Trace Analysis by ESCA-Electrochemical Measurements

Abstract

A procedure which combines electrochemical deposition and ESCA measurements for the determination of trace levels of metals in solution is described. The method is demonstrated by an experiment in which microgram quantities of Pb, Cd, and Bi are deposited on a mercury-coated platinum electrode and then detected by ESCA measurements.     

Application of a new separation analyzer for the characterization of dispersions stabilized with clay derivatives

The performance of a newly developed separation analyzer is demonstrated by characterization of the sedimentation kinetics and the swelling behavior of organoclays in binary solvent mixtures (toluene/cyclohexane) as well as by investigation of oil-in-water and water-in-oil emulsions stabilized by clay derivatives. The automatic measuring system – LUMiFuge 114 – is based on a special centrifuge with an integrated optoelectronic sensor system which allows the measuring of spatial and temporal changes of light transmission during the rotation in a variable centrifugal field (12–1200g). By using the new device a fast and reproducible characterization of settling behavior and dispersion stability was possible. Moreover, the evaluation of the long-term stability of samples with high separation stability as well as of the heterogenity of the separation process, the sample transparency and the elasticity of the sediment as a function of alternating centrifugal force could be carried out. Received: 27 August 1999/Accepted in revised form: 24 November 1999     

Single-electron transfer upon formation of 2,2,3,3-tetracyano-1-phenyl-7,8-dithiabicyclo[3.2.1]octane from allyl dithiobenzoate and tetracyanoethylene. Electrochemical investigations

Studies by the method of cyclic potential scanning from 0.2 to 1.9 V provided electroanalytical evidence that the reaction of allyl dithiobenzoate with tetracyanoethylene (TCNE) in MeCN proceeds as the reaction of the TCNE^.− radical anion with the PhSSAll^.+ radical cation to form phenyl-substituted 2,2,3,3-tetracyano-7,8-dithiabicyclo[3.2.1]octane. When current is not applied, the reaction does not proceed at 20°C for 3 days. However, this reaction in boiling MeCN occurs without electrochemical activation and, apparently, involves intermediate formation of the above radical ions. It was established by semiempirical PM3 calculations that allyl dithiobenzoate and TCNE form a stable charge-transfer complex that precedes chemical electron transfer. Dedicated to the memory of Professor Viktor Nikolaevich Drozd. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 78–81, January, 1999.     

Rotating polarizer analyzer ellipsometer with a fixed compensator

Recently, a rotating polarizer analyzer ellipsometer (RPAE) in which the two optical elements rotate in opposite directions with the same angular speed has been proposed. Calculated optical constants of some materials obtained from this ellipsometric configuration were in good agreement with published values except in the low photon energy regime. We here improve the structure by adding a fixed compensator after the rotating polarizer. The optical constants of c-Si calculated from the noisy signal in the low energy regime show a high reduction in the percent error when compared to that obtained from the previous structure that does not contain the fixed compensator.     

Photodegradation of Ciprofloxacin,Clarithromycin and Trimethoprim: Influence of pH and Humic Acids

In view of the rising relevance of emerging pollutants in the environment, this work studies the photodegradation of three antibiotics, evaluating the effects of the pH of the medium and the concentration of dissolved organic matter. Simulated light (with a spectrum similar to that of natural sunlight) was applied to the antibiotics Ciprofloxacin (Cip), Clarithromycin (Cla) and Trimethoprim (Tri), at three different pH, and in the presence of different concentrations of humic acids. The sensitivity to light followed the sequence: Cip > Cla > Tri, which was inverse for the half-life (Tri > Cla > Cip). As the pH increased, the half-life generally decreased, except for Cla. Regarding the kinetic constant k, in the case of Cip and Tri it increased with the rise of pH, while decreased for Cla. The results corresponding to total organic carbon (TOC) indicate that the complete mineralization of the antibiotics was not achieved. The effect of humic acids was not marked, slightly increasing the degradation of Cip, and slightly decreasing it for Tri, while no effect was detected for Cla. These results may be relevant in terms of understanding the evolution of these antibiotics, especially when they reach different environmental compartments and receive sunlight radiation.     

Synthesis and electrochemical characterization of carbon nanotubes decorated with nickel nanoparticles for use as an electrochemical capacitor

Attachment of nickel nanoparticles on multiwalled carbon nanotubes (MWCNTs) was conducted to explore the influence of Ni loading on the electrochemical capacitance of MWCNT electrodes. A chemical impregnation leaded to homogeneously disperse Ni particles onto the surface of MWCNTs, and the Ni particles were found to be an average size of 30–50 nm. The capacitive behavior of the MWCNT electrodes was investigated in 6 M KOH, by using cyclic voltammetry (CV), charge–discharge cycling, and ac electrochemical impedance spectroscopy. CV measurements showed that the Faradaic current was found to increase with the Ni coverage, indicating that the presence of Ni would enhance the pseudocapacitance through the redox process. Equivalent circuit analysis indicated that both of electrical connection and charge transfer resistances accounted for the major proportion of the overall resistance and were found to decrease with the amount of nickel. A linearity relationship between the total capacitance and the Ni population reflected that each Ni particle exhibits an identical electrochemical activity in enhancing the electrochemical capacitance. The overall electrochemical capacitance (including double layer capacitance and pseudocapacitance) of Ni-MWCNT electrode can reach a maximum of 210 F/g over 500 cycles.     

Damage track detectors in radioprotection dosimetry: a novel approach

Limited sensitivity and unpredictable background are the major drawbacks of damage track detectors in the assessment of low neutron doses and low concentrations of alpha emitters in biological and environmental samples. The simplest way to increase the sensitivity of the damage track detectors is to increase both the exposure time and the detector area. However, the strong variability of the background may make this task often impossible. This background problem has been finally solved by a new registration method based on counting coincidence spots in geometrically matched pair of detectors. By using spark counting and electrochemical etching, both of which produce spots visible at low magnification, coincidences induced in two matched detector-surfaces by a few-microns-long tracks can be easily seen. This novel counting approach can be considered just the converse of those used in the past with Bi-fission detectors and cosmic ray stacks.     

Electron Transfer Reactions in Pyrazole-2-in-5-ones and Their Thioanalogues

Electron transfer processes in a series of pyrazole-2-in-5-ones (pyrazolones-5) and some of their thioanalogues are studied experimentally and theoretically for the first time. The electrochemical behavior of thiopyrazolones and oxygen analogues is found to differ. The effect the nature of the electrode, heteroatom, and substituents exerts on the redox potentials is established. Schemes of redox processes for the compounds studied in nonaqueous media are suggested. The schemes take account the ability of pyrazolones-5 to autoprotonation. Electrooxidation of 1-phenyl-3-methyl-4-benzoylpyrazole-2-in-5-one and its thioanalogues is shown to occur similarly to electroless oxidation with the formation of various dimers. It is established that, in the absence of active substituents during the electroreduction of compounds under study, the reaction center is the pyrazolone cycle, whose reduction does not involve decomposition of heterocyclic system.     

停流—荧光反应速率仪及其应用于测定维生素B1

本文利用国产930型荧光光度计作为检测器,采用自已制作的流通池,配以蠕动泵及记录仪,组装成停流-荧光动力学分析装置,并用以进行了停流荧光反应速率法测定维生素B_1的研究,利用Hg~(2+)离子在碱性介质中将硫胺素(维生素B_1)氧化为硫胺荧的动力学反应,测定了维生素B_1片剂中维生素B_1的含量。结果表明,这套仪器用于动力学分析,操作简便、省时,维生素B_1的测定结果与标准值基本相符,相对标准偏差为1.28％,其准确度和重现性均令人满意。