Optimization of analytical procedures for the simultaneous voltammetric determination of total Hg(II) in presence of Cu(II) in environmental matrices

The present work reports the critical comparison about the employment of three different supporting electrolytes (0.1 mol L⁻¹ HClO₄, 0.01 mol L⁻¹ EDTA-Na² + 0.06 mol L⁻¹ NaCl + 2.0 mol L⁻¹ HClO₄ and 0.1 mol L⁻¹ KSCN + 0.001 mol L⁻¹ HClO₄) and their instrumental and chemical optimisation for the simultaneous voltammetric determination of total mercury(II) and copper(II) in sediments and sea water at gold electrode, especially discussing the reciprocal interference problems.     

Study and Application of Electrochemical Behavior of Calcium-ARS on a Mercury Film Electrode

A sensitive complex absorptive wave of Ca-ARS was obtained by using differential pulse voltammetry when a mercury film glass carbon electrode was immersed in 0.1 mol L⁻¹ KOH and 4.5×10⁻⁴ mol L⁻¹ ARS solution. The peak potential obtained was −1.17 V (vs Ag-AgCl). The peak current was proportional to the concentration of calcium in the range of 5.0×10⁻⁸−4.2×10⁻⁵ mol L⁻¹. The detection limit was 2.0×10⁻⁸ mol L⁻¹. This method was applied successfully to determining traces of calcium in blood serum. The electrochemical behavior of the system was also studied by cyclic voltammetry, and the experiment results showed that the electrode process was an irreversible absorptive with two electrons participating. Translated from Journal of Beijing Normal University (Natural Science Edition), 2005, 41(2) (in Chinese)     

Determination of human serum albumin using aurothiomalate as electroactive label

A new electroactive label has been used to monitor immunoassays in the determination of human serum albumin (HSA) using glassy-carbon electrodes as supports for the immunological reactions. The label was a gold(I) complex, sodium aurothiomalate, which was bound to rabbit IgG anti-human serum albumin (anti-HSA-Au). The HSA was adsorbed on the electrode surface and the immunological reaction with gold-labelled anti-HSA was then performed for one hour by non-competitive or competitive procedures. The gold(I) bound to the anti-HSA was electrodeposited in 0.1 mol L⁻¹ HCl at −1.00 V for 5 min then oxidised in 0.1 mol L⁻¹ H₂SO₄ solution at +1.40 V for 1 min. Silver electrodeposition at −0.14 V for 1 min followed by anodic stripping voltammetry were then performed in aqueous 1.0 mol L⁻¹ NH₃–2.0×10⁻⁴ mol L⁻¹ AgNO₃. For both non-competitive and competitive formats, calibration plots in the ranges 5.0×10⁻¹⁰ to 1.0×10⁻⁸ mol L⁻¹ and 1.0×10⁻¹⁰ to 1.0×10⁻⁹ mol L⁻¹ HSA, respectively, with estimated detection limits of 1.5×10⁻¹⁰ mol L⁻¹ (10 ng mL⁻¹) and 1.0×10⁻¹⁰ mol L⁻¹ (7 ng mL⁻¹), respectively, were obtained. Levels of HSA in two healthy volunteer urine samples were also evaluated, using both immunoassay formats.     

Simultaneous determination of phenylenediamine isomers and dihydroxybenzene isomers in hair dyes by capillary zone electrophoresis coupled with amperometric detection

The simultaneous determination of three isomers of phenylenediamines (o, m, and p-phenylenediamine) and two isomers of dihydroxybenzenes (catechol and resorcinol) in hair dyes was performed by capillary zone electrophoresis coupled with amperometric detection (CZE–AD). The effects of working electrode potential, pH and concentration of running buffer, separation voltage, and injection time on CZE–AD were investigated. Under the optimum conditions the five analytes could be perfectly separated in 0.30 mol L⁻¹ borate–0.40 mol L⁻¹ phosphate buffer (pH 5.8) within 15 min. A 300 μm diameter platinum electrode had good responses at +0.85 V (versus SCE) for the five analytes. Their linear ranges were from 1.0 × 10⁻⁶ to 1.0 × 10⁻⁴ mol L⁻¹ and the detection limits were as low as 10⁻⁷ mol L⁻¹ (S/N = 3). This working electrode was successfully used to analyze eight kinds of hair dye sample with recoveries in the range 91.0–108.0% and RSDs less than 5.0%. These results demonstrated that capillary zone electrophoresis coupled with electrochemical detection using a platinum working electrode as detector was convenient, highly sensitive, highly repeatable and could be used in the rapid determination of practical samples. Figure Electropherograms obtained from 10 mg mL⁻¹ hair dye sample solutions at a platinum working electrode under optimum CZE–AD conditions: (a) natural black (I), (b) golden: (1) p-phenylenediamine, (2) m-phenylenediamine, (3) o-phenylenediamine, (4) resorcinol, and (5) catechol     

Preconcentration and Voltammetric Determination of the Antihypertensive Doxazosin on a C8 Modified Carbon Paste Electrode

 An electrochemical study of the doxazosin oxidative process at carbon paste electrodes using different voltammetric techniques has been carried out. The process is irreversible and controlled by adsorption, giving rise to an oxidation wave around 1.0 V in citric acid-citrate buffer (pH 3.0). A mechanism based on the oxidation of the amine group is postulated. Two methods based on adsorptive stripping (AdS) of doxazosin at the C₈-modified carbon paste electrode (C₈-MCPE), before its voltammetric determination, are studied, using differential pulse voltammetry (DPV) and square wave voltammetry (SWV) as redissolution techniques. By means of AdS-DPV and C₈-MCPE, doxazosin can be determined over the 1.0 × 10⁻⁹ to 3.0 × 10⁻⁸ mol L⁻¹ range with a variation coefficient of 2.2% (2.0 × 10⁻⁸ mol L⁻¹) and a limit of detection of 7.4 ×10⁻¹⁰ mol L⁻¹. If AdS-SWV is used, a linear range from 1.0 × 10⁻⁹ to 4.0 × 10⁻⁸ mol L⁻¹ is obtained, the variation coefficient being 2.8% (2.0 × 10⁻⁸ mol L⁻¹, and the limit of detection reached 7.7 × 10⁻¹⁰ mol L⁻¹. The AdS-DPV procedure was applied to the determination of doxazosin in urine and formulations. Received March 13, 1999. Revision December 23, 1999.     

Mechanism of the reduction of molecular nitrogen to hydrazine by niobium (iii) hydroxide

The effect of the concentration of water on the rate of reduction of molecular nitrogen to hydrazine by niobium(iii) hydroxide in alkaline H₂O−MeOH and D₂O−MeOD mixtures was studied. In both cases, the reaction rate is maximum when [H₂O]=4 mol L⁻¹, and the inverse isotopic effect (K ^D/k ^H>1) is observed when [H₂O]<20 mol L⁻¹. Similar regularity was observed for the reaction of hydrogen elimination. It was found that HD is formed in the H₂O−MeOH system in the presence of D₂. The conclusion was made that the ratedetermining stage in hydrazine formation is the transfer of a hydride ion to the dinitrogen molecule coordinated to the binuclear Nb^III center. A kinetic scheme satisfactorily explaining the effect of the concentration of water ([H₂O]=1.5−49.0 mol L⁻¹) on the reaction rate constant was proposed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1600–1604, September, 1997.     

A comparative spectroelectrochemical study of the redox electrochemistry of nitroanilines

The oxidative and reductive electrochemistry of the three isomeric nitroanilines has been studied in neutral (0.1 mol L⁻¹ KClO₄) and acidic (0.1 mol L⁻¹ HClO₄) aqueous electrolyte solutions by cyclic voltammetry and surface enhanced Raman spectroscopy (SERS). The cyclic voltammograms recorded for o- and p-nitroanilines with a gold electrode in acidic solution, scanning toward negative potentials, revealed formation of phenylenediamine not observed in neutral solution. Similar behavior of nitroanilines and phenylenediamines was observed on gold and platinum electrodes. An oxygen–gold adsorbate stretching mode was detected between 400 and 430 cm⁻¹ in the SER-spectra of the three isomeric nitroanilines in both electrolyte solutions at positive electrode potentials, implying perpendicular adsorption via the nitro group.     

Determination of p-nitroaniline by the tartrate-acetone-Mn2+-KBrO3-H2SO4 double organic substrate oscillating system using non-equilibrium stationary state

This paper described the determination of p-nitroaniline in a double organic substrate oscillating system of tartrate-acetone-Mn²⁺-KBrO₃-H₂SO₄. Under the optimum conditions, temperature was chosen as a control parameter to design the bifurcation point and proposed a convenient method for determination of p-nitroaniline. Results showed that the system consisting of 3.5 mL 0.06 mol L⁻¹ tartrate, 4.0 mL 0.7 mol L⁻¹ H₂SO₄, 1.5 mL 1.5×10⁻⁴ mol L⁻¹ MnSO₄, 4.0 mL 0.4 mol L⁻¹ acetone and 7.0 mL 0.05 mol L⁻¹ KBrO₃ was very sensitive to the surrounding at 33.5°C. A good linear relationship between the potential difference and the negative logarithm concentration of p-nitroaniline was obtained to be in the range of 2.50×10⁻⁷∼3.75×10⁻⁵ mol L⁻¹ with a lower detection limit of 2.50×10⁻⁸ mol L⁻¹.      

Adsorptive anodic stripping voltammetry of zirconium(IV)-alizarin red S complex at a carbon paste electrode

A sensitive adsorptive anodic stripping procedure for the determination of trace zirconium at a carbon paste electrode (CPE) has been developed. The method is based on adsorptive accumulation of the Zr(IV)-alizarin red S(ARS) complex onto the surface of the CPE, followed by oxidation of adsorbed species. The optimal experimental conditions include the use of 0.10 mol · L⁻¹ ammonium acetate buffer (pH 4.3), ARS, an accumulation potential of 0.20 V (versus SCE), an accumulation time of 2 min, a scan rate of 200 mV · s⁻¹ and a second-order derivative linear scan mode. The oxidation peak for the complex appears at 0.69 V. The peak current is proportional to the concentration of Zr(IV) over the range of 1.0 × 10⁻⁹–2.0 × 10⁻⁷ mol · L⁻¹, and the detection limit is 3 × 10⁻¹⁰ mol · L⁻¹ for a 2 min adsorption time. The relative standard deviations (n = 8) for 5.0 × 10⁻⁸ and 5.0 × 10⁻⁹ mol · L⁻¹ Zr(IV) are 3.3 and 4.8%, respectively. The proposed method was applied to the determination of zirconium in ore samples with satisfactory results.     

Voltammetry of resazurin at a mercury electrode

Electrochemical behavior of resazurin on HMDE in Britton-Robinson (B-R) buffers (pH 2.0–10.0) was studied using the square-wave voltammetry (SWV), square-wave adsorptive stripping voltammetry (SWAdSV), and cyclic voltammetry (CV) techniques. The voltammogram of resazurin in B-R buffer at pH < 4.0 exhibited two cathodic reduction peaks. The voltammetric peaks were obtained at −0.144 V (reversible) and −1.250 V (irreversible) at pH 3.2, and correspond to the reduction of resorufin to dihydroresorufin and to the catalytic hydrogen wave, respectively. At pH > 4.0, a new irreversible cathodic reduction peak, assigned to the protonation of N-oxide on the phenoxazin ring, was observed. Electrochemical parameters (I _p/E _p, I _p/v, I _p/pH, I _p/t _acc) of the compound were determined. From the voltammetric data, electrochemical reduction mechanisms for all peaks have been suggested. Maximum peak current for the reversible peak was obtained at pH 4.1. A linear relationship between the current and concentration was determined, and also the lowest detection limit was found as 3.25 × 10⁻⁸ mol L⁻¹ and 1.98 × 10⁻¹⁰ mol L⁻¹ for SWV and SWAdSV, respectively.     

Development of a sequential injection anodic stripping voltammetry (SI-ASV) method for determination of Cd(II), Pb(II) and Cu(II) in wastewater samples from coatings industry

This paper describes the development and validation of a sequential injection (SI) anodic stripping voltammetry (ASV) method using the hanging mercury drop electrode for accumulation of the heavy metal cations Cu(II), Pb(II) and Cd(II). The method was applied to wastewater samples after proper acid digestion in open vessels to eliminate matrix effects. For a deposition time of 90 s at the flow rate of 10 μl s⁻¹, the detection limits of the method were 0.06, 0.09 and 0.16 μmol L⁻¹ for Cd, Pb and Cu, respectively. Under these conditions the linear dynamic range was between 0.20 and 9.0 μmol L⁻¹ and the sampling frequency was 30 analyses per hour. The relative standard deviation of the method was 3%(n=7) at the concentration level of 2.0 μmol L⁻¹. The accuracy of the method was evaluated by spiking the samples with known amounts of the metal cations, and by comparison with an independent analytical technique, the inductively coupled plasma atomic emission spectroscopy (ICP-AES). Average recoveries were around of 84%, and the results showed no evidence of systematic errors in comparison to the ICP-AES.     

Voltammetry and amperometric detection of tetracyclines at multi-wall carbon nanotube modified electrodes

The voltammetric behaviour and amperometric detection of tetracycline (TC) antibiotics at multi-wall carbon nanotube modified glassy carbon electrodes (MWCNT-GCE) are reported. Cyclic voltammograms of TCs showed enhanced oxidation responses at the MWCNT-GCE with respect to the bare GCE, attributable to the increased active electrode surface area. Hydrodynamic voltammograms obtained by flow-injection with amperometric detection at the MWCNT-GCE led us to select a potential value E _det = +1.20 V. The repeatability of the amperometric responses was much better than that achieved with bare GCE (RSD ranged from 7 to 12%), with RSD values for i _p of around 3%, thus demonstrating the antifouling capability of MWCNT modified electrodes. An HPLC method with amperometric electrochemical detection (ED) at the MWCNT-GCE was developed for tetracycline, oxytetracycline (OTC), chlortetracycline and doxycycline (DC). A mobile phase consisting of 18:82 acetonitrile/0.05 mol L⁻¹ phosphate buffer of pH 2.5 was selected. The limits of detection ranged from 0.09 μmol L⁻¹ for OTC to 0.44 μmol L⁻¹ for DC. The possibility to carry out multiresidue analysis is demonstrated. The HPLC-ED/MWCNT-GCE method was applied to the analysis of fish farm pool water and underground well water samples spiked with the four TCs at 2.0 × 10⁻⁷ mol L⁻¹. Solid-phase extraction was accomplished for the preconcentration of the analytes and clean-up of the samples. Recoveries ranged from 87 ± 6 to 99 ± 3%. Under preconcentration conditions, limits of detection in the water samples were between 0.50 and 3.10 ng mL⁻¹.     

Determination of rutin using a CeO2 nanoparticle-modified electrode

CeO₂ nanoparticles approximately 12 nm in size were synthesized and subsequently characterized by XRD, TEM and UV-vis spectroscopy. Then, a gold electrode modified with CeO₂ nanoparticles was constructed and characterized by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The modified electrode demonstrated strong catalytic effects with high stability towards electrochemical oxidation of rutin. The anodic peak currents (measured by differential pulse voltammetry) increased linearly with the concentration of rutin in the range of 5.0 × 10⁻⁷–5.0 × 10⁻⁴ mol · L⁻¹. The detection limit (S/N = 3) was 2.0 × 10⁻⁷ mol · L⁻¹. The relative standard deviation (RSD) of 8 successive scans was 3.7% for 5.0 × 10⁻⁶ mol · L⁻¹ rutin. The method showed excellent sensitivity and stability, and the determination of rutin in tablets was satisfactory.     

Role of mononuclear rare earth metal complexes in promoting the hydrolysis of p-nitrophenyl phosphate (NPP)

Two long-chain multidentate ligands: 2,9-di-(n-2′,5′,8′-triazanonyl)-1,10-phenanthroline (L₁) and 2,9-di-(n-4′,7′,10′-triazaundecyl)-1,10-phenanthroline (L₂) were synthesized. The hydrolytic kinetics of p-nitrophenyl phosphate (NPP) catalyzed by complexes of L₁ and L₂ with La(III) and Gd(III) have been studied in aqueous solution at 298 K, I = 0.10 mol · dm⁻³ KNO₃ at pH 7.5–9.1, respectively. The study shows that the catalytic effect of GdL1 was the best in the four complexes for hydrolysis of NPP. Its k_LnLH−1, k _LnL and pK _a are 0.0127 mol⁻¹ dm³ s⁻¹, 0.000022 mol⁻¹ dm³ s⁻¹ and 8.90, respectively. This paper expounds the result from the structure of the ligands and the properties of the metal ions, and deduces the catalysis mechanism.     

Effect of Constant Glucose Feeding on the Production of Exopolysaccharides by <Emphasis Type="Italic">Tremella fuciformis</Emphasis> Spores

A fed-batch culture system with constant feeding (glucose 80 g L⁻¹, 0.25 ml min⁻¹) was used to study the influence of glucose on cell dry weight and exopolysaccharides production from submerged Tremella fuciformis spores in a 5-L stirred-tank bioreactor. The results showed that high levels of cell mass (9.80 g L⁻¹) and exopolysaccharides production (3.12 g L⁻¹) in fed-batch fermentation were obtained after 1 h of feeding, where the specific growth rate (μ) and exopolysaccharides yield on substrate consumed (YP/S) were 0.267 d⁻¹ and 0.14 g g⁻¹. Unlike batch fermentation, maximal cell mass and exopolysaccharides production merely reached 7.11 and 2.08 g L⁻¹; the specific growth rate (μ) and exopolysaccharides yield on substrate consumed (YP/S) were 0.194 d⁻¹ and 0.093 g g⁻¹, respectively. It is concluded that the synthesis of exopolysaccharides can be promoted effectively when feeding glucose at a late exponential phase.     

Receptor properties of calix[4]resorcinarenes toward tetramethylammonium and choline cations in micellar solutions of sodium dodecyl sulfate

Concentration range of solubilization of calix[4]resorcinarene (H₈L) in sodium dodecyl sulfate (SDS) micelles was found. The interaction of the deprotonated form of H8L (tetraanions [H₄L]⁴⁻) with tetramethylammonium (TMA) and choline cations in micellar solutions of SDS was studied by pH-metry and NMR spectroscopy. The concentration dependences of the change in the cloud point in a multicomponent system TMA (choline)-[H₄L]⁴-SDS-tetrabutylammonium bromide were determined. A correlation of these dependences with host-guest binding processes was found. The sharp change in the cloud points of the corresponding micellar solutions in concentration regions of TMA (0-5·10⁻⁴ mol L⁻¹) and choline (0–1.1· 10⁻³ mol L⁻¹) is caused by the formation of inclusion complexes TMA (choline)-[H₄L]⁴⁻ at the interface of the aqueous and micellar pseudophases. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1366–1371, August, 2006.     

Determination of sulfanilamide based on the Mn(II)-catalyzed oscillating chemical reaction

A convenient and sensitive method for determination of sulfanilamide (SNA) was described based on the Mn(II)-catalyzed oscillating chemical reaction. Under optimum conditions, a linear relationship existed between the changes of oscillating period or amplitude and the negative of logarithm of SNA concentration in the range of 4.27 × 10⁻⁸ mol ·L⁻¹ ∼ 7.41 × 10⁻⁶ mol ·L⁻¹ (RSD, 0.85%) and 9.33 × 10⁻⁸ mol ·L⁻¹ ∼ 3.02 × 10⁻⁶ mol ·L⁻1 (RSD, 1.08%), respectively. The lower limit of detection was found to be 2.69 × 10⁻⁸ mol ·L⁻¹ and 6.03 × 10⁻⁸ mol ·L⁻¹, respectively.      

Effect of Ionic Strength and Temperature on the Protonation of Oxidized Glutathione

The protonation constants for oxidized glutathione, H _i−1L^(4−i+1)−, K _i ^H=[H _i L⁽⁴⁻ⁱ⁾⁻]/[H _i−1L^(4−i+1)−][H⁺] i=1,2,…,6 have been measured at 5, 25 and 45 °C as a function of the ionic strength (0.1 to 5.4 mol⋅[kg(H₂O)]⁻¹) in NaCl solutions. The effect of ionic strength on the measured protonation constants has been used to determine the thermodynamic values (K _i ^H0) and the enthalpy (ΔH _i ) for the dissociation reaction using the SIT model and Pitzer equations. The SIT (ε) and Pitzer parameters (β ⁽⁰⁾, β ⁽¹⁾ and C) for the dissociation products (L⁴⁻, HL³⁻, H₂L²⁻, H₃L⁻, H₄L, H₅L⁺, H₆L²⁺) have been determined as a function of temperature. These results can be used to examine the effect of ionic strength and temperature on glutathione in aqueous solutions with NaCl as the major component (body fluids, seawater and brines).     

Fluorescence-detecting cationic surfactants using luminescent CdTe quantum dots as probes

A novel fluorescence quenching method for the determination of cationic surfactants (CS), specifically cetyltrimethylammonium bromide (CTAB), dodecyltrimethylammonium bromide (DTAB), and cetylpyridinium chloride (CPC), has been developed using water-soluble luminescent CdTe quantum dots (QDs) modified with thioglycolic acid (TGA). The possible interference from heavy and transition metals (HTM) has been efficiently eliminated through simple sample treatment with mercapto cotton made in-house. Under optimum conditions, the extent of fluorescence quenching of CdTe QDs is linearly proportional to the concentration of CS from 2.0 × 10⁻⁷ to 7.0 × 10⁻⁶ mol L⁻¹ with a detection limit of 5.0 × 10⁻⁸ mol L⁻¹. The relative standard deviation for 1.0 × 10⁻⁶ mol L⁻¹ CTAB is 2.5% (n = 6). The proposed method exhibits high sensitivity and selectivity and furthermore avoided the use of toxic organic solvents and tedious solvent extraction procedures. It has been applied to the determination of trace CS in natural river water and commodity samples with satisfactory results. Potential interference from heavy and transition metals is eliminated during photoluminescence detection of CS through simple sample pre-treatment with mercapto cotton     

Simplified cloud point extraction for the preconcentration of ultra-trace amounts of gold prior to determination by electrothermal atomic absorption spectrometry

A simple and sensitive cloud point extraction method has been developed for the preconcentration of ultra-trace amounts of gold as a prior step to its determination by electrothermal atomic absorption spectrometry. It is based on the extraction of gold in hydrochloric acid medium using the non-ionic surfactant polyethyleneglycolmono-p-nonylphenylether (PONPE 7.5) without adding a chelating agent. The preconcentration of a 50 mL sample solution was thus enhanced by a factor of 200. The resulting calibration graph was linear in the range of 10–200 ng L⁻¹ with a correlation coefficient of 0.9993. The limit of detection (3s) obtained under optimal conditions was 2.0 ng L⁻¹. The relative standard deviation for 10 replicate determinations at a 100 ng L⁻¹ Au level was 3.6%. The method was applied to the ultra-trace determination of gold in water and copper samples.