This paper reviews literary data on electrochemical monitoring of biogenic microelements in natural and industrial objects. Special attention is paid to the questions of the trace determination for a number of transition metals by stripping voltammetry (SVA) after the adsorptive enrichment of these elements as dimethylglyoxime (DMG) complexes on the surface of a working electrode. The analytical possibilities of adsorptive SVA with the use of mercury-based and mercury-free electrodes are compared. The paper is dedicated to the memory of Vadim Mikhailovich Ivanov.

This paper reviews the literature data on mercury-less electrodes for determining traces of transition elements by stripping voltammetry after the adsorption enrichment of these elements as dimethylglyoxime complexes on the surface of the electrode. A brief discussion of the design and characteristics (detection limit, adsorption time) of mercury-less electrodes and mercury-based electrodes is given.

The oxidation of sodium diethyldithiocarbamate at a glassy carbon electrode in water and water-organic mixtures in the presence of KCl and NaOH is investigated. Using cyclic square-wave voltammetry and rotated electrode voltammetry it is shown that, in water and water-ethanol mixtures, the electrode reaction is irreversible. In the case of KCl water solution, the oxidation reaction involves one electron. In the case of KCl in a water-ethanol mixture, two electrons are involved. The possible oxidation mechanism is discussed.

Possible approaches to the flow-injection determination of europium(III) in the presence of other lanthanides are studied. One of the approaches is based on the direct amperometric detection of europium(III) in a flow-injection system with a glassy-carbon electrode at a potential of –0.85 V (against a saturated calomel electrode). The linear calibration range is 5.0 × 10 –5 –5.0 × 10 –4 M of europium, and the limit of detection is 1.8 × 10 –5 M (2.8 μg/mL). The throughput capacity is 90 h –1 for a sample volume of 600 μL. Another approach involves the online reduction of europium(III) to europium(II) in a flow Jones mini-reductor filled with amalgamated zinc, followed by the spectrophotometric detection of europium(II) using redox reactions between europium(II) and iron(III) in the presence of 1,10-phenanthroline, molybdophosphoric acid, or Methylene Blue. In the latter case, the calibration curve is linear in the range 0–5.0 × 10 –6 M europium(III), the limit of detection is 9.0 × 10 –8 M (0.014 μg/mL). The throughput capacity is 180 h –1 for a sample volume of 200 μL. The performance parameters of the proposed flow-injection methods are estimated using the analysis of artificial mixtures and dissolved samples of samarium(III) oxide and lanthanum(III) fluoride containing europium impurities as an example.

The data on determination methods for molybdenum published from 1985 to 2001 are surveyed.

Procedures were developed for the determination of molybdenum in steels, soil, and sea water using o , o "-dihydroxyazo compounds [Lumogallion IREA (LG) and Magneson IREA (MG)] and heterocyclic azo compounds [4-(2-pyridylazo)resorcinol (PAR) and 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PAAP)], including their determination in the presence of the third component, hydoxylamine (HA). The addition of the third component improved the selectivity of determination. The advantage of the developed procedures is that they do not involved extraction and, thus,shorten the analysis. Molybdenum can be determined in sea water in the presence of LG or LG + HA by diffuse reflectance spectroscopy or adsorption voltammetry using the added–found method or using visual tests. In the presence of MG, PAR, and 5-Br-PAAP, the standard addition method was used to eliminate the matrix effect.

The chromaticity characteristics of molybdenum(VI) complexes with o.o’-dihydroxyazo compounds (Lyumogallion IREA and Magnezon IREA) and heterocyclic azo compounds [4-(2-pyridylazo)resorcinol and 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol] were studied in the presence and in the absence of the third component, hydroxylamine. It was demonstrated that chromaticity characteristics can be used for analyzing samples with low molybdenum concentrations. The calibration graph equations for the color characteristics of complexes both in solution and in the immobilized exhibit sensitivity coefficients more than an order of magnitude higher than for absorbance and the Kubelka-Munk function

Linear sweep voltammograms of Lumogallion IREA (pH 2), Magneson IREA (pH 2), 4-(2-pyridylazo)resorcinol (pH 4.8), and 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (pH 4.8) in the presence of molybdenum(VI) exhibit peaks at potentials more negative than the potentials of reduction peaks of the reagents by approximately 0.1 V. In all of the above cases, the heights of these peaks linearly increased with an increase in the concentration of molybdenum(VI) in the range from 5 x 10 -7 to 2.5 x 10 -6 M; thus, these peaks can be used for the determination of molybdenum. The simultaneous proportional decrease in the heights of the cathodic peaks of the reagents can be used for indirect determination of molybdenum(VI). The limits of detection without preliminary accumulation at a dropping mercury electrode with a drop time of 5 s are (1.5-3.9) x 10 -7 M, depending on the nature of the reagent and the technique used for determining the concentration.

The electrochemical behavior of palladium(II) was studied by differential pulse, linear sweep, and alternating-current square-wave voltammetry in HC1, HNO3, H2SO4, and HC1O4 solutions in the presence of dimethylglyoxime. A peak with a height linearly depending on the concentration of palladium(II) was observed in voltammograms. Typical relationships between the height and potential of a peak and pH, dimethylglyoxime concentration, the potential and time of adsorption accumulation suggested that the observed peak was due to the hydrogen liberation catalyzed by palladium(II) dimethylglyoximate adsorbed on the electrode surface. The detection limits for palladium(II) accumulated for 120 s at -0.2 V were 2 x 10-8, 5 x 10-9, and 8 x 10-10 M for differential pulse, linear sweep, and alternating-current square-wave voltammetry, respectively.

The kinetics of complexation in the system molybdenum(VI)-azo compound-hydroxylamine was studied. o,o'-Dihydroxyazo compounds, such as Lyumogallion IREA and Magnezon IREA, and heterocyclic azo compounds, such as 4-(2-pyridylazo)resorcinol and 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol, were studied. The formation of mixed-ligand complexes with the ratio of components 1 : 1 : 1 was detected. Rate constants, activation energies, and stability constants of the forming compounds were evaluated. It was concluded that the reagents under study are promising for the analytical chemistry of molybdenum.

The possibilities for the amperometric detection of Cu(II), Fe(III), and Ni(II) complexes with 4-(2-pyridylazo)resorcinol (PAR) after their separation by chromatography were examined. The optimum conditions for the amperometric detection of metals were chosen. It was found that the metals can be quantitatively determined with the limits of detection (grams per sample) 4.0 x 10(-9) for Cu(II), 2.9 x 10(-9) for Fe(III), and 1.1 x 10(-9) for Ni(II); RSD = 3-8%. A procedure for determining Cu(II), Fe(III), and Ni(II) in apple juice was developed.