Effects of pretreatments and modifiers on electrochemical properties of carbon paste electrodes

The electrochemical properties of carbon paste electrodes (CPEs), including unmodified and modified with protein and polycations, were investigated by impedance spectroscopy (IS) using ferricyanide and ferrocene monocarboxylic acid (FcMA) as redox probes. Various electrochemical pretreatments were applied to the unmodified CPE. The heterogeneous charge transfer rate constant of ferro/ferricyanide couple is enhanced by 2 to 10 times compared with that obtained at untreated electrodes. It was found that for ferricyanide the more suitable pretreatments are successive cyclic voltammetric scans, cathodization and a square wave-like stepping rather than high-potential anodization. However, the pretreatment only exhibits a slight effect on the kinetics of FcMA. At the CPEs containing modifier, the electron transfer rate of the redox couple depends more on the pH of electrolyte solution if ferro/ferricyanide is used. The results can be explained by the differently charged states of the CPEs that were caused by the protonation or deprotonation of the modifiers in various pH solutions and demonstrate the importance of the electrostatic interaction on the kinetics of the highly polar species such as ferricyanide. The different adsorptive behavior of ferricyanide and FcMA is also discussed.     

Electrochemical Study of the Lipid‐Transfer Protein

Electrochemical study of barley grain lipid‐transfer protein (LTP) revealed that it may belong to the metal‐binding protein class. Using differential pulse polarography the presence of Cu(II) and Zn(II) ions in the native LTP structure was proved, as well as its affinity for binding Ni(II) ion. Application of a more sensitive electroanalytical technique, such as anodic stripping voltammetry with analyte preconcentration, revealed the presence of Pb(II) and Cd(II) ions and also enabled the following Hg(II) ion binding. Possible biological role of LTP in plant stress response and its contribution to barley phytoextraction potential are discussed.     

铜膜差分脉冲阳极溶出伏安法测定药物中铋的含量

研究了铜膜电极代替汞膜电极测定重金属铋的差分脉冲溶出伏安法。实验了同位镀膜法测定铋的条件。在最佳实验条件下,Bi3+浓度在5×10-8~2×10-5mol/L范围内,其溶出峰峰高与浓度呈线性关系,检出限达到1×10-8mol/L。测定了一些药物中铋的含量,结果准确。     

Selective Determination of Quercetin Using Carbon Nanotube‐Modified Electrodes

Electrochemical detection of quercetin has been carried out on glassy carbon electrodes modified with carbon nanotubes and Nafion (GC/Nafion‐CNT). GC/Nafion‐CNT electrodes did not show passivation effect that occurs on the unmodified electrodes and displayed better stability and reproducibility. Quercetin oxidation was most favorable in acidic conditions and current gradually decreased as the solution pH increased. No oxidation was observed when two ? OH groups in a catechol moiety were fully deprotonated. These electrodes enabled selective determination of quercetin in the presence of interfering species such as ascorbic acid, uric acid, glucose, and catechol in large excess. Quantification of quercetin in a yellow onion has been made and favorably compared with reported values. Good selectivity and high sensitivity obtained by Osteryoung sSquare‐wave voltammetry can open new possibilities of direct quercetin determination in vegetables with a minimal sample treatment.     

米非司酮的吸附伏安特性

在ＰＨ７．０Ｂｒｉｔｔｏｎ－Ｒｏｂｉｎｓｏｎ缓冲溶液中，米非司酮（Ｍｉｆｅｐｒｉｓｔｏｎｅ，ＲＵ４８６）在汞电极上有２个不可逆的线性扫描还原峰，峰电位分别为－１．２３Ｖ和－１．６８Ｖ。本文探讨了第一峰的电化学行为。     

Simultaneous determination of trace uranium(VI) and zinc(II) by adsorptive cathodic stripping voltammetry with aluminon ligand

Uranium(VI) complexed with aluminon (3-[bis(3-carboxy-4-hydroxy-phenyl)methylene]-6-oxo-1,4-cyclohexadiene-1-carboxylic acid triammonium salt) was determined by adsorptive cathodic stripping voltammetry (ACSV) using a hanging mercury drop electrode. Trace uranium(VI) and zinc(II) can be simultaneously determined in a single scan in the presence of aluminon and urea. Optimal conditions were found to be: accumulation time; 180–200 s, accumulation potential; 50 mV versus Ag/AgCl, scan rate; 40 mV s⁻¹, supporting electrolyte; 0.1 M sodium acetate buffer at pH 6.5–7.0, and concentration of aluminon; 1×10⁻⁶ M. The linear range of uranium(VI) and zinc(II) were observed over the concentration range 2–33 and 30–120 ng ml⁻¹, respectively. The detection limit (S/N=3) are 0.2 ng ml⁻¹ (uranium) and 30 ng ml⁻¹ (zinc). A good reproducibility shows RSDs of 2.5–4.0% (n=10). The procedure offers high selectivity, with the presence of urea masking some metal ions.     

Sulfide ion electrooxidation catalysed by cobalt phthalocyanine microcrystals

Electrooxidation of sulfide ion catalysed by microcrystals of cobalt phthalocyanine was investigated by cyclic voltammetry in 0.5M KNO₃ at pH 9.22. Traces of catalyst were immobilized at the surface of a paraffin-impregnated graphite electrode by the mechanical transfer of its powder. The electro-oxidation of HS^– proceeds in two irreversible steps, with the first peak between 0 V and –0.12 V and the second at 0.17 V. The first step is second order in HS^– and its product is the adsorbed disulfide, which may further dissociate to give adsorbed sulfur atoms. The reduction of sulfur occurs at –0.1 V.     

Catalytic Adsorptive Stripping Voltammetry of Germanium(IV) in the Presence of Gallic Acid and Vanadium(IV)‐EDTA

A highly sensitive and selective catalytic adsorptive cathodic striping procedure for the determination of trace germanium is presented. The method is based on adsorptive accumulation of the Ge(IV)‐gallic acid (GA) complex onto a hanging mercury drop electrode, followed by reduction of the adsorbed species. The reduction current is enhanced catalytically by addition of vanadium(IV)‐EDTA. The optimal experimental conditions include the use of 0.03 mol/L HClO₄ (pH1.6), 6.0×10^?3 mol/L GA, 3.0×10^?3 mol/L V(IV), 4.0×10^?3 mol/L EDTA, an accumulation potential of ?0.10 V(vs. Ag/AgCl), an accumulation time of 120 s and a differential pulse potential scan mode. The peak current is proportional to the concentration of Ge(IV) over the range of 3.0×10^?11 to 1.0×10^?8 mol/L and the detection limit is 2×10^?11 mol/L for a 120 s adsorption time. The relative standard deviation at 5.0×10^?10 mol/L level is 3.1%. No serious interferences were found. The method was applied to the determination of germanium in ore, mineral water and vegetable samples with satisfactory results.     

Electrochemical Behavior of Diphenyl Disulfide and Thiophenol on Glassy Carbon and Gold Electrodes in Aprotic Media

The investigation of the electrochemical reduction processes of C₆H₅SSC₆H₅ and C₆H₅SH in CH₃CN using cyclic voltammetry indicates a different behavior on GC and Au electrodes. On GC surface adsorption phenomena are absent, the electrochemical reduction process is irreversible and diffusion controlled. For both the starting molecules the same species, C₆H₅S^?, is formed upon reduction. The E° values of the reduction processes were determined by convolution method and the standard free energy of the S? S bond of C₆H₅SSC₆H₅ estimated. On Au surface instead, a self‐assembled monolayer of C₆H₅SAu_ads originated after the S? S or S? H bond breaking can be observed by simply dipping the electrode in solution of C₆H₅SSC₆H₅ and C₆H₅SH, respectively. The properties of the SAM were investigated by electrochemical reduction of the adsorbed thiolates. On Au electrode the reduction processes involve C₆H₅SAu_ads and give rise to desorbed C₆H₅S^?. A neutral radical is obtained by electrochemical oxidation of thiolate anion. It reacts rapidly with the electrode surface to give the S‐Au bond again.     

Voltammetric behaviour of bromhexine and its determination in pharmaceuticals

A complete electrochemical study and a novel electroanalytical procedure for bromhexine quantitation are described. Bromhexine in methanol/0.1 mol L⁻¹ Britton–Robinson buffer solution (2.5/97.5) shows an anodic response on glassy carbon electrode between pH 2 and 7.5. By DPV and CV, both peak potential and current peak values were pH-dependent in all the pH range studied. A break at pH 5.5 in E_P versus pH plot revealing a protonation–deprotonation (pK_a) equilibrium of bromhexine was observed. Spectrophotometrically, an apparent pK_a value of 4.3 was also determined.

An electrodic mechanism involving the oxidation of bromhexine via two-electrons and two-protons was proposed. Controlled potential electrolysis followed by HPLC–UV and GC–MS permitted the identification of three oxidation products: N-methylcyclohexanamine, 2-amino-3,5-dibromobenzaldehyde and 2,4,8,10-tetrabromo dibenzo[b,f][1,5] diazocine.

DPV at pH 2 was selected as optimal pH for analytical purposes. Repeatability, reproducibility and selectivity parameters were adequate to quantify bromhexine in pharmaceutical forms. The recovery was 94.50 ± 2.03% and the detection and quantitation limits were 1.4 × 10⁻⁵ and 1.6 × 10⁻⁵ mol L⁻¹, respectively. Furthermore, the DPV method was applied successfully to individual tablet assay in order to verify the uniformity content of bromhexine. No special treatment of sample were required due to excipients do not interfered with the analytical signal. Finally the method was not time-consuming and less expensive than the HPLC one.