Flow-through Electrochemical Cell Research Articles

Stripping Voltammetry with a Flow-through Electrochemical Cell for Simultaneous Trace Determination of Nickel(II) and Copper(II) in Concentrated Alkaline Solutions

A highly sensitive method for the on-line simultaneous determination of trace CuII and NiII in high-purity alkaline solutions has been demonstrated by anodic stripping voltammetry (ASV) using a radial flow cell Au electrode (RFCE-Au). In this method, the electrolysis process is performed while alkaline solutions flow through RFCE-Au without any pretreatment. Under the optimum conditions, linear calibration graphs for CuII and NiII were obtained in the range of 0.5-6.0 μg dm−3 in KOH solution and 0.5-5.0 μg dm−3 in NaOH solution, respectively. The detection limits of CuII and NiII, defined as 3 σ of the 0.5 μg dm−3 signal, were 0.047 μg dm−3 and 0.13 μg dm−3 in KOH solution, and 0.058 μg dm−3 and 0.15 μg dm−3 in NaOH solution, respectively. The proposed method is applicable to on-line trace analysis of CuII and NiII, which has been urgently needed in the semiconductor manufacturing process.

Measurement of pH in high-temperature nickel laterite pressure acid leach process solutions

The performance of a flow-through electrochemical cell with an yttria-stabilized zirconia (YSZ) sensor for pH measurement in high-temperature nickel laterite pressure acid leach (PAL) process solutions was evaluated. Very good agreement was observed between the measured pH values and those theoretically predicted using the OLI Systems software package calibrated independently based on solubility measurements. PAL process solutions were simplified to ternary MgSO 4–Al 2(SO 4) 3–H 2SO 4 solutions by combining all the divalent metal sulphates into one with the properties of MgSO 4. The experimental results support the postulation that the high-temperature behaviour of nickel, cobalt and manganese sulphates can be satisfactorily approximated with that of MgSO 4. The experimental findings also support the postulation that acid should be added to a PAL process so that the solution pH is around 1 at the leach temperature, regardless of the feed composition.

Determination of Dissociation Constants of Hexanoic, Heptanoic, and Benzoic Acids to 673 K and 30 MPa by Potentiometric pH Measurements

Potentiometric pH measurements of 0.01 mol·kg−1 aqueous solutions of hexanoic, heptanoic, and benzoic acids in 0.1 mol·kg−1 NaCl media were performed using a flow-through electrochemical cell with two pressure-balanced hydrogen platinum electrodes. The dissociation constants of these carboxylic acids were determined on the basis of pH values measured at temperatures from (322 to 673) K and pressures of (23 and 30) MPa. The dissociation constants were consistent with the literature values determined from calorimetry, electrical conductivity, and potentiometry at temperatures up to 523 K. A simple Born model for describing temperature and pressure dependence of the dissociation constants was used to correlate the data.

Determination of Tetracyclines Based on the Flow-through Electrochemical Detection Cell with Nickel Electrode

Determination of Tetracyclines Based on the Flow-through Electrochemical Detection Cell with Nickel Electrode

Electrochemical Degradation of Benzoquinone in a Flow through Cell with Carbon Fibers

The anodic degradation of benzoquinone(BQ), a model compound for wastewater treatment was carried out using a home-made flow-through electrochemical cell with carbon fibers. To optimize the controlled current electrolysis condition of an aqueous BQ solution, the experimental variables affecting the degradation of BQ, such as the applying current, pH, reaction time, and flow rate of the BQ solution were examined. The degradation products of the oxidation reaction were identified by High Performance Liquid Chromatography and Inductively Coupled Plasma Atomic Emission Spectrometer. Low molecular weight aliphatic acids, and CO2 were the major products in this experiment. The removal efficiency of BQ from the solution increased with the applying current and time. 99.23% of 1.0 × 10-2 M BQ was degraded to aliphatic acids and CO2 when the applying current is 175 mA in a 12 hr electrolysis.

Determination of dissolved sulphides in waste water samples by flow-through stripping chronopotentiometry with a macroporous mercury-film electrode

Sulphides in water samples were determined by stripping chronopotentiometry in a computer controlled flow system with a flow-through electrochemical cell. The working electrode was a porous glassy carbon electrode coated with Nafion and mercury. The sample was diluted with 0.1 mol L −1 NaOH and analysed. Sulphides in the sample were collected in the porous electrode as mercury sulphide and then stripped by a current of −500 μA. The limit of detection was found to be 1.6 μg L −1 and 0.5 μg L −1 for 1 mL and 5 mL of preconcentrated sample, respectively. The linear range for 1 mL sample was found to be 5–400 μg L −1. The repeatability and reproducibility was found to be 2.6% and 4.8%, respectively. The method was applied to analyses of waste water samples from a tannery.

Determination of As(III) and total as in water by graphite furnace atomic absorption spectrometry after electrochemical preconcentration on a gold-plated porous glassy carbon electrode

AbstractArsenic(III) was preconcentrated in a flow-through electrochemical cell on a gold coated porous carbon electrode. On stripping, arsenic was eluted with diluted nitric acid and determined off-line by GF AAS. The deposition and stripping steps were optimized. The limit of detection and limit of quantification were found to be 1.9 µg L−1 and 6.4 µg L−1, respectively. The repeatability and reproducibility were found to be 5.3 % and 9.3 %, respectively. Total arsenic was determined after a microwave assisted chemical reduction of As(V) to As(III) making the procedure suitable for speciation analysis. The method was applied in analysis of water samples.

Polymer-coated bismuth film electrodes for the determination of trace metals by sequential-injection analysis/anodic stripping voltammetry

In this work, the utility of polymer-coated bismuth film electrodes (BiFEs) was assessed for the simultaneous on-line determination of Cd(II), Pb(II) and Zn(II) by square-wave anodic stripping voltammetry (SWASV) in the sequential-injection analysis (SIA) mode. The polymeric Nafion coating was initially plated on a glassy carbon electrode forming part of the flow-through electrochemical cell. The subsequent steps of the bismuth layer generation, analyte preconcentration, voltammetric measurement and electrode cleaning were conducted on-line. For a sample volume of 1.2 mL, the limits of detection (at S/N = 3) were 2 μg L −1 for Cd(II) and Pb(II) and 6 μg L −1 for Zn(II), the coefficients of variation at 20 μg L −1 were 5.1% for Cd(II), 5.9% for Pb(II) and 6.2% for Zn(II) and the measurement frequency was between 10 and 20 h −1 (depending on the sample volume aspirated). It is demonstrated that the polymer-coated BiFEs, combined with SIA, can provide an environmentally friendly, sensitive and robust tool to perform rapid and cost-efficient on-line monitoring of trace metals by ASV, even in the presence of surface-active compounds.

Direct electrochemistry and bioelectrocatalysis of H2O2 reduction of recombinant tobacco peroxidase on graphite. Effect of peroxidase single-point mutation on Ca2+-modulated catalytic activity

Direct electron transfer (DET) reactions and bio(electro)catalytic reduction of H2O2 catalysed by native and recombinant forms of tobacco peroxidase (nTOP and rTOP) were studied in homogeneous-phase catalysis and when TOPs were adsorbed on graphite electrodes. Non-glycosylated wild type and Glu141 → Phe mutant forms of rTOP were produced using an Escherichia coli expression system. Mutation was introduced to explore the mechanisms for modulation of the catalytic activity of TOP by Ca2+ ions. At the pH optimum of 5.0, direct electrochemical Fe3+/2+ transformation of the peroxidase heme was characterised by potentials of −208 mV (nTOP) and −239 mV vs. Ag|AgCl (rTOP), and 0.9 ± 0.1 and 1.1 ± 0.4 pmoles of adsorbed nTOP and rTOP, correspondingly, were in DET contact with graphite. Kinetic analysis of amperometric (at +50 mV) data on H2O2 reduction at TOP-modified electrodes, placed in a wall-jet flow-through electrochemical cell, yielded 82% (nTOP) and 88% (rTOP) of adsorbed TOP molecules active in the DET reaction. The efficiency of DET (and bioelectrocatalysis) increased 3.5-fold when changing from glycosylated nTOP to rTOP. The Glu141 → Phe mutation in the heme-binding pocket of rTOP enabled to achieve a Ca2+-tolerance of TOP in the reaction with H2O2, which is characteristic of other plant peroxidases, and to a large extent in heterogeneous DET and reaction with a second substrate catechol. The results promote further applications of TOP for biosensor- and solid-phase biocatalysts development.

Direct electrochemistry of recombinant tobacco peroxidase on gold

Direct electron transfer (DET) reactions of recombinant tobacco peroxidase (rTOP), namely direct electroreduction of Compound I/Compound II and heme Fe 3+/2+ conversion, were studied on gold electrodes. rTOP of wild type, non-glycosylated, was produced using an Escherichia coli expression system. At pH 5.0, the redox potential for direct electrochemical transformation of the Fe 3+/2+ of the peroxidase heme was −143 mV vs. Ag∣AgCl, and 0.26 ± 0.07 pmol of the adsorbed rTOP were in DET contact with the gold electrode. The total amount of the adsorbed rTOP estimated from QCM data was 53 ± 5 pmol/cm 2 or 1.67 pmol when referred to the surface area of the electrodes used for electrochemical measurements. Of 1.67 pmol of adsorbed rTOP, only 0.76 pmol were catalytically active. DET between Au and the enzyme was also studied in the reaction of the bioelectrocatalytic reduction of H 2O 2 by cyclic voltammetry and amperometric detection of H 2O 2 at +50 mV with rTOP-modified Au electrodes placed in a wall-jet flow-through electrochemical cell. Maximal bioelectrocatalytic current response of the rTOP-modified gold electrodes to H 2O 2 was observed at pH 5.0 and stemmed from its bioelectrocatalytic reduction based on DET between Au and the active site of rTOP. Kinetic analysis of the DET reactions gave 52% of the adsorbed rTOP molecules active in DET reactions (0.4 pmol of adsorbed catalytically active rTOP, correspondingly), which correlated well with the non-catalytic-voltammetry data. DET was characterised by a heterogeneous ET rate constant of 13.2 s −1, if one takes into account the QCM data, and 19.6 s −1, if the amount of rTOP estimated from the data on DET transformation of Fe 3+/2+ couple of rTOP is considered. The sensitivity for H 2O 2 obtained for the rTOP-modified Au electrodes was 0.7 ± 0.1 A M −1 cm −2. These are the first ever-reported data on DET reactions of anionic plant peroxidases on bare gold electrodes.

Photoelectrochemical investigation of methylene blue immobilised on zirconium phosphate modified carbon paste electrode in flow injection system

The adsorption of methylene blue (MB) onto zirconium phosphate (ZrP) was studied and the adsorption capacity value and isotherms were determined. The adsorption capacity of ZrP was increased after being exposed to gas phase n-butyl amine. The adsorbed MB on ZrP was used as a modifier material in carbon paste electrode (MCPE), which in turn was used in voltammetric investigations and flow injection (FI) amperometric determination of ascorbic acid (AA). A quasi-reversible electrode reaction of adsorbed MB was exhibited. A home made flow-through electrochemical cell with a suitable transparent window for irradiation of the electrode surface was constructed and used for amperometric FI studies. The photoamperometric-FI conditions were optimised as 1.5 mL min −1 flow rate, a 25 cm transmission tubing length, a 100 μL injection volume, and a constant applied potential of +100 mV versus SCE. The calibration curve for AA was linear over the concentration range from 1.0 × 10 −6 to 4.0 × 10 −5 M. The relative standard deviation of four replicate injections of 3.0 × 10 −5 M AA was 1.2%. The results obtained for AA determination in pharmaceutical products are in good agreement with those obtained using the procedure involving the reaction between triiodide and AA.

Anodic Degradation of Toxic Aromatic Compound in the Flow Through Cell with Carbon Fiber Electrode

The anodic degradation of aqueous phenol and 2-chlorophenol, which are model organic pollutants, has been studied using a flow-through electrochemical cell (FTEC) with carbon fibers as an anode material. For a controlled current electrolysis of an aqueous solution of 2-chlorophenol and phenol, experimental variables, including such as, the current input, the reaction time, and the flow rate of the solutions were considered. The degradation products of the oxidation reactions were identified with HPLC, GC/MS. Small aliphatic acids and CO2 were the main products. Depending on the values of the applied current and reaction time, a complete degradation or the partial oxidation of the compounds to other intermediates was obtained. At the flow rates of 2-6 mL/s were effective on the degradation efficiencies of the 2-chlorophenol and phenol.

Separation of Lead and Zinc Using a Flow-Through Electrochemical Cell and Their Simultaneous Spectrophotometric Determination

Separation of Lead and Zinc Using a Flow-Through Electrochemical Cell and Their Simultaneous Spectrophotometric Determination

The development of a flow injection analysis method for the quantification of free cyanide and copper cyanide complexes in gold leaching solutions

The quantification of free cyanide, Cu(CN) 2 − and Cu(CN) 3 2−, is essential in investigating the leaching of copper–gold ores in cyanide solutions. A flow injection analysis (FIA) method has been developed for this purpose which utilizes a flow-through electrochemical cell containing silver and platinum electrodes. The basis of this method is that the measured charge due to either silver oxidation or copper reduction at an applied potential is related to the species concentration. A potential of −150 mV was chosen for free cyanide measurement, at which the measured charge due to silver oxidation is related to the free cyanide concentration. While silver oxidation at 100 mV and copper reduction at −650 mV were used for Cu(CN) 3 2− and Cu(CN) 2 − analysis, respectively. For the solution containing both free cyanide and Cu(CN) 3 2−, a two-step analysis technique was developed for the quantification of each of these species. A pretreatment method was also developed to remove the interference of sulfide ions. The versatility of the FIA method developed in this study is demonstrated by measuring the cyanide species formed during the leaching of both Cu 2O and Cu 2S in cyanide solutions.

Determination of Carboxylic Acid Dissociation Constants to 350 °C at 23 MPa by Potentiometric pH Measurements

The dissociation constants of aqueous ethanoic, propanoic, butanoic, pentanoic, and hexanoic acids were determined in 0.1 mol·kg-1 NaCl media at temperatures from (19.4 to 350.4) °C and 23 MPa by a flow-through electrochemical cell with a pressure-balanced hydrogen platinum electrode. Temperature dependence of K values shown by each acid were qualitatively similar. The values are consistent with the literature values determined from potentiometry, calorimetry, and electrical conductivity. An empirical density model for the description of the temperature dependence of the equilibrium constants was used to correlate the data.

Determination of sulfuric acid first dissociation constants to 400 °C and 32 MPa by potentiometric pH measurements

pH of 10 −3 mol kg −1 H 2SO 4 aqueous solutions at subcritical and supercritical conditions was measured at temperatures from 375.0 to 400.6 °C and pressures from 25.8 to 32.5 MPa by a flow-through electrochemical cell and it changed from 3.90 at 375.2 °C and 32.5 MPa to 6.37 at 399.8 °C and 28.1 MPa. The molal equilibrium constant for the dissociation reaction of H 2SO 4 was determined based on the measured pH and the value varied from 1.67×10 −7 at 399.8 °C and 28.1 MPa to 4.53×10 −4 at 375.2 °C and 32.5 MPa. The values were between the literature values determined from UV-Vis spectroscopy and electrical conductivity. An empirical density model for the description of the temperature and pressure dependence of the equilibrium constants was used to correlate the data.

Determination of Acetic Acid Dissociation Constants to 400 °C and 32 MPa by Potentiometric pH Measurements

pH of 0.01 mol·kg-1 CH3COOH aqueous solutions at subcritical and supercritical conditions was measured at temperatures from (21.8 to 400.4) °C and pressures from (23.0 to 32.3) MPa by a flow-through electrochemical cell, and it changed from 3.38 at 21.8 °C and 23.0 MPa to 6.90 at 400.4 °C and 28.1 MPa. The molal equilibrium constant for the dissociation reaction of CH3COOH was determined on the basis of the measured pH, and the value varied from 4.29 × 10-10 at 400.4 °C and 28.1 MPa to 1.79 × 10-5 at 21.8 °C and 23.0 MPa. At subcritical conditions, the values showed good agreement with the literature values determined from potentiometry with a static cell, from calorimetry, and from electrical conductivity. An empirical density model for the description of the temperature and pressure dependence of the equilibrium constants was used to correlate the data.

Apparatus for direct pH measurement of supercritical aqueous solutions

A flow-through electrochemical cell was developed for direct pH measurement of supercritical aqueous solutions, based on our previously reported cell [Sue et al., Rev. Sci. Instrum. 72 (12) (2001) 4442]. The cell consisted of two high-temperature and high-pressure flow-through platinum hydrogen electrodes. The distance between tips of the two electrodes was minimized to reduce the streaming potential and to simplify the experimental procedure. Flow characteristics were considered in the design by placing orifices near the center of the cell to reduce concentration fluctuations around the electrodes. With the newly-designed cell, pH measurements were made for HCl+NaCl aqueous solutions at flow rates from 0.5 to 6.0 g/min, temperatures from 19.6 to 392.9 °C and pressures from 0.1 to 29.8 MPa. Comparison of the experimentally determined and theoretically calculated pH was better than ±0.02 logarithmic units over the range of conditions studied.

Use of disposable gold working electrodes for cation chromatography–integrated pulsed amperometric detection of sulfur-containing amino acids

We have prepared disposable thin-film gold working electrodes on polymeric substrates. Our microfabrication process allows for inexpensive and reproducible mass production of such electrodes. We utilize this new type of electrode in flow-through electrochemical cells to replace the conventional non-disposable gold working electrodes for integrated pulsed amperometric detection (IPAD) of compounds separated by high-performance cation-exchange chromatography. Using two S-containing amino acids (homocysteine and cysteine) as test compounds, we have modified a previously reported waveform for optimum performance with disposable gold electrodes. With the help of the same two test substances we have characterized the analytical performance of disposable gold electrodes under the new conditions. Compared to non-disposable working electrodes, the disposable working electrodes generated equal or better results in the limit of detection, linearity of calibration and reproducibility. When used with a new IPAD waveform, the disposable electrodes functioned reproducibly for 3 days. At the end of the specified usage period of 3 days, the disposable electrodes are simply replaced. Reconditioning by polishing is thus no longer required.

Effect of cysteine mutations on direct electron transfer of horseradish peroxidase on gold

Surface exposed cysteines were genetically engineered in the structure of recombinant horseradish peroxidase (rHRP). Recombinant forms of HRP with either a His-tag or a Strep-tag at the C-terminus were produced, which additionally had cysteines at positions 57, 189 or 309 (C-terminus) of the polypeptide chain. An E. coli expression system was exploited. The effect of these mutations on the direct electron transfer (ET) between Au and the enzyme was studied in the reaction of the bioelectrocatalytic reduction of H 2O 2, at –50 mV versus Ag|AgCl, on rHRP-modified Au electrodes placed in a wall-jet flow-through electrochemical cell. Adsorptive immobilisation of rHRPs on pre-oxidised Au from the protein solution at pH 6.0 provided a high and stable current response to H 2O 2 due to its bioelectrocatalytic reduction based on direct (mediatorless) ET between Au and the active site of the rHRPs. Comparative analysis of the direct ET rate constants, estimated from the amperometric data on direct and mediated ET in the presence of catechol at pH 7.4 and 6.0, gave evidence that the introduction of the His-tag or cysteine in the C-terminal area of the enzyme resulted in an increased efficiency of direct ET due to a favourable coupled electron and proton transfer pathway. Due to the high efficiency of direct ET, the sensitivity was independent on the addition of the mediator or change of pH indicating that the response to H 2O 2 is determined solely by the mass transfer of the analyte to the active site of HRP. The sensitivities obtained for the Au electrodes modified with rHRPs (2.0±0.1 A M −1 cm −2) and the low detection limit for H 2O 2 (10 nM) paves the way to develop the P-chip (peroxidase chip) - a biosensors system of a microscopic size for a mediatorless detection of H 2O 2 based on direct ET between Au and the recombinant forms of HRP.