Miloslav Kopanica

Determination of uranium by adsorptive stripping voltammetry

Abstract A voltammetric method for the determination of uranium in natural waters in the concentration range from 0.4 μg 1 −1 to 0.2 mg 1 −1 is described. The method is based on adsorptive accumulation of the uranium(VI)—pyrocatechol complex on the hanging mercury drop electrode followed by the reduction of the absorbed complex.

Voltammetry with disc electrodes and its analytical application: II. Anodic stripping voltammetry of trace concentrations of silver and copper employing a glassy carbon electrode*

Summary The rotating GCE has been found to be a suitable electrode for anodic stripping voltammetry of submicro amounts of metal ions. Sharp mono peaks were obtained corresponding to the anodic dissolution of silver and copper in acidic, neutral and alkaline media. The effect of various factors (rotation velocity of the electrode during the deposition and stripping process ‘scan rate’ deposition potential and deposition time, composition of the supporting electrolyte) on the peak current and peak potential was studied. The peak height has been found to be a linear function of the concentration of the metal analyzed and the log of the peak potential to depend linearly on the concentration of silver and copper, respectively. The determination of trace amounts of silver (10−6–10−9 M) and copper (10−6–10−8 M) can be carried out in acidic, neutral or alkaline media. A procedure was established for the determination of micro amounts of silver in the presence of a large excess of copper. Ammonia/ammonium nitrate solution was employed as supporting electrolyte and when the concentration of copper was too high, the sample solution was replaced by a pure solution of ammoniacal buffer after the deposition step. 5×10−7 M silver solutions containing copper at a concentration of 5×10−3 M were analyzed successfully; the error did not exceed ±5%. The application of GCE in anodic stripping voltammetry is being further studied.

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.

DIFFERENTIAL-PULSE VOLTAMMETRIC DETERMINATION OF MOLYBDENUM IN NITRATE MEDIUM IN THE PRESENCE OF 8-HYDROXYQUINOLINE

Abstract The determination of Mo(VI) by differential-pulse voltammetry based on catalytic currents in nitrate medium is described. The existence of catalytic currents in the system Mo(VI)NO 3 − in the presence of 8-hydroxyquinoline was proved by various polarographic techniques. The optimum background electrolyte is 20 ml 0.5 M KNO 3 −0.005 M HNO 3 with the addition of 1 ml of 1 × 10 −2 M 8-hydroxyquinoline. The detection limit is 7 × 10 −10 M under these conditions. Cr(VI), Cu(II), Cd(II) and Pb(II) interfere when present at higher concentrations then Mo(VI) and W(VI) interferes at an equal concentration to Mo(VI). The method was successfully used in analyses of environmental samples.

Determination of cyadox and its metabolites in plasma by adsorptive voltammetry

Adsorptive stripping voltammetry was combined with separation of the surfactants by gel chromatography or solvent extraction for determination of the growth promotor, cyadox, and its metabolites in the plasma from pig blood. The procedure permits determination of the substance at concentrations from units to hundreds of ng per ml of plasma. The accuracy of the results was checked by comparison with radiochemical measurements.

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.

Thin-layer polarographic detector for the high-performance liquid chromatographic detection of thiourea derivatives

A polarographic thin-layer detector for high-performance liquid chromatography, was devised. The detector has a sandwich structure with a working surface area of the mercury electrode of 0.25 mm2. The detector was applied to the direct amperometric detection and for detection with reactivation of the electrode surface by voltage pulses. It is shown that the latter detection mode is suitable for the determination of compounds that form insoluble or complex mercury salts, e.g., thiourea. Thiourea and its derivatives can be separated and determined in a wide concentration range and the detection limit for thiourea corresponds to 2 ng injected. Thiourea can be determined directly in urine.