Christian Neuhold

An improved voltammetric method for the determination of trace amounts of uric acid with electrochemically pretreated carbon paste electrodes.

Carbon paste electrodes, preanodized in alkaline medium at 1.4 V vs SCE for a short period of time, exhibit a great shift of the oxidation potential of uric acid in cathodic direction and a marked enhancement of its current response, compared to unpretreated electrodes. These effects are dependent on the preanodization potential and the time imposed on the electrodes as well as on the alkalinity of the supporting electrolyte. The enhanced voltammetric response can be used to determine uric acid in the concentration range of 5.0-4.0 x 10(4) mug/1 (3.0 x 10(-8)-2.4 x 10(-4)M) with a detection limit (3sigma) of 2.0 mug/1 (1.2 x 10(-8)M). Ascorbic acid in less than 30-fold excess does not interfere. For multiple determinations (5 runs), the relative standard deviation is 2.1% at a concentration of 1 mg/1 uric acid. The proposed procedure can be used to determine uric acid in human urine and serum without any preliminary treatment of the samples in an accurate, rapid and simple way.

Anodic stripping voltammetric determination of titanium(IV) using a carbon paste electrode modified with cetyltrimethylammonium bromide.

A method is described for the voltammetric determination of titanium(IV) using a carbon paste electrode modified in situ with cetyltrimethylammonium bromide. The cationic micellar surfactant adsorbs onto the electrode particularly at negative potentials, simultaneously preconcentrating titanium(IV) as the oxalate complex with reduction to titanium(III). Anodic stripping voltammetry exploiting reoxidation can be used for the determination of trace levels of titanium(IV). Linearity between current and concentration exists between 5 and 160 mug l(-1) Ti(IV) (preconcentration time 2 min). The limit of detection (calculated as 3sigma) is 0.1 mug l(-1), with a preconcentration time of 10 min.

Voltammetric determination of trace amounts of mercury with a carbon paste electrode modified with an anion-exchanger

SummaryA carbon paste electrode modified with a liquid anion exchanger (Amberlite LA2) was used for the voltammetric determination of mercury(II). Mercury is preconcentrated, as tetrachloromercurate(II), onto the surface of the modified electrode only by the ion-exchange effect of the modifier without application of potential. After exchange of the medium the accumulated amount of mercury(II) is determined by differential pulse anodic stripping voltammetry in a blank electrolyte solution. The response depends on the concentration of mercury in the bulk solution, preconcentration time, and other parameters. The detection limit was 1 μg Hg(II)/l when a suitable time for preconcentration was chosen. Preconcentration for 5 min yields a linear calibration graph for concentrations up to 1000 μg Hg(II)/l. The effect of other ions on the determination of mercury and the applicability of the method to the analysis of phenylmercury compounds in pharmaceutical preparations were investigated.

Simultaneous determination of uric acid, xanthine and hypoxanthine with an electrochemically pretreated carbon paste electrode

A simple method is presented for the simultaneous differential pulse voltammetric determination of uric acid, xanthine and hypoxanthine. It is based on the improved current responses of the three analytes at carbon paste electrodes polarized in a dilute alkaline medium (0.002 mol/l NaOH, 0.1 mol/l NaClO4) at 1.3 V vs. SCE for a short time. Compared with the methods reported in the literature, this procedure has a much wider linear range (2 to 3 orders of magnitude in concentration), lower detection limits (5 to 10 μg l−1) and less interference by ascorbic acid. The electrochemical responses were found to be dependent on the pre-anodization potential and the time imposed on the electrodes as well as on the alkalinity of the supporting electrolyte. The proposed procedure was used to determine uric acid, xanthine and hypoxanthine in human urine without any preliminary treatment.

Voltammetric determination of gold using a carbon paste electrode modified with thiobenzanilide

Gold(III) can be preconcentrated from acidic solution on a carbon paste electrode chemically modified with thiobenzanilide under open-circuit conditions. Using cathodic differential pulse voltammetry, 0.05–6 µg ml–1 of gold can be determined after preconcentration for 1 or 2 min. Except for mercury(II) and platinum group metals, common ions have little effect on the determination of gold(III). The preparation and regeneration of the modified electrode as well as the various methodical parameters for the preconcentration and measurement of gold(III) were investigated.

Screen-printed electrodes for nitrite based on anion-exchanger-doped carbon inks

Disposable voltammetric sensors for nitrite have been microfabricated by screen-printing an anion-exchanger (Aliquat 336)-modified carbon ink. The modifier ability to accumulate low levels of nitrite is not compromised by the screen-printing and low-temperature drying processes. Experimental variables influencing the preconcentration/voltammetric performance are elucidated and optimized. A short (30 s) accumulation period yields a detection limit of 30 µg l–1 nitrite. Monitoring of micromolar levels of nitrite in drinking- and groundwater samples is illustrated. Prospects of mass production of other preconcentrating modified electrodes are discussed.

Voltammetric behaviour of thallium (III) at a solid heterogeneous carbon electrode using ion-pair formation

Thallium in its trivalent state forms very stable [TIX4]– complexes with halides in corresponding acidic media. Tetrahalogenothallates (III) are able to produce ion pairs with suitable positively charged counter-ions in aqueous solutions. These ion pairs, each possessing distinct lipophilic character, could be accumulated directly onto the surface of carbon paste electrodes without applying a potential (open circuit conditions). The nature of the electrode material and of the counter-ion respectively, influences the adsorptive behaviour of the ion pair significantly. Best results were obtained by using a new type of heterogeneous carbon electrode employing a solid binder (phenanthrene) instead of the commonly used pasting liquid. With respect to the counter-ions, tetraphenylphosphonium and tetrabutylammonium proved to be the best. Parameters influencing the optimum conditions for the analytical procedure (pH, counter-ion concentration, etc.) were investigated. The relationship between current signal and concentration of thallium, and its dependence on the accumulation time were studied. The detection limit was found to be ⩽ 10 µg kg–1, signals were reproducible within an sr of 7.7% for a thallium concentration of 1 mg kg–1. The interference of other ionic species was also evaluated. The method was employed for the determination of thallium in lead- and cadmiumcontaining zinc alloys.

Stripping voltammetric determination of trace amounts of mercury using a carbon paste electrode modified with 2-mercapto-4(3H)-quinazolinone

A carbon paste electrode modified with 2-mercapto-4(3H)-quinazolinone was used for the voltammetric determination of mercury(II). Mercury was preconcentrated onto the surface of the modified electrode only by the complexing effect of the modifier without application of potential (i.e. in open-circuit conditions). After exchange of the medium, the accumulated amount of mercury(II) was determined by differential pulse anodic stripping voltammetry. The response depended on the concentration of mercury in the bulk solution, preconcentration time, and other parameters. The detection limit was 0.1 μg 1–1 Hg(II) for a preconcentration time of 15 min. Preconcentration for suitable times yielded a linear calibration graph from 0.5 to 6000 μg 1–1 Hg(II). For multiple determinations (5 runs), the relative standard deviation was 5% for a concentration of 100 μg 1–1 Hg(II). The proposed procedure was used to determine trace mercury in plant and sewage sludge samples with good results.

Studies on the voltammetric behaviour of a 2-mercaptoimidazole containing carbon paste electrode: determination of traces of silver

The voltammetric behaviour of a 2-mercaptoimidazole (2-MI) containing carbon paste electrode was studied. When mixed to carbon paste as an electrode modifier, 2-MI can be reduced at negative potentials (−1 V vs. SCE), but it does not give a response in the potential range where Ag(0) is oxidized to Ag(I). Silver could be accumulated from 0.1 mol l−1 acetate buffer onto a 2-MI modified carbon paste electrode without a potential applied; after medium exchange, it was reduced at −1 V vs. SCE in 0.1 mol l−1 acetate buffer solution and determined by differential pulse anodic stripping voltammetry. With suitable preconcentration times, the detection limit was 0.1 μg l−1; a linear relation between current and concentration was found to exist within a range of 0.5 to 1000 μg l−1. In the presence of EDTA, common metal ions have no or only little effect on the voltammetric determination of silver.

Catalytic Determination of Perchlorate Using a Modified Carbon Paste Electrode

Abstract A method for the catalytic voltammetric determination of perchlorate using a carbon paste electrode modified with a liquid anion exchanger is presented. The fundamental catalytic effect is based on the chemical reoxidation of electrochemically generated Tl(O) by perchlorate which can be monitored by an increase of the corresponding current flow. Perchlorate can be preconcentrated, together with tetrachlorothallate(III) as a catalyst, from hydrochloric acid solutions onto the modified carbon paste electrode under open circuit conditions. For analytical purposes, the increase of the current response for the reoxidation of Tl(O) to Tl(I) is exploited for quantifications. Methodical parameters such as pH, ionic strength of the media, preconcentration time and thallium concentration are investigated; the influence of interferents is studied. The dependence of the current increase on the concentration of perchlorate with different accumulation times is presented. The detection limit (3σ) is 50 μg˙l−1 C...