Chronopotentiometric Analysis Research Articles

Determination of Ni (II) in beverages without any sample pretreatment by adsorptive stripping chronopotentiometry (AdSCP).

The purpose of this paper was to use adsorptive stripping chronopotentiometry for the determination of Ni (II) in worldwide consumed beverages without any sample pretreatment, using dimethilglyoxime (DMG) as complexing agent and a glassy carbon mercury film electrode as the working electrode. Ni (DMG)2 complex is adsorbed onto the mercury film at an electrolysis potential of -500 mV for 60 s and then reduced by a -5 microA constant cathodic current. The sensitivity of the method was studied for certified reference water and black tea in the pH range 6.5-11. At pH 9.5 in ammonia buffer, a detection limit of 0.2 microg L(-1) was achieved; the instrumental precision (expressed as rsd %) was 1.5%, and the accuracy, expressed as obtained recoveries both from certified and not certified matrixes, ranged from 93.0 to 95.5 %. The chronopotentiometric analysis executed on commercial beverages provided evidence that black tea samples were the richest source of Ni (II) (1500-3700 microg L(-1)), followed by coffee (100.0-300.5 microg L(-1)); bottled mineral water showed a Ni (II) concentration lower than 4.6 microg L(-1). Among alcoholic beverages, red wines presented the highest content of Ni (II) (55.5-105.0 microg L(-1)). Significant differences were noticed between Ni (II) levels of fermented and distillated alcoholic beverages; moreover, canned cola and beer did not show higher Ni (II) levels with respect to the glass-bottled products.

Chronopotentiometric analysis of tocopherols using a glassy carbon vessel as the working electrode

Using a specific electrochemical cell with a glassy carbon vessel as the working electrode, a significant enhancement of the chronopotentiometric determination of tocopherols was achieved. Due to the large area of the working electrode, the method sensitivity was increased approximately 50-fold compared to the common electrochemical cell with a glassy-carbon disc electrode. Limits of quantitation of approximately 0.35 mg/dm 3 for α-, γ- and δ-tocopherol were obtained. The influence of the most important experimental factors was investigated and the method defined was applied for tocopherol determination in crude vegetable oils. The tocopherol content was determined by standard solutions prepared in a matrix of animal fat. An absolute method for γ- and δ-tocopherol quantitation is also described.

Recent Advances on DNA Biosensors

DNA biosensors are realised immobilising a DNA structure on a suitable transducer to obtain selective information. In this paper we show how the determination of low-molecular weight compounds with affinity for DNA was measured by their effect on the oxidation signal of the guanine peak of calf thymus DNA immobilised on the electrode sensor and investigated by chronopotentiometric analysis. The DNA biosensor is able to detect known intercalating and groove binding compounds. Applicability to river water samples was demostrated. Moreover, a piezoelectric sensor coupled to a short oligonucleotide can be used as detector of the hybridisation reaction. We show as a model the detection of a specific mutation in apolipoprotein E (ApoE) gene. Biotinylated 23-mer probes were immobilised on the streptavidin coated gold surface of a quartz crystal; the protein was covalently bound to the thiol/dextran modified gold surface. The device was able to distinguish different synthetic oligonucleotides. The hybridisation reaction was also performed using real samples of DNA extracted from human blood and amplified by Polymerase chain reaction PCR. The extension of such procedure to samples of environmental interest is discussed.

DNA electrochemical biosensors.

Disposable electrochemical DNA-based biosensors are reviewed; they have been used for the determination of low-molecular weight compounds with affinity for nucleic acids and for the detection of the hybridisation reaction. The first application is related to the molecular interaction between surface-linked DNA and the target pollutants or drugs, in order to develop a simple device for rapid screening of toxic or similar compounds. The determination of such compounds was measured by their effect on the oxidation signal of the guanine peak of calf thymus DNA immobilised on the electrode surface and investigated by chronopotentiometric analysis. The DNA biosensor is able to detect known intercalating compounds, such as daunomycin, polychlorinated biphenyls (PCBs), aflatoxin B1, and aromatic amines. Applicability to river and waste water samples is also demonstrated. Disposable electrochemical sensors for the detection of a specific sequence of DNA were realised by immobilising synthetic single-stranded oligonucleotides onto a graphite screen-printed electrode. The probes became hybridised with different concentrations of complementary sequences present in the sample. The hybrids formed on the electrode surface were evaluated by chronopotentiometric analysis using daunomycin as indicator of the hybridisation reaction. The hybridisation was also performed using real samples. Application to apolipoprotein E (ApoE) is described, in this case samples have to be amplified by PCR and then analysed by DNA biosensor. The extension of such procedures to samples of environmental interest or to contamination of food is discussed.

Electrochemical DNA biosensor for environmental monitoring

A disposable electrochemical DNA biosensor for the determination of toxic aromatic amines has been developed. The device relies on the intercalative or electrostatic collection of aromatic amines onto an immobilized dsDNA or ssDNA layer (obtained from several sources), followed by a chronopotentiometric analysis. The anodic signal of the guanine bases of DNA coated screen printed electrodes (SPEs) is strongly influenced by structural or conformational modifications of the DNA layer accrued from DNA–analyte association. So the variation in the oxidative signal of guanine is taken as an index of the molecular recognition. When the analyte is electroactive, its oxidation peak gives an additional information; in fact interesting correlation have been found between the amount of analyte trapped on the electrode and the guanine peak variation. Submicromolar detection limits have been obtained for molecules with more than two aromatic rings after a 2 min accumulation. The biosensor has also been tested on wastewater real samples; the comparison with results of classical genotoxicity tests has confirmed the applicability of the method for real samples.

Characterization and Electrochemical Behavior of a Copper Ferrite Obtained by In Situ Precipitation from Aqueous Solutions

The chemical composition, crystalline structure and properties of copper-bearing ferrites obtained by in situ coprecipitation from aqueous solutions depend on the experimental conditions. Under optimum conditions of pH, temperature, Fe(II)/Cu(II) ratio, stirrer rate and air flow, a copper-bearing ferrite was obtained, whose chemical composition, determined by potentiometric titration, ICP-AES and XRF, is Cu0,13FeII0,87FeIII2O4ċnH2O. An investigation into the crystalline structure of the ferrite by thermal analysis (TGA, DTG and DSC) and XRD pointed to CuxFe3-xO4ċFe3O4ċCuOċnH2O as the most probable structure. Electrochemical analysis of the solid ferrite was performed in HCl and HClO4 solutions using a carbon paste electrode (CPE). Successive cyclic voltammograms were registered and results suggest that solid species only participate in electrodic reactions during the first scan. In the second and subsequent scans only dissolved species are involved, thus pointing to dissolution phenomena of the solid ferrite in the CPE: In perchloric acid, only peaks corresponding to the redox systems Feads3+ + 1e– ⇔ Feads2+ and Cuads2+ + 2e– ⇔ Cu0 were scanned, whereas in hydrochloric acid, the peaks corresponded to the systems FeCl2,ads+ + le– ⇔ FeClads+ + Cl–, Cuads2+ + Cl– + le– ⇔ CuCl(s) and CuCl)(s) + 1 e– ⇔ Cu(s) + Cl–. The values of the half-peak potentials and their shift with the activity of hydrochloric acid confirmed these reactions. Chronopotentiometric analysis at a CPE containing copper ferrite further confirmed the proposed mechanisms.

Disposable DNA electrochemical biosensors for environmental monitoring

Two disposable electrochemical biosensors are described; the first one is useful for the detection of the hybridization of DNA and the second one is able to detect low-molecular weight compounds (toxins, pollutants, drugs) with affinity for nucleic acids. Synthetic single-stranded oligonucleotides have been immobilized onto graphite screen-printed electrodes using adsorption at a controlled potential. The probes were hybridized with different concentrations of the complementary sequences. The hybrids formed on the electrode surface were evaluated by chronopotentiometric stripping analysis using daunomycin hydrochloride as an indicator of the hybridization reaction. The DNA sensor was able to detect 0.2 mg l −1 of a 21-base target sequence. The determination of low-molecular weight compounds with affinity for DNA was measured by their effect on the oxidation signal of the guanine peak of calf thymus immobilized on the electrode sensor and investigated by chronopotentiometric analysis. The DNA biosensor is able to detect known intercalating and groove binding compounds. Detection limits of 0.3, 0.2 and 10 mg l −1 were obtained for daunomycin, polychlororinated biphenyls (PCBs) and aflatoxin B1, respectively. Applicability to river water samples was demonstrated.

New aspects of enzyme chronopotentiometry

Chronopotentiometric analysis is a novel method of measuring substrate concentrations with oxidoreductase electrodes. In contrast to conventional enzymatic potentiometric measurements where the analytical signal is due to a potential difference of redoxactive species, in enzyme chronopotentiometry the potential of the working electrode as a function of time is studied during two galvanostatic steps. The application of this technique to the determination of hydrogen peroxide with a peroxidase electrode is presented.

Programmable analyzer for voltammetric and chronopotentiometric experiments

A computerized system for voltammetric and chronopotentiometric analysis is described. The system consists of a microcomputerized electrochemical unit and a personal computer (PC) with peripherals. The eletrochemical unit is programmed and activated via a command list entered from the PC keyboard, and then operates independently. Meanwhile, the PC is available for data processing. The system can execute user-defined operations, e.g., voltage steps, measurement of relaxation current, at a frequency of 50 kHz in real time. Thus it affords a flexible means for implementation of standard as well as non-standard voltammetric techniques. The system facilitates a rapid comparison of voltammetric and chronopotentiometric techniques under analogous experimental conditions. Because electrode potentials can be sampled at a frequency as high as 660 kHz, the system is well suited for potentiometric stripping analysis of solutions with a high content of oxidants. The analytical possibilities of the instrument are demonstrated by two examples. In one, square-wave voltammetric and potentiometric stripping approaches are compared for the determination of traces of lead (II) in non-deaerated media. In the other, constant-current chronopotentiometry and square-wave voltammetry are compared for the quantitation of nickel (II) deposited by chemical interaction with a chemically modified electrode.

The indoor exposure of thin sputtered copper-aluminium films

Sputter-deposited CuAl films have been exposed indoors. Corrosion products were analysed by chronopotentiometric analysis, electrical contact resistance measurements, reflectance measurements and visual inspection. Tarnishing is inhibited on CuAl films with respect to pure copper films. The tarnish film does contain cuprous oxide but cuprous sulphide is not found. The inhibiting action is attributed to cuprous ion diffusion inhibition in an aluminium-doped cuprous oxide film.

Redox mechanisms in electronic conducting polymers: Voltammetric and chronopotentiometric analysis of polypyrrole and polythiophene

Electrochemical investigations on poly(N-methylpyrrole) in CH 3CN and on poly(methylthiophene) in carbonate propylene are described. Two electroanalytical techniques, Linear Sweep Voltammetry and Constant Current Chronopotentiometry are used and their respective advantages are discussed. When possible Linear Sweep Voltammetry associated with a deconvolution technique for isolating each phenomena contribution is a suitable technique for elucidating a redox mechanism as in case of polypyrrole. For polythiophene this does not work as well and Chronopotentiometry appears to be a better technique.

Considérations sur la surface de γ-MnO 2

The active surface area of γ-MnO 2 was determined using lithium chemisorption results and chronopotentiometric analysis. These values were compared with the surface obtained by physisorption of liquid nitrogen (Brunauer-Emmett-Teller (BET) method).

Application of chronopotentiometric analysis to the anodic treatment of copper sulphides

The analytical techniques of chronopotentiometry have been applied to previously published data on the electroleaching of the copper sulphides chalcocite, Cu2S, and digenite, Cu9S5. The results can be explained by a model that combines solid state diffusion with chemical interaction between the solid and the solution. Quantitative evaluation of the reactions indicates that the first stage of reaction is preferential dissolution of copper, resulting in a compound of approximately CuS composition, but the Cu:S ratio of which apparently decreases with increasing temperature of treatment. The second state of reaction is the decomposition of this material to cupric ion and sulfur.

Systematic Studies on Chronopotentiometric Analysis. I-II. I. Characteristics of Copper, Lead, Cadmium and Bismuth Ions in Various Supporting Electrolytes

各種支持電解質における銅, 鉛, カドミウムおよびビスマスイオンのクロノポテンシオメトリー的挙動について検討し, 四分波電位, せん移時間定数, 還元電極反応に関与する電子数を測定した。これらの定数をそれぞれポーラログラフ法における半波電位および拡散電流定数と比較し, 概して良好な対応をうることができたが, 通常ポーラログラフ法では 0.01% のゼラチンを共存させるため, ゼラチンが電極反応にいちじるしく関与するような場合の拡散電流定数は, ゼラチンを加えないクロノポテンシオメトリーでのせん移時間定数に対して良い対応を示さなかった。またクロノポテンシオグラムには, ポーラログラムと異なり極大現象はないが, 1N 過塩素酸,硫酸, 硝酸中における銅イオンの場合, O.OO1% ポリアクリルアミドの共存により波形が改善されることを見いだした。

Systematic Studies on Chronopotentiometric Analysis. III-VI. VI. The General Consideration of Chronopotentiometry of Two-component Mixtures and the Determination of Small Amounts of Lead and Cadmium in Zinc Oxide

0.1N 塩化カリウム溶液中の鉛, カドミウムイオンの2成分系について, カドミウムイオンのクロノポテンシオグラムに及ぼす鉛イオンの影響について分析化学的見地より検討した。2成分系のおのおのの濃度が一定の場合,それらのせん移時間の比は電流密度に無関係に一定である。また後放電物質の濃度,電流密度を一定にして前放電物質の濃度を変化させた場合,第2波のせん移時間は前放電物質の濃度と直線関係をもって変化する。前放電物質の濃度,電流密度を一定にして後放電物質の濃度を変化させた場合, 第2波のせん移時間は後放電物質の濃度と放物線関係をもって変化する。これらの関係を示す式を誘導し,実験によりこれらの関係を証明した。微量のカドミウムィオンに適当な濃度の鉛イオンを共存させることにより, そのせん移時間を増大させて測定した値は理論式を用いての計算値とかなりよい一致を示した。多墨の前放電物質の存在は後放電物質の定壁を妨害する。たとえば普通のペン記録計を用いてカドミウムィオンを定量する場合, カドミウムィオンのせん移時間を 6 秒以上にするためには 15 倍以上の鉛イオンが共存しないことが必要である。2成分系における定量分析例として純亜鉛華中の 0.013%Cb および O.OO2%Cd を定量し,ポーラログラフ法による定量結果と良好な一致をうることができた。