Wojciech Jedral

Application of palladium- and rhodium-plating of the graphite furnace in electrothermal atomic absorption spectrometry

Palladium- and Rh-plating of the graphite furnace has been evaluated as a method of introducing the metallic form of Pd and Rh for chemical modification in electrothermal atomic absorption spectrometry. It is shown that by electroplating Pd and Rh onto the inner surface of the tube, the pre-treated graphite surface may resemble the behaviour of the corresponding modifier. The resulting metallic layer is very effective in inhibiting the loss of volatile elements (e.g., As and Se), as well as reducing the influence of oxide and carbide formation (e.g., Si). The advantage of the proposed procedure of introduction of a solid chemical modifier is that the pre-treated tubes exhibit an extended analytical lifetime for the determination of As and Se, up to 80 and 160 firings in the presence of Pd and Rh, respectively. In the case of Si the Rh-plated graphite tube could last for about 100 firings.

Electrochemical and Raman spectroscopic studies of the influence of chlorinated solvents on the corrosion behaviour of iron in borate buffer and in simulated groundwater

The remediation, by contact with granular iron, of groundwater contaminated with chlorinated halocarbons necessitates a flow of electrons at the iron/solution interface. To refine our understanding of the mechanism and kinetics of the charge transfer process, electrochemical and spectroscopic measurements were performed on iron electrodes in borate buffer and in simulated groundwater solutions of calcium carbonate and potassium bromide, before and after exposure to carbon tetrachloride. The results of these measurements highlighted the combined influence of the organic contaminant and inorganic ions on the corrosion behaviour of iron as well as on the nature of the films formed in their presence.

Open ablation cell for LA-ICP-MS investigations of historic objects

Laser ablation (LA) of historic objects (glass and ceramic) as well as reference materials (NIST610 and SRM679) was executed in standard closed or home made open ablation cells of different intrinsic volume (c.a. 60 cm3 and 9 cm3 or 4.5 cm3 respectively). Comparative investigations with different ablation cells were carried out with argon as the carrier gas and registration of transient signals during the subsequent multielemental analysis by means of inductively coupled plasma mass spectrometry (ICP-MS). Open cells allowed direct ablation from surface of historic objects and demonstrated suitability for mapping and quantitative elemental analysis. Resolution of created maps showing distribution of selected elements was improved by using open cells for which the rinse out time of aerosol was noticeably shortened (by a factor of 3) compared to the standard cell. The accuracy of quantitative elemental results obtained for ceramic objects was tested using SRM679 Brick Clay in the form of pressed pellet and was found to be satisfactory. It was demonstrated that the applicability of LA-ICP-MS in archaeometry as well as conservation science could be enhanced by popularization of open cells for the direct ablation from objects, which are too big to fit to standard cells and require strict limitation of sampling.

Secondary ion mass spectrometry for characterizing antimony, arsenic and selenium on graphite surfaces modified with noble metals and used for hydride generation atomic absorption spectrometry

The surface and sub-surface distribution of noble metals (after electrodeposition of 600 μg or thermal reduction of 10 μg as modifiers), as well as Sb, As and Se (200 ng) as analytes after their deposition on the graphite surface was investigated using secondary ion mass spectrometry (SIMS) in the dynamic mode. This permitted simultaneous observation of the depth profile distribution of modifier and analyte with a depth resolution of down to approximately 25 nm, limited however, by the surface roughness of the samples. Hydride generation was intentionally used for this purpose because in this approach the investigated system: graphite–modifier with added analyte is free from matrix components. This was essential for the evaluation of this novel approach using SIMS for surface analysis. Investigations concerning the distribution of analytes were performed on the graphite surface modified with palladium, iridium or rhodium. It was found that after deposition at 400 °C, all analytes partially penetrated the graphite surface and their distribution overlaps the distribution pattern of the noble metals. The degree of penetration differs for each analyte and depends on the modifier used.

Oxidation of ascorbic acid by copper(II) and the ferrocenium ion in acetonitrile–water mixtures

Measurements are reported on the rates of oxidation of ascorbic acid (H2A) by copper(II) and the ferrocenium cation [Fe(C5H5)2]+ in mixtures of water and acetonitrile. The stabilisation of Cu+ by acetonitrile results in a rapid increase in E⊖ for Cu2+–Cu+ and a simple, irreversible reaction between copper(II) and ascorbic acid to form copper(I) and dehydroascorbic acid upon addition of small amounts of acetonitrile to water. The acid dependence of the rate constant is consistent with the involvement of complexes of the monoanion, [CuII(HA)]+, and the dianion [CuIIA], the former becoming relatively more important with increasing acetonitrile content of the solvent. The solvent dependence of the rate constant is discussed in relation to the solvation of copper(II) and copper(I) in the mixtures and it is suggested that the dominant influence is the solvation of copper(I) formed in the initial electron-transfer process. Redox potentials, E for [Fe(C5H5)2]+–[Fe(C5H5)2], and rates of oxidation of ascorbic acid by ferrocenium, a typical outer-sphere oxidant, have been measured under the same conditions and are compared with those of the copper(II)–copper(I) system.

Square-wave amperometric monitoring of reaction rates

The application of square-wave amperometry, an analytical technique involving the use of two indicator electrodes polarized by a square wave, in the determination of reaction rates is described. Kinetic measurements have been made on the bromination of anisole in aqueous solution. Square-wave amperometry in conjunction with a stopped-flow apparatus equipped with Pt electrodes in the observation tube was used to monitor the disappearance of bromine in dilute solutions. Reaction rates with half-lives down to ca. 5 ms can be measured using relatively high-frequency square waves (ca. 800–1000 Hz). The results are in good agreement with values measured spectrophotometrically at higher bromine concentrations. The rate of dissociation of Ag+ from its macrobicyclic cryptand 211 complex was also measured, using a pair of silver indicator electrodes. Free Ag+ may be monitored selectively in the presence of its cryptate complex by using a low-amplitude square wave (± 150 mV). The advantages and limitations of the amperometric technique for kinetic measurements are discussed.

Improved electronic equipment for square-wave amperometry

Abstract Improved electronic equipment for square-wave amperometry suitable for both analytical and kinetic applications is described. The equipment consists of a polarizing unit for generation of the symmetrical square-wave polarizing potential and a measuring circuit which involves integration of the cell signal and output to a sample-and-hold unit. The operation of the polarizing unit and the cell measuring circuits are controlled by a logic unit which generates appropriate time intervals corresponding to the following: the frequency of the polarizing potential, a waiting time between the beginning of each square-wave cycle and sampling of the cell current to allow for decay of the capacitance current, and time intervals for the integration of the cell current, and the output of the contents of the integrator circuit to a sample-and-hold unit. An important feature of the equipment is the inclusion of iR -drop compensation. This is particularly advantageous for measurements in solution of low ionic strength, and for measurements on more concentrated solutions (> 10 −4 M reactant).

The effect of platinum(IV) on the coulometric generation of tin(II)

The current efficiency of tin(II) generation has been measured for various electrolytes and electrodes. The best results (>99.9%) were obtained with the paraffin-impregnated graphite electrode in 2M CaCl(2) + 0.2M HCl + 0.2M SnCl(4). In titrations of platinum(IV) with electrogenerated tin(II) in an electrolyte containing chloride or bromide it was found that the presence of platinum(II) or (IV) interferes in the cathodic generation of tin(II). The platinum is reduced to the elemental state at the electrode and causes simultaneous generation of hydrogen which decreases the current efficiency. This effect is more pronounced in chloride media.

Biamperometric determination of platinum in some technical products

A method is described for the determination of platinum based on the reduction of platinum (IV) to platinum (II) with an excess of copper (I) chloride and titration of the resulting solutions with a strong oxidant, such as cerium (IV). The reaction proceeds in two steps, the first corresponding to oxidation of an excess of copper (I), and the second to oxidation of platinum (II). The electrochemical characteristics of the systems involved appear to be ideal for biamperometric titration. Up to the first end-point the reversible systems copper (II)-copper(I) is present, after which the next system, platinum(IV)-platinum (II), formed in the course of further additions of the titrant, is strongly irreversible. An excess of the oxidant in acidic solution forms a reversible cerium (IV)-cerium (III) couple. In these systems a double bend titration curve is formed when a small voltage (100 mV) is applied. The only standardised solution required is cerium (IV) sulphate and removal of oxygen is, in general, unnecessary. The reaction is much faster at elevated temperatures and it is therefore convenient to warm the solution up to 80 °C before titration.Depending on the concentration of the titrant (0·1 or 0·01 M) an amount of platinum from 5 to 50 mg can be determined with a relative standard deviation of about 0·14 per cent. This method has been applied to the determination of platinum in platinum-nickel alloys and platinum catalyst.

Determination of the silver error in the coulometric titration of acids in various media

The dependence of the silver error in coulometric titrations of acids in a one-compartment cell on the solubility of silver halide has been established. The silver error is proportional to the total solubility of silver and inversely dependent on the current density. Because the main contribution to the total titration error is given by the silver error, it is necessary to choose conditions that produce the minimum value. The silver error is below 0.1% when the current density of the cathodic process is at least 10 mA cm–2. This relationship was tested in solutions of chloride, bromide and iodide in water, methanol, ethanol and acetone. For the aqueous solutions the bromide electrolyte is recommended and, for methanolic solutions, the best results are obtained with chloride ions.