Carlo Bragato

Determination of Lead and Copper in Wine by Anodic Stripping Voltammetry With Mercury Microelectrodes: Assessment of the Influence of Sample Pretreatment Procedures

The results obtained using different pretreatment procedures for determining total lead and copper in wine by anodic stripping voltammetry were compared. The measurements were performed by employing mercury microelectrodes as the working electrode and linear sweep voltammetry as the voltammetric technique in the oxidation step. The pretreatments considered were (A) wine plus concentrated HCl to pH 1.5; (B) wine plus 30% m/m H 2 O 2 (2.5:1 v/v) and UV irradiation for 2 h; and (C) wine plus concentrated HCl to pH 1.5 and UV irradiation for 4 h. The three procedures gave different results, and the efficiency of the different procedures in releasing the cations from inorganic and organic complexes can be ranked as B > C > A. Moreover, data obtained with procedure B agreed within 15% with the values found by AAS measurements. For the quantification of the metal ions, both the multiple standard addition method, based on current or charge versus concentration graphs, and an absolute method, based on the charge involved in the stripping peak, were employed. The latter procedure allowed measurements to be carried out even when the calibration plots were not linear. When all the quantitative methods could be applied, data agreement within 5% was found.

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.

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.

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%.

Steady state voltammetry in the process of hydrogen evolution in buffer solutions

Abstract The process of hydrogen evolution occurring at a platinum microelectrode was studied in buffer solutions made by either mono- or polyprotic acids. Buffers formed either by mixing equal amounts of the weak acid and its conjugate base, or an excess of acid or base were considered. The results demonstrated that the steady state limiting current, I l , is related to the amount of both free proton and undissociated acid present in the mixture, even if the pH of the solution is constant or changes to a negligible extent. Linear trends, between the steady-state limiting current and the analytical concentration of the weak acid, C A , present in the mixture, was found in the buffers, while comparative measurements carried out in solutions containing the weak acids alone, showed that the plots are not linear. Theoretical relationships relating I l and C A directly were also derived, and they allowed to rationalise the fact that all the buffers formed from a given monoprotic acid display plots having a common slope, while the buffers obtained from a polyprotic acid may show plots with different slopes.

A steady-state voltammetric investigation on the oxidation of ferrocene in ethanol–water mixtures

Abstract The oxidation process of ferrocene was studied in ethanol–water mixtures, under steady-state conditions, with platinum and carbon microdisk electrodes. The electrode process was found to be essentially reversible in mixtures up to 60 vol.% water. In media with a larger water content, a weak adsorption of the reactant was involved. Half-wave potentials determined with both microelectrodes, quoted against an aqueous Ag|AgCl (KCl saturated) reference electrode, shifted towards less positive values as the water content was increased. In contrast, the half-wave potential referenced against the cobaltocenium|cobaltocene system was calculated to be −1.318 (±0.005) V, independent of the medium composition. The limiting current, at a given concentration of ferrocene, strongly depended on the medium composition and mainly reflected the change of viscosity of the media. The overall results allowed us to conclude that ferrocene can be used as a voltammetric standard in the mixtures examined.

Stability of mercury-coated platinum microelectrodes upon touching a solid surface in scanning electrochemical microscopy (SECM) experiments

The features of experimental approach curves, recorded in scanning electrochemical microscopy measurements, and acquired with a series of sphere-cap tips while approaching a solid insulating substrate, are examined in detail. Sphere-caps are prepared by electrodeposition of liquid mercury onto a platinum microdisk substrate 10 μm radius, and having RG=R/a=10 (R is total tip dimension, which includes the shield thickness and the electrode radius a). From the data obtained, it is established that sphere caps with aspect ratio h/a>0.3 (h is the sphere cap height) are able to touch, or even be squeezed against the surface of the solid substrate. Under these latter conditions, for the sphere caps, the negative feedback process at low tip–substrate distances, is more pronounced than that of the corresponding naked microdisk. A good tip stability and no mercury loss is observed upon touching the substrate for sphere caps with h/a<1.3. This circumstance allows one to perform “contact” voltammetric measurements of lead ions adsorbed onto a thin layer chromatography plate.

Square wave voltammetry of strong acids at platinum microelectrodes

Abstract The hydrogen evolution reaction 2H + +2e=H 2 has been studied by square wave voltammetry (SWV) with a Pt microdisk 12.5 μm radius, in HClO 4 solutions over the concentration range 0.01–10 mM. The SW measurements have been performed at low frequencies, over the range 0.5–60 Hz, where non-planar diffusion prevails at the microelectrode. Under these conditions, as expected, the forward and the backward current profiles are sigmoidal, whereas the net current profile is peak shaped. The analysis of typical square wave parameters as a function of frequency ( f ) has shown that peak potential ( E p ) and width at half height ( W 1/2 ) stay constant, while the dimensionless peak current ( ψ ) depends linearly on the parameter θ 1/2 (θ=4Dτ/a 2 ,τ=1/f) , as expected for a reversible electrode process. W 1/2 values lower than that expected for a 1:1 stoichiometry of the electrode process, and a shift of E p by 30 mV towards less negative values for a 10-fold increase of acid concentration have been explained as due to the 2:1 stoichiometry that characterises the electrode process investigated here. The net peak current has been found to depend linearly on acid concentration over the entire range investigated.

Steady-state voltammetry for the reduction of labile metal complexes in the absence and presence of different concentrations of supporting electrolyte

Abstract Steady-state voltammetry for the reduction of labile complexes of lead, cadmium and mercury, in the absence of deliberately added supporting electrolyte and with different concentrations of supporting electrolyte, has been studied at mercury (lead and cadmium) and platinum disc (mercury) microelectrodes. The effects of ionic strength and nature of an inert electrolyte on the migrational component, which usually adds to diffusion in the mass transport in solutions with low electrolyte, have been examined. The results obtained have demonstrated that the ratio of limiting current at different concentrations of supporting electrolyte to diffusive limiting current I lm / I ld , depended not only on the charge of the free ion, but also on the amount of labile complexes formed between the metal ion with ligands which may arise from the counter-ion accompanying the metal in its starting salt, from small amounts of supporting electrolyte and from the autoprotolysis of water, i.e., OH − . Moreover, the I lm / I ld ratio is affected by the background electrolyte, which is almost unavoidable in water, and by the hydrolysis of the metal ion which yields protons. A comprehensive theoretical equation, based on already known treatments, has been also derived to predict the limiting current ratios in the presence of all the effects cited above.

A novel approach for the determination of the total concentration of acids in aqueous solutions by simultaneous diffusion limited current for reduction of acids and pH measurements

Abstract A novel method is presented for the detection of acid content in aqueous solutions. The method is based on simultaneous measurements, in the test solution, of the overall steady-state limiting current arising from the reduction of the acids, and pH, along with the use of a theoretical expression. The current is recorded with platinum microelectrodes, while pH is measured with glass electrodes. To relate current and pH data, an “amperometric calibration” of the glass electrodes is performed. Calibration equations are obtained from a series of combined current/pH measurements in solutions containing a range of concentrations of the strong acid HClO4. This procedure overcomes the problems associated with the unknown activity coefficient of H+ and liquid junction potential contributions involved in the pH measurements. The new methodology is tested in several aqueous solutions containing mono- and polyprotic acids either alone or in mixture; buffered solutions are also considered. It is shown that, under the latter conditions, special cases exist, in which the total acid concentration is achieved by overall steady-state limiting current measurements alone. The accuracy of the proposed method is tested against classical titration procedures. The discrepancies between the two methods do not exceed ∼5%.