Huang Huiliang

Flow potentiometric and constant-current stripping analysis for arsenic(V) without prior chemical reduction to arsenic(III): Application to the Determination of Total Arsenic in Seawater and Urine

Abstract Arsenic(V) is reduced to elemental arsenic on a gold-coated platinum-fibre electrode at electrolysis potentials below −1.60 V vs. Ag/AgCl and subsequently re-oxidized, either by means of a constant current, or chemically, with gold(III) as oxidant. Total arsenic in acidified seawater can be determined by means of electrolysis for 60 s at −1.80 V vs. Ag/AgCl and subsequent stripping in 4 M hydrochloric acid containing 2.5 M calcium chloride. The detection limit obtained after 60 s of electrolysis (ca. 0.1 μg1−1) is about ten times lower than that obtained by the electrochemical stripping methods for arsenic(III) reported hitherto. Total arsenic in urine is determined after digestion with nitric acid and hydrogen peroxide.

Carbon fibre electrodes in flow potentiometric stripping analysis

Abstract The construction of carbon fibre flow electrodes suitable for use in connection with potentiometric and constant-current stripping is described, and the fibre electrodes are compared with a glassy carbon disc thin layer cell. The signal-to-background ratio is approximately 1.6 times higher for an 8–10 μm carbon fibre compared to the glassy carbon disc electrode. If an Ag/AgCl tube is used as both counter and reference electrode, the signal-to-noise ratio of the fibre electrode is approximately five times better than for a glassy carbon disc electrode with a calomel reference; the latter electrode design, however, gives slightly better precision. The dead volume and internal potential drop of the fibre electrodes are more than one order of magnitude smaller than for the glassy carbon disc electrode. Because of the simplicity of the manufacturing process and low material cost, the fibre cells can be used as disposable electrodes and the polishing process necessary in connection with glassy carbon disc electrodes can be omitted.

Flow constant-current stripping analysis for antimony(III) and antimony(V) with gold fibre working electrodes : Application to Natural Waters

Abstract Antimony(III) is determined by means of electrolysis at −0.40 V vs. Ag/AgCl on a gold-coated gold fibre electrode for 0.5–10 min in a redox buffer containing 0.01 M iron(II) in 0.10 M hydrochloric acid, and subsequent stripping with a constant current of 0.50μA either in 2 M hydrochloric acid or in 4 M hydrochloric acid/4 M calcium chloride. Antimony(V) is determined by the same procedure in 4 M hydrochloric acid medium. Bismuth(III) is masked by the addition of iodide to the sample prior to electrolysis. Antimony(III) and antimony(V) are determined by standard addition methods; the whole procedure including digital and graphical evaluation of the results is fully automated. The antimony(V) concentrations in the river water reference sample SLRS-1 and the seawater reference sample NASS-1 were found to be 0.63 and 0.31 μg l−1 with standard deviations of 0.046 and 0.051 μg l−1, respectively (n=15). The certified value for SLRS- 1 is 0.63±0.05 μg l−1; no certified value is available for NASS-1.

Computerized flow potentiometric stripping analysis with nafion-modified carbon fibre electrodes

Abstract Five to ten carbon fibres (10-μm diameter) are mounted perpendicularly to the flow direction in polyethylene tubes (0.8 mm inner diameter) and modified simultaneously by sucking five 10-μl portions of Nation dissolved in ethanol through the flow electrodes; after each portion, air is sucked through the tubes for some minutes. The modified electrodes can be used for several hundred electrolysis/stripping cycles in solutions containing albumin or Triton X-100. Analysis of a urine reference sample shows that zinc(II), cadmium(II) and lead(II) can be determined after acidification and electrolysis for 5, 35 and 100 s, respectively. For the determination of lead(II), 2 M hydrochloric acid is used as stripping medium but for the two other elements the sample itself is used. The electrochemically available fraction of the trace elements can be assessed by analyzing unacidified samples.

Simultaneous determination of mercury(II), copper(II) and bismuth(III) in urine by flow constant-current stripping analysis with a gold fibre electrode

Abstract Urine samples are treated with concentrated nitric acid and potassium permanganate ar 70°C for 10 min prior to injection. The flow electrode system consists of a 10-μm diameter gold fibre working electrode, a glassy carbon reference electrode and a platinum counter electrode. In the fully automated constant-current stripping procedure, the gold fibre is first covered with a fresh gold film after which the sample is electrolyzed for 1 min prior to stripping in 0.1 M hydrochloric acid with a current of 0.1μA. The procedure is repeated on a spiked sample after which the sample analyte concentrations are evaluated and presented digitally and graphically on a printer/plotter. The results obtained for bismuth, copper and mercury in a urine reference sample were 36.9, 39.7 and 47.7 μg l −1 with standard deviations ( n =10) of 3.2, 4.2 and 2.1, respectively. The certified values for copper and mercury were 45 and 51 μg l −1 ; no certified value was available for bismuth.

Flow potentiometric and constant-current stripping analysis for mercury(II) with gold, platinum and carbon fibre working electrodes : Application to the Analysis of Tap Water

Abstract Gold, platinum and carbon fibres with 10-μm diameter were mounted in PVC tubes and used as flow sensors in computerized potentiometric and constant-current stripping analysis for mercury, after electroplating ofa gold film onto the fibre surfaces. Compared to gold and glassy carbon disc electrodes, the fibre electrodes gare increased sensitivity and stability and were considerably simpler to handle. The gold-coated carbon fibre electrode gave a higher background than the gold fibre electrode, in both the potentiometric and constant-current stripping modes. Mercury(II) could be determined in presence of a 10 5 -fold (molar) amount of copper(II) by constant-current stripping in media with chloride concentrations below 0.05 M. The detection limit for mercury(II) after 10 min of electrolysis was 45 ng l −1 at the 3 σ level.

Computerized flow constant-current stripping analysis for nickel(II) and cobalt(II) with carbon fibre and modified carbon fibre electrodes

Nickel (II) and cobalt (II) are determined by constant-current stripping analysis, with mercury- coated carbon fibre electrodes, in non-deoxygenated solutions after potentiostatic adsorptive accumulation of their dimethylglyoxime complexes. By adding several stripping scans, each obtained after a short period of potentiostatic deposition, instead of a single scan obtained after the same total time of potentiostatic adsorption, the linear range between the analytical signal and the analyte concentration can be extended. By using pulsed-potential procedures for adsorptive accumulation, cobalt (II) can be determined in the presence of a larger amount of nickel (II) and vice versa. Carbon fibre electrodes were modified by sucking ethanolic solutions of either dimethylglyoxime (of Nafion and dimethylglyoxime) through the electrodes; Nafion did not affect the general behaviour of this electrode. The background of the modified electrodes was lower than that of the unmodified electrodes. The accuracy of the method was confirmed by analysis of sea- water reference samples. The detection limit for nickel (II) was about 40 ng l−1 for a total potentiostatic deposition time of 100 s (5×20 s); the reagent blank was about 150 ng l−1 under these conditions.

Automated determination of lead in urine by means of computerized flow potentiometric stripping analysis with a carbon-fibre electrode

Five different digestion procedures have been investigated. In the recommended procedure 5 ml of concentrated hydrochloric acid-nitric acid mixture (1:1) were added to a 5-ml urine sample. After 1 min the sample was diluted to 100 ml with distilled water. The sample was divided into two 50-ml samples and to one of them a standard addition of 10 microg/l. lead(II) was made. The two samples were analysed fully automatically by means of computerized flow potentiometric stripping analysis, the main features of the procedure being mercury-film precoating, electrolysis of the sample for 90 sec and subsequent stripping in 1M calcium chloride/0.1M hydrochloric acid. Tin and copper were found not to interfere if present at concentrations below 50 mg/l. but high concentrations of tin had to be masked by the addition of copper in order to form a copper-tin intermetallic compound. Complexing agents used in lead poisoning therapy did not interfere with the determination. The lead concentration in a Seronorm reference urine sample was found to be 93 microg/l. with a standard deviation of 8 microg/l. (n = 5), the certified value being 88 microg/l.

Determination of mercury in air by means of computerized flow constant-current stripping analysis with a gold fibre electrode

Abstract Mercury in air was determined after collection in potassium permanganate or sodium carbonate solution. The mercury concentration in these solutions was determined in a computerized flow potentiometric stripping analyzer with a 10-μm gold fibre working electrode, a glassy carbon reference electrode and a platinum counter electrode. After sample electrolysis for 1–10 min, stripping was done in a 1 mg l −1 gold(III) solution in 0.01 M nitric acid/0.01 M sodium nitrate with a constant stripping current of 0.50 μA. Results obtained for flue gas samples were in good agreement with results from cold-vapor atomic absorption spectrometry.

Computerized flow constant-current stripping analysis for gold(III) with carbon or platinum fibre electrodes

Gold(III) is determined in hydrochloric acid media to which 1–5 mg l−1 of mercury(II) and copper(II) (if not already present in the sample) have been added. Potentiostatic deposition of gold onto the carbon or platinum fibre electrode is achieved by means of pulsed-potential electrolysis; e.g., each pulse is composed of electrolysis for 9 s at −0.60 V vs. mercury drop electrode followed by oxidation of copper and mercury by applying a potential of 0.30 V for 1 s. For constant-current stripping (0.5−1μA), 2 M hydrochloric acid in 40% (v/v) ethanol is the best medium. A detection limit (three sigma level) of 1 μg l−1 gold is estimated for a total electrolysis time of 900 s.