SWASV.