Luis Deban

Determination of nickel and cobalt in refined beet sugar by adsorptive cathodic stripping voltammetry without sample pretreatment

Abstract Nickel and cobalt have been analysed in refined beet sugar at the μg kg −1 level by adsorptive cathodic stripping voltammetry in the differential-pulse mode (DPCSV) at the hanging mercury drop electrode (HMDE) as their dimethylglyoxime complexes, M(DMG) 2 . Measurements were directly carried out on untreated sugar solutions. The performance of the procedure was compared with electrothermal atomic absorption spectrometry (ETAAS) applied to digested sugar samples, and showed in general better accuracy. The procedure was applied to the determination of these toxic elements in refined beet sugar samples, and concentrations below 50 μg kg −1 nickel and 10 μg kg −1 cobalt were found, which are much lower than the toxic levels of these elements.

Determination of Zinc, Cadmium and Lead in Untreated Sugar Samples by Anodic Stripping Voltammetry

Trace amounts of zinc, cadmium and lead were determined in refined beet sugar by differential-pulse anodic stripping voltammetry (DPASV) at a hanging mercury drop electrode. Measurements were performed on samples without previous treatment. The optimised procedure allowed the direct determination of metals at the m g kg-1 level. The performance of the procedure was compared with electrothermal atomic absorption spectrometry and DPASV applied to digested samples, and showed better accuracy. The procedure was applied to the determination of toxic elements in commercial beet sugar samples and levels of metals below 35 m g kg-1 Pb, 80 m g kg-1 Zn and 10 m g kg-1 Cd were found.

Determination of mercury in refined beet sugar by anodic stripping voltammetry without sample pretreatment

Abstract Total mercury has been analysed in refined beet sugar at the μg kg−1 level by differential-pulse anodic stripping voltammetry (DPASV) at a rotating gold disk electrode (RGDE). DPASV measurements of mercury are based on its accumulation onto the RGDE as a Hg–Au amalgam and followed by the reoxidation of Hg(0) to Hg(II). Measurements were directly made on untreated sugar solutions. The DPASV procedure was compared with cold-vapour atomic absorption spectrometry carried out at a flow-injection mercury system when applied to digested sugar samples. The DPASV method has been demonstrated to perform conveniently for metal determination in beet sugar of good quality (low metal concentration) produced by Spanish sugar refineries. Mercury concentrations below 5 μg kg−1 were found in all commercial beet sugar samples analysed, which are much lower that the maximum permissible contents derived from the provisional tolerable weekly intake recommended by the World Health Organization.

Modification of the methylene blue method for spectrophotometric selenium determination

The spectrophotometric method for Se(IV) determination based on its catalytic effect in the reduction of Methylene Blue (MB) by sulphide is modified. The variables that affect the decolorization of MB were taken into account: reagent concentrations, order of addition, mixing and standing times, pH, ionic strength, temperature, solution volume, wavelength, etc. The results of this study allowed a decrease of the determination limit and, by selection of the appropriate analytical conditions, choice of optimum linear range according to the selenium content in the sample. The lower limit range is 15-75 mug/1., with 3% relative standard deviation and no systematic errors. Procedures for overcoming several potential interferences were studied. The proposed method was applied to several environmental samples and the results were compared with those obtained by other standard methods.

IDENTIFICATION OF DIFFERENT PRODUCTS OBTAINED BY ELECTROCHEMICAL AND PHOTOCHEMICAL REDUCTION OF THE HERBICIDE METAMITRON

The electrochemical reduction of the herbicide Metamitron (4-amino-4,5-dihydro-3-methyl-6-phenyl-1,2,4-triazin-5-one) has been carried out at different pHs using a mercury pool cathode. The final product of these reductions depends both on the electrolysis potential and on the pH. All processes are irreversible and the reductions of the CN and the NNH2 (pH > 3) or NNH+3 bond (pH < 3) are inferred. An aqueous solution of Metamitron is also degraded by sunlight leading to 4,5-dihydro-3-methyl-6-phenyl-1,2,4-triazin-5-one (deaminometamitron). This reaction can be monitored polarographically.

Chemical and electrochemical behaviour of lead ions in the ZnCl22NaCl mixture at 450 °C

Abstract The stability of lead chloride and oxide as well as the electrochemical behaviour of Pb 2+ ions in the ZnCl 2 2NaCl mixture at 450 °C has been studied using the techniques of potentiometry, cyclic voltammetry, chronoamperometry and chronopotentiometry. The lead oxidation states 0 and II have been shown to exist in this melt, showing that Pb(IV) is a powerful oxidizing agent which oxidizes the chloride ion of the melt according to the reaction: Pb ( IV ) + 2 Cl − ↔ Pb ( II ) + Cl 2 ( g ) The standard potential of the redox couple Pb(II) Pb(0) has been determined by potentiometry ( E 0 Pb(II) Pb(0) = −1.383 ± 0.01 rmV) (vs. Cl 2 (1 atm ) Cl − ). It was shown that PbO is not stable and enters acid-base reaction with the melt according to the reaction: Zn ( II ) + PbO ( s ) ↔ ZnO ( s ) + Pb ( II ) The solubility products of lead oxide have been determined ( pKs = − 2.6 ± 0.1 on a molality scale). All these results have enabled the construction of the E - pO 2− equilibrium diagram. Using a tungsten electrode, it has been observed that the Pb(II)//Pb(0) exchange is quasi-reversible; log k s 0 and α values for this reaction are (−4.3 ± 0.1) and (0.48 ± 0.06) respectively. Mass transport towards the electrode is a simple diffusion process, the average diffusion coefficient D pb ( II ) is (7.5 ± 0.7) × 10 −6 , and the thickness of the diffusion layer δ by thermal convection is (7.5 ± 0.2) × 10 −3 cm.

Inorganic arsenic speciation by differential pulse anodic stripping voltammetry using thoria nanoparticles-carbon paste electrodes

Two novel thoria (ThO2) nanoparticles-carbon paste electrodes were used to evaluate an anodic stripping voltammetric method for the direct determination of arsenite and total inorganic arsenic (arsenite plus arsenate) in water samples. The effect of Ag((I)), Cu((II)), Hg((II)), Sb((III)) and Se((IV)) ions on the electrochemical response of arsenic was assayed. The developed electroanalytical method offers a rapid procedure with improved analytical characteristics including good repeatability (3.4%) at low As((III)) concentrations, high selectivity, lower detection limit (0.1 μg L(-1)) and high sensitivity (0.54 μA μg(-1) L). The analytical capability of the optimized method was demonstrated by the determination of arsenic in certified reference materials (trace elements in natural water, trace elements in water and coal fly ash).

Electrochemical determination of the effect of lead(II) on the photochemical degradation of the pesticide metamitron

The complexation of the pesticide Metamitron by lead(II) was investigated by polarography and stripping voltammetry. The influence of the pH on the complexation was determined and a maximum value for the conditional stability constant of the lead‐metamitron complex (logβ’ = 3.1) was obtained at pH 5.6. An investigation into the photochemical degradation of the pesticide demonstrated that lead decreases the degradation rate of metamitron by 13% within the first 30 days, due to stabilisation by complexation.

Determination of arsenic by ICP-MS after retention on thoria nanoparticles

A procedure for arsenic determination by ICP-MS in environmental and biological samples was developed using thoria nanoparticles for arsenic retention. Both inorganic arsenic species, As(III) and As(V), show similar behaviour regarding this thoria nanosorbent. The experimental data related to the amount of arsenic retained per gram of nanoparticles were theoretically modelled using mechanistic and empiric equations. The best kinetic theoretical relationship allowed us to obtain a feasible quantification of the retention process before reaching steady-state. In addition, calculation of the overall retention constant was also possible at steady-state. The maximum effective retention capacity for arsenic was about 30 mg As per g nanomaterial. The detection limit of the developed procedure was 0.07 μg As L−1 and the relative standard deviation 2.9%. The accuracy of the developed procedure was assayed by the determination of arsenic in two certified reference materials.

Determination of Pb(II) with a dithizone-modified carbon paste electrode.

A dithizone (DTZ) modified carbon paste electrode was developed for the sensitive and selective determination of Pb(II) using differential pulse anodic stripping voltammetry as well in batch as in FIA system. The analysis procedure is based on an open circuit accumulation step in a stirred sample solution. This was followed by a medium exchange to a clean solution and subsequently by a proper anodic stripping. The analytical performance was evaluated with respect to the quantity of modifier in the paste, accumulation time, background electrolyte, Pb(II) concentration and other variables. When the accumulation time applied was 5 minutes, linear calibration graphs were obtained in the concentration range 1 × 10−7–1 × 10−5 M and 8 × 10−8–5 × 10−6 M for batch and Flow Injection Analysis (FIA), respectively. The detection limits found were 8.65 × 10−8 M in batch and 4.45 × 10−8 M in FIA. A convenient and rapid renewal of electrode surface allows the use of a single modified electrode surface in multiple analytical determinations. Several coexisting metals ions such as Cd(II), Hg(II), Cu(II) and Zn(II) had no interference on the determination of Pb(II). The proposed method was applied in the determination of lead in soils located in the vicinity of metallurgic transformation industry. The results obtained were in accordance to the ones supplied by Atomic Absorption Spectroscopy (AAS).