Miroslav Rievaj

Voltammetric determination of caffeine in beverage samples on bare boron-doped diamond electrode

A sensitive and selective electrochemical method for the caffeine determination using bare boron-doped diamond electrode was developed. It was found that caffeine provided highly reproducible and well-defined irreversible oxidation peak at very positive potential. The effects of supporting electrolyte, pH and scan rate on the voltammetric response of caffeine oxidation were studied to select the optimum experimental conditions. Linear response of peak current on the concentration in the range from 4×10(-7) to 2.5×10(-5)M, good repeatability (RSD of 2.1%) and detection limit of 1.5×10(-7)M without any chemical modifications and electrochemical surface pretreatment were evaluated. The effect of possible interferents appeared to be negligible which evidently proved very good selectivity. The proposed method was successfully applied for the caffeine determination in commercially available beverage samples, with results in a close statistical agreement to those declared by manufacturer and HPLC used as independent method.

Voltammetric determination of penicillin V in pharmaceutical formulations and human urine using a boron-doped diamond electrode

Simple, sensitive and selective differential pulse voltammetric method for determination of penicillin V on a bare (unmodified) boron-doped diamond electrode has been developed. Penicillin V provided highly reproducible and well-defined irreversible oxidation peak at very positive potential of +1.6V (vs. Ag/AgCl). The optimum experimental conditions for oxidation of penicillin V were achieved in acetate buffer solution (pH 4.0). The modulation amplitude of 0.1V, modulation time of 0.05s and scan rate of 0.05Vs(-1) were selected as optimum instrumental parameters for differential pulse voltammetry. Linear response of peak current on the concentration in the range from 0.5 to 40μM with coefficient of determination of 0.999, good repeatability (RSD of 1.5%) and detection limit of 0.25μM were observed without any chemical modifications and electrochemical surface pretreatment. The effect of possible interferents such as stearic acid, glucose, urea, uric acid and ascorbic acid appeared to be negligible which evidently proved the good selectivity of method. The practical analytical utility of proposed method was demonstrated by determination of penicillin V in pharmaceutical formulations (tablets) and human urine samples with satisfactory recoveries (from 98 to 101% for tablets and 97 to 103% for human urine).

Indirect voltammetric detection of fluoride ions in toothpaste on a comb-shaped interdigitated microelectrode array

A novel technique based on dynamic electrochemistry for the detection of fluoride ions was developed. It is based on its strong complexation with ferric ion. Formed fluoroferric complex is cathodically inactive at the potential of the reduction of free ferric aquo ion. The voltammetric and amperometric response of platinum comb-shaped interdigitated microelectrode array is decreased after fluoride addition. This decrease serves for the quantification of fluoride ions added to the solution. The detection limit of 4.5×10(-5) mol dm(-3) was achieved when one of the segments of interdigitated microelectrode array (IDA) was used as an indicating electrode. The detection limit is about one order of magnitude lower than in the case of conventional platinum macroelectrode. In comparison with ISE electrodes this method is faster and also avoiding large error resulting from the antilogarithmization of ISE Nerstian response. The method was applied to the analysis of toothpaste.

Study of cyanoaquonitrosyl complexes of chromium. III. Formation of adducts of Cr(CN)(H2O)4NO+ with Ag+ and Hg2+ and their decomposition

The paper describes the conditions for the preparation of Cr(CN)(H2O)4NO+, its isolation in solution and its identification on the basis of the analysis of the components, spectrophotometric and elctrochemical properties and the kinetics of acid hydrolysis. The reactions of Cr(CN)(H2O)4NO+ with Ag+ and Hg2+ ions were studied from the point of view of the stoichiometry and equilibrium. The products of these reactions were characterized by the electronic absorption spectra and sorption on ion-exchangers. On the basis of the product properties, the course of the reactions and of analogous reactions, the structures characterized by groups CrNCHg and CrNCAg were proposed. The kinetics of the decomposition reactions of the Cr(CN)(H2O)4NO+ adducts with Ag+ and Hg2+, giving Cr(H2O)5NO2+ and Hg(CN)2, are described and the factors affecting them are discussed.

Determination of mercury in high-purity gallium arsenide using a gold-fibre voltammetric microelectrode

A gold-fibre microelectrode is proposed for the determination of mercury in high-purity gallium arsenide by stripping analysis. The precision and accuracy were tested using mercury-spiked gallium arsenide samples and were found to be satisfactory for mercury contents higher than approximately 10 ppb. Both the precision and accuracy exceed those of the method applying a compact gold rotating disc electrode. De-aeration of the sample solution and its stirring during accumulation are unnecessary in the analyses with gold-fibre microelectrodes.

Study of cyanoaquonitrosyl complexes of chromium. IV. Electrochemical reactions and complexation equilibria of the NO(H2O)4CrCN+ adducts with mercury(II)

Abstract The electrochemical reactions of bond isomers cis-NO(H 2 O) 4 Cr-CN + and cis-NO(H 2 O) 4 Cr-NC + and of their complexes with Hg 2+ are described. The reaction schemes proposed are based chiefly on polarographic and coulometric measurements. The measurement of the potentials of the reversible reduction of Hg(II) complexed in {NO(H 2 O) 4 Cr-NC} 2 Hg 4+ was used for the calculation of the formation constant of this complex from NO(H 2 O) 4 Cr-NC + and Hg 2+ , K = 1.43 X 10 21 M −2 at 25°C and an ionic strength of 1 M (ClO − 4 , Na + ), and of the acid-base dissociation constant of NO(H 2 O) 4 Cr-NCH 2+ , K a = 2.75 X 10 −2 M for the same conditions. An analysis of the limiting diffusion current of reduction of the complexed Hg(II) yielded the value of the equilibrium constant for the reaction of Hg 2+ with {NO(H 2 O) 4 Cr-NC} 2 Hg 4+ , Q = 7 ± 4 for 25°C and an ionic strength of 1 M (ClO − 4 , Na + , H + ).

Study of cyanoaquonitrosyl complexes of chromium. VI. Study of the NO(H2O)4CrNCHg3+ formation kinetics and of the NO(H2O)4CrNC+ isomerization☆

It has been found from a kinetic and spectrophotometric study that the formation of {NO[H2O}4CrNCHg3+ involves two steps: 1) rapid equilibrium association of NO(H2O)4CrCN+ with Hg2+_(the equilibrium constant K = (7.2 ± 0.8) X 102 M−1 for 278 K, an ionic strength of 1M and [H+] = 0.1 M); 2) the rate-determining step, independent of [Hg2+], with a rate constant of k = 6.0 X 10−2 s−1 for 278 K, an ionic strength of 1 M and [Hg+ = 0.1 M. The spectrophotometric measurements extrapolated to the instant of mixing of the reactants verify the formation of the associate and enable the calculation of the equilibrium constant in good agreement with the value obtained kinetically, as well at the construction of the intermediate visible spectrum. It has been found that the rate of the NO(H2O)4CrNC+ isomerization to the more stable NO(H2O)4CrCN+ is pH-dependent, as the isomerization rate constant for the reactant protonated form, kp, is given by kp < 0.05 kn, where kn = (26 ± 2) X 10−2 s−1 is the isomerization rate constant for the unprotonated form at 293.16 K and an ionic strength of 1 M. The equilibrium constant for this protonation is Qp = [NO[H2O)4CrNCH2+]/[NO(H2O)4CrNC+] [H+ = 45 ± 4 M−1 for the above conditions the NO(H2O)4CrNC+ visible spectrum was as the obtained. The two reactions studied are discussed from the point of view of the common mechanistic features and are compared with analogous reactions of cyanochromium(III) complexes that do not contain the NO ligand.

Study of cyanoaquonitrosyl complexes of chromium. V. Electrochemical properties of the cyanide bond isomers of NO(H2O)4CrCN+ and of their complexes with silver(I) and a study of their equilibria☆

Abstract The equilibrium of the CN bond isomers of NO(H 2 O) 4 Cr(CN) + was studied polarographically using the fast formation of the NO(H 2 O) 4 CrNC + complex with Hg 2+ . The isomerization equilibrium constant. Q i = [NO(H 2 O) 4 CrCN + ]/[NO(H 2 O) 4 CrNC + ] = (2.6 ± 0.1) X 10 2 , and the deprotonation equilibium constant, K a = [NO(H 2 O) 4 CrNC + [H + ]/ [NO(H 2 O) 4 CrNCH 2+ ] = (2.5 ± 0.2) X 10 −2 M, (both values at 25 °C and an ionic strength of 1 M ), obtained from these mesurements are in a good agreement with the values obtained by other methods. The electrochemical reactions of {NO(H 2 O) 4 CrNC} 2 Ag 3+ were described and explained from the viewpoint of the stochiometry and the mechanim of the formation reaction. The schemes proposed are based on voltammetric and coulometric measurements. From an analysis of the polarographic limiting diffussion current for the reduction of complexes Ag(I), the value of the equilibrium constant, Q = [NO(H 2 O) 4 CrNCAg 2+ ] 2 /[Ag + ][{NO(H 2 O) 4 CrNC} 2 Ag 3+ ] = (0.12 ± 0.08), was obtained for 25 °C and an ionic strength of 1 M .

MERCURY(II)-PROMOTED FORMATION AND AQUATION OF LINKAGE ISOMERS OF TRANS-DICYANOTETRAAMMINECHROMIUM(III). A POLAROGRAPHIC AND SPECTRAL STUDY

Polarography and spectrophotometry reveal various interactions between trans-Cr(NH3)4(CN)2+ and Hg2+ in acidic aqueous solution. There is evidence for formation of two trinuclear adducts with Cr:Hg proportions of 2:1 and 1:2, accompanied by linkage isomerization (CrNCHg) of the cyanide bridging groups. These processes are followed by Hg2+-induced aquation of CN−. The six Cr(III) species occurring in the reaction sequence are identified by their half-wave reduction potentials. The trans configuration of the final aquation product, Cr(NH3)4(H2O)23+, indicates complete stereoretention during all the reactions, consistent with the observed stoichiometry of CrHg association which differs from that of a cis-dicyano analogue. The linkage isomerization of trans-Cr(NH3)4(CN)2+ is much faster than that of trans-Cr(NH3)4(H2O)(CN)2+, while the rates of the respective Hg2+-promoted aquation reactions are the same, denoting trans effects of CN− in the former process but not in the latter.

Interaction of cyanopentaamminechromium(III) with mercuric ion. Electrochemical, spectral and preparative study of adduct formation and linkage isomerism

Abstract The reaction of Cr(NH 3 ) 5 (CN) 2+ with Hg 2+ was investigated in aqueous medium by various electrochemical techniques, as well as by absorption and emission spectrophotometry. Evidence was found for formation in solution of a stable 2:1 adduct, the polarographic and spectral properties of which indicate linkage isomerization (CrNCHg) of the cyano bridging group. In addition, two solids containing 4:1 and 2:1 Cr:Hg adduct species were isolated and characterized. The kinetics of adduct formation and subsequent Hg 2+ promoted CN − aquation were studied by monitoring the phosphorescence of cyanopentaamminechromium(III) end by polarography, respectively. The former reaction obeys the rate law: −d[Cr]/d t = k 1 [Cr]+ k 2 [Cr] 2 , with k 1 independent of the [Hg 2+ ]:[Cr] ratio in the 5–700 range, and k 2 decreasing as this ratio increases. The aquation rate is independent of both [Hg 2+ ] and [H 3 O + ]. The factors affecting the two reaction stages are briefly discussed.