Showing papers on "Polarography published in 1994"

Polarographic oxygen electrodes and their use in plant aeration studies

Abstract: The electrolytic reduction of oxygen which occurs at a wetted ‘unattackable’ cathodic electrode of platinum or gold when polarised in conjunction with a Ag/AgCl anode, forms the basis of most polarographic oxygen measurements in plant biological work.Various types of polarographic electrode and their uses are reviewed. These include cylindrical sleeving electrodes for quantifying localised oxygen fluxes from intact roots, ‘bare’, membrane-coated, and Clark-type microelectrodes suitable for measuring concentrations and profiles both inside roots and in the rhizosphere, and macro-Clark electrodes most frequently used in respiratory studies.Some details concerning equipment and electrode construction are given and some pitfalls in electrode application are discussed.

Electrochemical determination of ultralow levels (<10−12 M) of mercury by anodic stripping voltammetry using a chemically modified electrode

Abstract: A glassy carbon electrode (GCE) coated with 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane, Kryptofix-222, has been used for the very sensitive and selective analysis of very low levels of mercury. A detection limit lower than 10−12 M (<0.2 ppt) was obtained applying anodic stripping voltammetry (ASV). The parameters that governed the sensitivity and possible interferences by other ions have been examined in detail. The electrode has been successfully applied for determining mercury in seawater, wastewater, and also in human saliva and amniotic fluid. The modified electrode exhibited high stability and, therefore, could be used repeatedly without mechanical regeneration.

Study of the Redox Behavior of Anthraquinone in Aqueous Medium

Abstract: The electrochemical reduction mechanism of anthraquinone (AQ) has been studied in N,N-dimethyl formamide (DMF) and/or its water mixtures by means direct current polarography (DCP) and differential pulse polarography (DPP). The polarograms consisted of two well-defined waves in anhydrous DMF. The presence of small amounts of water does not effect the first wave but causes the second wave to shift toward more positive potentials; large amounts of water (solutions with at least 40% water) give a single wave. Half-wave potentials for the one-electron step in DMF are more negative than those in aqueous solutions and have been used to calculate the disproportionation constant, K D , of the radical anion in each solution

Adsorption voltammetric techniques for the determination of uranium(VI) with 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone as complex forming reagent

Abstract: Uranium (VI) can be determined by adsorptive voltammetric techniques, as its chloranilic acid complex, over a wide concentration range. Differential pulse polarography is useful for quantification of uranium between 0.1 and 1.5 mg/l; for the range from 10 to 500 μg/l differential pulse voltammetry and for ultra-trace analysis between 0.024 and 40 μg/l adsorptive stripping voltammetry are preferred. The standard deviation for the 3σ-detection limit of 24 ng/l uranium was found to be 8%. In the trace analysis of metals in aquatic environmental systems by adsorptive stripping voltammetry it is normally necessary to decompose polluted water samples by UV irradiation or microwave digestion. The advantage of the developed method is the fact that no sample pretreatment is necessary.

Hexazinone: polarographic reduction and adsorption on lignin.

Abstract: Hexazinone [3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4(1H,3H-dione] (I) is reduced in acidic media at the dropping mercury electrode in two two-electron steps. The first step corresponds to a reduction of a protonated azomethine bond which is complicated at pH 2-4 by the establishment of a hydration-dehydration equilibrium. Measurement of the first wave at pH 3.7 is suitable for analytical purposes and was used for following the adsorption of hexazinone on lignin. In comparison with acifluorfen, thiram, and DCNA, which had been studied earlier, hexazinone is less strongly adsorbed and is rapidly desorbed

Review: interactions of metal ions with nucleic acids and their subunits. an electrochemical approach

Abstract: Different electrochemical methods (differential pulse polarography, sweep voltammetry, alternating current voltammetry, chronocoulometry, cyclic voltammetry) can be used to investigate the interactions of metal ions such as osmium tetroxide, platinum complexes, Cu(II), Ni(II), Zn(II), Cd(II), Pb(II) and Eu(III) with DNA, t-RNA and their subunits. Modern electrochemical methods can detect very small perturbations in a double-helical structure as well as the behavior of single stranded (denatured) fragments of nucleic acids. The effectiveness of polarographic methods is comparable to enzymatic techniques which use specific nucleases. The polarographic data can also be used for the quantitative evaluation of the metal ion nucleic acid interactions (e.g., to calculate the association or stability constants and the number of binding sites). Some topics concerning these problems as well as general aspects of metal-nucleic acid interactions are discussed in this review.

Electrochemical determination of carbaryl oxidation in natural water and soil samples

Abstract: An electrochemical method for the determination of carbaryl, after prior oxidation to 1,4-naphthoquinone in natural water and soils is reported. The coulometric oxidation of carbaryl at a platinum electrode was studied using 0.024 mol/L Britton-Robinson buffer (pH 7.0). The reduction of the oxidation product 1,4-naphthoquinone at a dropping mercury electrode was used for the indirect determination of carbaryl after separation on C18 Sep-pak cartridges by differential pulse polarography (detection limits: 0.41 mg L−1 of water and 0.47 mg kg−1 of soil) and directly without separation by adsorptive stripping voltammetry (detection limits: 5 μg L−1 of water and 7 μg kg−1 of soil, for 75 s preconcentration time). Relative errors were lower than 3.7% and relative standard deviations smaller than 4.5%.

Differential-pulse cathodic stripping voltammetric determination of sodium nitroprusside at a hanging mercury drop electrode aided by copper(II) and poly-L-lysine modification

Abstract: The nitroprusside anion, Fe(CN)5NO2–, can be accumulated from pH 5 Britton–Robinson buffer solution on to the surface of a hanging mercury drop electrode (HMDE), which has been modified by means of adsorbed poly-L-lysine and which is held on open circuit. This was shown by closing the circuit at –0.2 V versus the Ag–AgCl electrode, after accumulation, and scanning cathodically by use of differential-pulse voltammetry. The voltammetric peak that was observed at about –0.6 V, however, was that for the adsorbed product of the first reduction step of nitroprusside, reported to be Fe(CN)4NO2–, which is the product of the one-electron reduction of the nitroprusside ion [to Fe(CN)5NO3–] followed by the elimination of CN–. When accumulation was carried out at the peak potential of the first polarographic reduction peak of nitroprusside (–0.35 V), the peak current at –0.6 V was enhanced 4-fold, which indicated that the Fe(CN)4NO2– was accumulated on poly-L-lysine more extensively than was the nitroprusside ion. From pH 2 (0.01 mol dm–3 hydrochloric acid) solution, nitroprusside can be accumulated very efficiently between 0 and –0.1 V at an unmodified HMDE in the presence of excess of copper(II). The cathodic stripping peak at about –0.35 V is due to the reduction of an adsorbed copper/reduced nitroprusside species, which has not been fully identified, but is most probably either the copper(I) or copper(II) salt of Fe(CN)4NO2–. This signal was enhanced slightly when accumulation was effected at a poly-L-lysine-modified HMDE. Linear calibration graphs were obtained for up to at least 1 × 10–7 mol dm–3 for the determination of nitroprusside at both pH 5 and pH 2, by using cathodic stripping voltammetric methods based on the above electrode processes, and the limit of detection was calculated to be about 5 × 10–10 mol dm–3 in both instances. Relative standard deviations were typically <2% at the 1 × 10–8 mol dm–3 level.

Electroanalytical study of the antioxidant tert-butylhydroquinone (TBHQ) in an oil-in-water emulsified medium

Abstract: An electroanalytical study of the oxidation of tert-butylhydroquinone (TBHQ) by differential pulse polarography (DPP) and square-wave voltammetry (SWV) in an emulsified medium formed with ethyl acetate and Pluronic F-68 is carried out. The optimization of the chemical variables of the emulsions by DPP allowed the choice of the following working conditions: pH 9.0, 0.04% Pluronic F-68. 2.0 mL of ethyl acetate and 3 minutes as ultrasonic stirring time. The demonstrated reversibility of the electrode process at the mercury electrode makes it suitable for the application of SWV. The influence of the square-wave (SW) characteristic parameters on the TBHQ net peak current at the hanging mercurv drop electrode (HMDE) was tested. Calibration graphs were obtained within the ranges 1.0 X 10" to 1.0 X lo-', 1.0 X mol L-'. An improved sensitivity with respect to that obtained by DPP was produced. The limit of detection was found to be 6.2 X mol L-'. Interferences from tevt-butylhydroxyanisole, tert-butylhydroxytoluene, propylene glycol, citric acid, propyl gallate, and ascorbic acid were tested. The proposed method was applied with good results to the determination of Tl3HQ in spiked popcorn samples by direct emulsification of the ethyl acetate sample extract.

Polarographic behaviour and determination of paracetamol and salicylamide after treatment with nitrous acid

Abstract: A polarographic procedure is described for the determination of paracetamol (acetaminophen) and salicylamide after treatment with nitrous acid. The different experimental parameters affecting the derivatization process and the polarographic analysis were studied. The derivatization products were found to be reduced at the dropping mercury electrode over the whole pH range in Britton-Robinson buffers. At pH 7.0, well defined diffusion-controlled cathodic waves were produced for both compounds. Plots of limiting current vs. concentration were linear over the ranges 0.05–0.75 and 0.25–1.5mM for paracetamol and salicylamide, respectively, in the d.c. mode, with minimum detectability of 2.5 × 10−6 and 1.25 × 10−5 M, respectively. The procedure was applied to the analysis of some pharmaceutical dosage forms and the results were in good agreement with those obtained by official and compendial methods.

Electrochemical reduction behavior and analysis of some organophosphorous pesticides

Abstract: The electrochemical reduction behavior of organophosphorous pesticides such as monocrotophos, phosphamidon, and dichlorvos have been studied by employing dc polarography, cyclic voltammetry, ac polarography, differential pulse polarography miilicoulometry, and controlled potential electrolysis The general reduction mechanism of these compounds has been suggested based on the results obtained A differential pulse polarographic method is developed for the determination of title compounds in agricultural formulations Both standard addition and calibration methods are used for the analysis experiments The lower detection limits are found to be 10 × 10−8, 125 × 10−8 and 12 × 10−8 respectively

Application of the Multiple Linear Regression Method to Differential Pulse Polarography

Abstract: The Multiple Linear Regression method has been used for treatment of the highly overlapped peaks obtained from the binary mixtures Cd-Ni and the ternary mixture Cd-Ni-Cu in KSCN 1 M and pH = 4.5 (Britton-Robinson buffer) by Differential Pulse Polarography. Metal concentrations were within the range of 6.8 to 33.2 μ.M for Cadmium and of 6.4 30.4 μM for Nickel and Copper. The obtained results prove that the hard multivariate calibration models such as Multiple Linear Regression can be applied in Differential Pulse Polarography when the baseline is subtracted; in addition, both the fitting parameters and relative errors are much better when the smoothed polarograms are used.

Polarographic behavior of the β-lactam antibiotic Cefuroxime and study of the reduction mechanism in acidic media

Abstract: Polarographic behavior of the cephalosporin Cefuroxime (CFXN) in aqueous solutions of PH ranging from 1 to 10 was investigated. The reactant is strongly adsorbed in acidic media, while in neutral and alkaline solution, electrode reaction is not affected by adsorption phenomena. A kinetic study of the methoxyimino group electroreduction to methanol and an aminic derivative in acidic media was carried out. The overall electrode process is controlled by the heterogeneous protonation step preceded by a fast electronation step in the pH range 1 to 3, whereas for 3 < pH < 5, the rate determining step (rds) is the first electronation step. Moreover, a fast protonation precedes the rds in the whole acidic medium. The best experimental conditions for the detection of CFXM in differential pulse plarography (DPP)were found. Owing to its high adsorptivity, CFXM is detectable at a concentration as low as 1 × 10−8M at a pH of about 2.