Věra Stará

Adsorptive stripping voltammetric determination of thiourea and thiourea derivatives

Abstract Adsorptive stripping voltammetry of thiourea, α-naphthylthiourea and diphenyl-thiourea is discussed. In perchlorate solution, these compounds are adsorbed at the hanging mercury drop electrode at positive potentials (or at open circuit) and can be stripped in a cathodic scan. Detection limits are 2.5 ng l −1 for thiourea, 80 ng l −1 for α-naphthylthiourea and 50 ng l −4 for diphenylthiourea. The method is applicable in the determination of thiourea in cattle-feed and in the direct analysis of urine.

Cathodic stripping voltammetry and adsorptive stripping voltammetry of selenium(IV)

Cathodic stripping voltammetry of selenium(IV) in 0.1 M hydrochloric acid media yielded a nonlinear calibration graph for the concentration range 10−9−10−8 M. In this concentration range, adsorptive stripping voltammetry based on adsorption of the selenium/3,3′-diaminobenzidine complex on the surface of the hanging mercury drop electrode at the deposition potential of +0.05 V (vs. SCE) is more convenient. A linear calibration graph is obtained for selenium concentrations of 3×10−9−3×10−8 M, with an accumulation time of 300 s.

Determination of some quinoxaline-N-dioxide derivatives by adsorptive stripping voltammetry

Abstract Some derivatives of quinoxaline-N-dioxides, which are used as growth promoters in animals (Carbadox, Cyadox, Olaquindox), can be determined at nanomolar concentrations by stripping volatammetry from a static mercury drop electrode after adsorptive accumulation on the electrode surface. With differential pulse voltammetry, in 0.1 M sodium perchlorate with 5% (v/v) dimethylformamide, the detection limit for Cyadox is 3 × 10−10 mol 1−1 after accumulation for 300 s in stirred solution; detection limits are 2 × 10−9 mol 1−1 (180 s accumulation) for Carbadox and 7 × 10 mol 1−1 (60 s accumulation) for Olaquindox. The relative standard deviations are 0.85% for Cyadox (4 × 10−9 mol 1−1), 0.54% for Carbadox (2 × 10−8 mol 1−1) and 0.95% for Olaquindox (2 × 10−8 mol 1−1). Surfactants interfere.

Differential pulse polarographic determination of tellurium. Application of catalytic hydrogen current

Abstract Tellurium deposited on the DME displaces the reduction potential of hydrogen and this results in the formation of a catalytic hydrogen current. Differential pulse polarographic measurement of the corresponding peak enables the determination of traces of tellurium down to the 1 ppb level. Using the method of standard addition, tellurium can be determined in the presence of all the metals usually accompanying tellurium in natural and industrial materials. In the cases when the material analyzed contains a larger excess of cadmium, silver or selenium, the separation of tellurium by extraction with methylisobutylketone should be employed.

Voltammetric determination of cyadox using adsorptive accumulation in a flow-through system

Cyadox, one of the quinoxaline-N-dioxide derivatives used as a grwoth promoter, has been determined voltammetrically after adsorptive accumulation on the surface of a hanging mercury drop electrode in a flow-injection system. A simple detector slipped onto the capillary of a static mercury drop electrode immersed in an electrolyte solution together with the reference and auxiliary electrodes were used for the detection. With a 1000 μl injected sample volume, a linear calibration plot was obtained at a Cyadox concentration level of 10−8 mol 1−1: slope of the calibration line 0.154 nA n M−1, intercept −0.453 nA, standard error 0.091 nA, correlation coefficient 0.999. Diluted blood plasma samples (from pigs) were analyzed using the flow-injection approach and adsorptive accumulation without the necessity of a separation procedure.

Differential pulse polarographic determination of selenium and tellurium and their mixtures

Abstract The presence of Se(IV) in the analyzed solution strongly depresses the differential pulse polarograhpic peak current of the metal ions reduced at more negative potential than −0.40 V in hydrochloric acid medium. The depression is due to the formation of an elementary selenium deposit on the surface of the DME during the reduction of Se(IV) to elementary selenium. The interferring effect of Se(IV) is eliminated by its extraction with diisopropylether from hydrochloric acid medium; Te(IV) and the other metal ions are determined in the aqueous phase with an error ±5%.

Fast-scan differential pulse polarography in pre-enriched solution

Abstract Fast-scan differential pulse polarography is a suitable method for the determination of trace amounts of amalgam forming metals in electrochemically enriched solution. The metal ions analyzed are first accumulated on the surface of a hanging mercury electrode by electrolysis at constant potential, approx. 200 mV more negative than is the half-wave potential of the corresponding metal ion. After a short oxidation interval the reformed metal ions are determined by cathodic potential scan with differential pulse technique. The sensitivity of the determination is equal to that of ASDPP determination; the sensitivity and selectivity can be improved by the addition of reagents forming insoluble compound with the metal ions analyzed.

Differential pulse polarographic determination of cyanuric chloride

Cyanuric chloride (2,4,6-trichloro-1,3,5-triazine) can be determined by fastscan differential pulse polarography in methanolic acetate buffer solution at pH 5.6 at a hanging mercury drop electrode. At positive potentials, the insoluble salt formed between cyanuric chloride and mercury(I) is adsorbed on the mercury surface and the d.p.p. current is enhanced. The detection limit is 0.2gmg ml−1. Cyanuric chloride in air can be determined after absorption in methanol.

Chromium(III)-complexes of some aminopolycarboxylic acids and their polarographic behaviour

Summary The complexes of ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid and triethylenetetraminehexaacetic acid with Cr3+ ions, which are slowly formed in slightly acidic solutions, yield well developed polarographic waves in the potential range around −1.20 V and poorly-developed waves at −1.60 V. If either of the ligands is added to a solution of Cr3+ ions, a wave appears immediately at −1.20 V; the corresponding current is due to the rate of the reaction between the reduced Cr2+ ions and the ligand at the surface of the electrode. The presence of NO3− ions causes increase of the wave of the complexes in the potential range around −1.20 V; this effect is due to the oxidation of the corresponding Cr(II) complexes with NO3− ions at the surface of the electrode. The presence of La3+ ions causes an increase in the height of the waves of the complexes in the potential range around −1.60 V. All the described chromium(III) complexes yield square-wave polarographic peaks at −1.20 V. These peaks are not influenced by the presence of NO3− ions. La3+ ions cause an increase in the square-wave polarographic peaks corresponding to the reduction of the Cr(III)-TTHA complex. This effect is attributed to the formation of the mixed binuclear complex, Cr-La-TTHA, which alters the rate of the electrode process involving the reduction of the Cr(III)-TTHA complex.