Detection of As(III) via oxidation to As(V) using platinum nanoparticle modified glassy carbon electrodes: arsenic detection without interference from copper

The electrochemical detection of As(III) was investigated on a platinum nanoparticle modified glassy carbon electrode in 1 M aqueous HClO4. Platinum nanoparticle modified glassy carbon electrodes were prepared by potential cycling in 0.1 M aqueous KCl containing 1 mM K2PtCl6. In each potential cycle, the potential was held at + 0.5 V for 0.01 s and at -0.7 V for 10 s. 25 cycles were optimally used to prepare the electrodes. The resulting electrode surfaces were characterized with AFM. The response to arsenic(III) on the modified electrode was examined using cyclic voltammetry and linear sweep voltammetry. By using the As(III) oxidation peak for the analytical determination, there is no interference from Cu(II) if present in contrast to the other metal surfaces (especially gold) typically used for the detection of arsenic; Cu(II) precludes the use of the As(0) to As(III) peak for quantitative anodic stripping voltammetry measurements due to the formation of Cu3As2 and an overlapping interference peak from the stripping of Cu(0). After optimization, a LOD of 2.1 +/- 0.05 ppb was obtained using the direct oxidation of As(III) to As(V), while the World Health Organization's guideline value of arsenic for drinking water is 10 ppb, suggesting the method may have practical utility.     

A stripping chronopotentiometric (SCP) method with a gold film electrode for determining inorganic arsenic species in seawater

An electrochemical method based on stripping chronopotentiometry (SCP) with a gold film electrode has been developed for determining arsenic in seawater. The detection limits were 0.053 ppb (0.71 nM) and 0.022 ppb (0.29 nM) for total inorganic As (As(T)) and As(III) after deposition times of 60 and 150 s, respectively. Compared to other stripping chronopotentiometric methods that use a gold macroelectrode to perform measurements of arsenic in seawater, the procedure described here exhibits better sensitivity and a fourfold shorter deposition time. Among the SCP methods, our procedure had proven its ability to analyse arsenic(III) in seawater. It therefore allows the concentrations of the various arsenic inorganic species in seawater—i.e. As(T), As(III) and As(V)—to be analysed. The proposed method is reliable, inexpensive and compact. It was successfully applied to the study of arsenic speciation along the salinity gradient of the Penzé estuary (NW France).     

Speciation of Arsenic by Potentiometric Stripping Analysis Using Gold(III) Solution as Chemical Reoxidant and a Wall‐Jet Flow Cell

A simple procedure is described for the potentiometric stripping of arsenic with a wall‐jet cell by means of potentiostatic co‐deposition of gold and arsenic at a glassy‐carbon electrode and subsequent chemical stripping with Au(III). Optimum medium containing 160 mg L^?1 of Au(III) in HCl 0.1 M, where it is possible to speciate As(III) and As(V). As(V) was electrodeposited directly without prior chemical reduction at working electrode. As(III) was first determined at an electrodeposition potential of ?0.1 V. Afterwards, total arsenic was determined by an electrodeposition potential of ?0.7 V, from the area of peak obtained of the differential stripping potentiogram by using the standard addition method. The original As(V) concentration in the sample was calculated by difference. The possibilities of the optimized method were demonstrated by determinations of As(III), As(V) and total arsenic in samples of polluted water.     

Voltammetric determination of arsenic in high iron and manganese groundwaters

Determination of the speciation of arsenic in groundwaters, using cathodic stripping voltammetry (CSV), is severely hampered by high levels of iron and manganese. Experiments showed that the interference is eliminated by addition of EDTA, making it possible to determine the arsenic speciation on-site by CSV. This work presents the CSV method to determine As(III) in high-iron or -manganese groundwaters in the field with only minor sample treatment. The method was field-tested in West-Bengal (India) on a series of groundwater samples. Total arsenic was subsequently determined after acidification to pH 1 by anodic stripping voltammetry (ASV). Comparative measurements by ICP-MS as reference method for total As, and by HPLC for its speciation, were used to corroborate the field data in stored samples. Most of the arsenic (78 ± 0.02%) was found to occur as inorganic As(III) in the freshly collected waters, in accordance with previous studies. The data shows that the modified on-site CSV method for As(III) is a good measure of water contamination with As. The EDTA was also found to be effective in stabilising the arsenic speciation for longterm sample storage at room temperature. Without sample preservation, in water exposed to air and sunlight, the As(III) was found to become oxidised to As(V), and Fe(II) oxidised to Fe(III), removing the As(V) by adsorption on precipitating Fe(III)-hydroxides within a few hours.     

Determination of As(III) and arsenic(V) in natural waters by cathodic stripping voltammetry at a hanging mercury drop electrode

A simple, fast and quantitative method was developed for the determination of As(III) and total inorganic arsenic (As (total)) in natural spring and mineral waters using square wave cathodic stripping voltammetry (SWCSV) at a hanging mercury drop electrode (HMDE). In the determination of As(III), pre-concentration was carried out on the electrode from a solution of 1 mol/l HCl in the presence of 45 ppm of Cu(II) at a potential of −0.39 V versus Ag/AgCl, and the deposited intermetallic compound was reduced at a potential of about −0.82 V versus Ag/AgCl. In the determination of As (total) the pre-concentration was carried out in 1 mol/l HCl in the presence of 400 ppm of Cu(II) at a potential of −0.40 V versus Ag/AgCl, and the intermetallic compound deposited was reduced at a potential of about −0.76 V versus Ag/AgCl. For determination of As(III) the quantification limit was 0.2 ppb for a deposition time of 40 s, and the relative standard deviation (R.S.D.) was calculated to be 6% (n=13) for a solution with 8 ppb of As(III). For As (total), the quantification limit was 2 ppb for a deposition time of 3 min, and the R.S.D. was calculated to be 3% (n=10) for a solution with 8 ppb of As(V). The method was validated by application of recovery and duplicate tests in the measurements of As(III) and As (total) in natural spring and mineral waters. For As (total), the results of the SWCSV method were compared with the results obtained by optical emission spectrometry with ICP coupled to hydride generation (OES-ICP-HG) good correlation being observed.     

Gold Nanoparticle Modified Screen Printed Electrodes for the Trace Sensing of Arsenic(III) in the Presence of Copper(II)

Gold ensembles for the trace level sensing of arsenic(III) in the presence of copper(II) are reported. The gold ensembles are fabricated using citrate capped gold nanoparticles which are chemically synthesised in an aqueous solution with an aliquot of this simply cast onto an economical and disposable screen printed electrode. After drying at room temperature, the gold ensembles are ready for use. The gold ensembles are explored towards the sensing of arsenic(III) in the presence of copper(II) using anodic stripping voltammetry where the corresponding stripping peaks are well resolved and using this protocol it is possible to readily detect 3 µg L^?1 (3 ppb) with a detection limit of 0.4 µg L^?1 (0.4 ppb). Proof‐of‐concept is also shown for the sensing of arsenic(III) in a canal water sample. Given the low cost of the sensor and ease of fabrication, the gold ensembles hold promise for the sensing of arsenic(III) in water samples where copper(II) may be present.     

Arsenic speciation in natural waters by cathodic stripping voltammetry

Contamination of groundwater with arsenic (As) is a major health risk through contamination of drinking and irrigation water supplies. In geochemically reducing conditions As is mostly present as As(III), its most toxic species. Various methods exist to determine As in water but these are not suitable for monitoring arsenic speciation at its original pH and without preparation. We present a method that uses cathodic stripping voltammetry (CSV) to determine reactive As(III) at a vibrating, gold, microwire electrode. The As(III) is detected after adsorptive deposition of As(OH)₃⁰, followed by a potential scan to measure the reduction current from As(III) to As(0). The method is suitable for waters of pH 7-12, has an analytical range of 1 nM to 100 μM As (0.07-7500 ppb) and a limit of detection of 0.5 nM with a 60 s deposition time. The As speciation protocol involves measuring reactive As(III) by CSV at the original pH and acidification to pH 1 to determine inorganic As(III) + As(V) by anodic stripping voltammetry (ASV) using the same electrode. Total dissolved As is determined by ASV after UV-digestion at pH 1. The method was successfully tested on various raw groundwater samples from boreholes in the UK and West Bengal.     

Simultaneous Determination of Copper(II), Lead(II) and Zinc(II) at Bismuth Film Electrode by Multivariate Calibration

In this work, simultaneous determination of Cu(II), Pb(II) and Zn(II) ions at low concentration levels (ppb) by square wave anodic stripping voltammetry on a Bi(III) film electrode plated in situ at a glassy carbon electrode (GCE) is described. A chemometric approach was used to overcome the overlapping peaks of Cu(II) and Bi(III), the competition of the electrodeposited Cu and Bi for the surface of the GCE and the formation of Cu‐Zn intermetallic compounds. The construction of the multivariate calibration models, based on partial least squares regression, allowed the simultaneous determination of Cu (in the concentration range 8.0 to 20.1 ppb), Pb (2.0 to 30.0 ppb) and Zn (29.7 to 90.4 ppb) with most of the prediction errors obtained in the external validation set for the three models lower than 16, 11 and 26 %, respectively. Finally, this method was used for the determination of these trace metal ions in surface river water samples with satisfactory results [errors below 10, 5 and 32 % for Cu(II), Pb(II) and Zn(II), respectively].     

Determination of As(III) and total inorganic arsenic by flow injection hydride generation atomic absorption spectrometry

A simple procedure was developed for the direct determination of As(III) and As(V) in water samples by flow injection hydride generation atomic absorption spectrometry (FI–HG–AAS), without pre-reduction of As(V). The flow injection system was operated in the merging zones configuration, where sample and NaBH₄ are simultaneously injected into two carrier streams, HCl and H₂O, respectively. Sample and reagent injected volumes were of 250 μl and flow rate of 3.6 ml min⁻¹ for hydrochloric acid and de-ionised water. The NaBH₄ concentration was maintained at 0.1% (w/v), it would be possible to perform arsine selective generation from As(III) and on-line arsine generation with 3.0% (w/v) NaBH₄ to obtain total arsenic concentration. As(V) was calculated as the difference between total As and As(III). Both procedures were tolerant to potential interference. So, interference such as Fe(III), Cu(II), Ni(II), Sb(III), Sn(II) and Se(IV) could, at an As(III) level of 0.1 mg l⁻¹, be tolerated at a weight excess of 5000, 5000, 500, 100, 10 and 5 times, respectively. With the proposed procedure, detection limits of 0.3 ng ml⁻¹ for As(III) and 0.5 ng ml⁻¹ for As(V) were achieved. The relative standard deviations were of 2.3% for 0.1 mg l⁻¹ As(III) and 2.0% for 0.1 mg l⁻¹ As(V). A sampling rate of about 120 determinations per hour was achieved, requiring 30 ml of NaBH₄ and waste generation in order of 450 ml. The method was shown to be satisfactory for determination of traces arsenic in water samples. The assay of a certified drinking water sample was 81.7±1.7 μg l⁻¹ (certified value 80.0±0.5 μg l⁻¹).     

Differential determination of arsenic (III) and total arsenic with L-cysteine as prereductant using a flow injection non-dispersive atomic absorption device

Speciation of arsenic in environmental samples gains increasingly importance, as the toxic effects of arsenic are related to its oxidation state. A method was developed for the determination of trace amounts of arsenic (III) and total arsenic by flow injection hydride generation coupled with an in-house made non-dispersive AAS device. The total arsenic is determined after prereduction of arsenic (V) to arsenic (III) with L-cysteine in a low concentration of hydrochloric, acetic or nitric acid. The conditions for the prereduction, hydride generation and atomization were systematically investigated. A quartz tube temperature of 800 degrees C was found to be optimum in view of peak shape and baseline stability. Pb(II), Ni(II), Fe(III), Cu(II), Ag(I), Al(III), Ga(II), Se(IV), Bi(III) were checked for interfering with the 2 microg/L As(V) signal. A serious signal depression was only observed for Se(IV) and Bi(III) at a 150-fold excess. With the above system, arsenic was determined at a sampling frequency of about 1/min with a detection limit (3sigma) of 0.01 microg/L using a 0.5 mL sample. The reagent blank was 0.001+/-0.0003 absorbance units and the standard deviation of 10 measurements of the 2 microg/l As signal was found to be 1.2%. Results obtained for standard reference materials and water samples are in good agreement with the certified values and those obtained by ICP-MS     

Determination of Arsenic(III) at a Nanogold Modified Solid Carbon Paste Electrode

A selective and sensitive electroanalytical method was developed for arsenic determination based on a nanogold (AuNP) modified solid carbon paste working electrode (SCPE) modified in two steps (i) physisorption and (ii) additional electrodeposition of nanogold particles in the presence of iodide. Copper(II) interference was solved by covering the gold layer by a self assembled mono layer (SAM) of glutathione. Using DPASV a linear response of the signal was obtained as a function of As(III) in the concentration range 0.05–20 µM (4–1498 ppb) with a limit of detection of 0.01 µM (0.9 ppb). Sample stirring and degassing were not needed. Application to the determination of arsenic(III) and (V) in underground water samples from Burkina Faso was successfully achieved.     

Redox Speciation of Inorganic Arsenic in Water and Saline Samples by Adsorptive Cathodic Stripping Voltammetry in the Presence of Sodium Diethyl Dithiocarbamate

This paper describes a new voltammetric procedure for the inorganic speciation of As(III) and As(V) in water samples. The procedure is based on the chemical reduction of arsenate [As(V)] to arsenite [As(III)] followed by the voltammetric determination of total arsenic as As(III) at the hanging mercury drop electrode (HMDE) by adsorptive cathodic stripping voltammetry (AdCSV) in the presence of sodium diethyl dithiocarbamate (SDDC). The reduction step involved the reaction with a mixture of Na₂S₂O₅ and Na₂S₂O₃ in the concentrations 2.5 and 0.5 mg mL^?1, respectively, and the sample heating at 80 °C for 45 min. The linear range for the determination of total arsenic as As(III) in the presence of SDDC was between 5 and 150 μg L^?1 for a deposition time of 60 s (r=0.992). A detection limit of 1.05 μg L^?1 for total As was calculated for the method in water samples using a deposition time of 60 s. The detection limits of 4.2 μg L^?1 and 15.0 μg L^?1 for total As in seawater and dialysis concentrates, respectively, were calculated using a deposition time of 60 s. The relative standard deviations calculated were 2.5 and 4.0% for five measurements of 20 μg L^?1 As(V) as As(III) in water and dialysis concentrates, respectively, after chemical reduction under optimized conditions. The method was applied for the determination of As(III) and total As in samples of dialysis water, mineral water, seawater and dialysis concentrates. Recovery values between 86.0 and 104.0% for As(III) and As(V) added to the samples prove the satisfactory accuracy and applicability of the procedure for the arsenic monitoring.     

New method of gold-film electrode preparation for anodic stripping voltammetric determination of arsenic (III and V) in seawater

A new method of efficient rotating gold-film glassy-carbon electrode preparation prior to the determination of As(III) and As(V) in seawater by anodic stripping voltammetry (ASV) is described. Factors affecting sensitivity and precision including pH, deposition time and potential, rotation and scan rate, and the nature of working electrode were investigated. Electroinactive As(V) was reduced to As(III) by gaseous SO(2) prior to ASV determination. For a deposition time of 4 min the determination limit was approximately 0.19 ppb. Precision of the proposed method was very good (RSD=2-0.6% at 1-5 ppb) and a relatively good accuracy determined by analysis of certified reference seawater (CASS-1) and seawater samples spiked with an arsenic standard solution, was also obtained.     

Highly Sensitive Voltammetric Speciation and Determination of Inorganic Arsenic in Water and Alloy Samples Using Ammonium 2‐Amino‐1‐Cyclopentene‐1‐Dithiocarboxylate

A simple, fast, reproducible (2.5% RSD at 3.0 μg/L), and sensitive method is described for quantifying As(III) (0.3 μg/L detection limit, 0.5–440 μg/L dynamic range). Anodic stripping voltammetry (ASV) is performed after accumulating arsenic at a mercury film electrode at ?0.350 V vs. Ag/AgCl (saturated KCl) for 20 s in 0.2 M HCl containing 8 μM ammonium 2‐amino‐1‐cyclopentene‐1‐dithiocarboxylate (AACD), without oxygen removal. This is the first report of using AACD in ASV and in electrochemical quantification of As(III). Total arsenic is determined after sodium‐sulfite‐reduction of As(V) to As(III). Interferences are minimal. Method validation involved water and metal alloy samples.     

Stripping Analysis of Trace Arsenic Based on the MnOx/AuNPs Composite Film Modified Electrode in Alkaline Media

A simple, rapid fabricated and sensitive modified electrode for detection of As(III) in alkaline media was proposed. The modified electrode was prepared by co‐electrodeposition of manganese oxides (MnO_x) and gold nanoparticles (AuNPs) on the glassy carbon electrode (GCE) with cyclic voltammetry. Linear sweep anodic stripping voltammetry (LS‐ASV) was employed for the determination of arsenic (III) without interference from Cu(II), Hg(II), and other coexisting metal ions. A lower detection limit of 0.057 µg L^?1 (S/N=3) were obtained with a accumulation time of 200 s. The proposed method was successfully applied to determine arsenic (III) in real water samples with satisfactory recoveries.     

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

Arsenic(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.     

Direct Estimation of Total Arsenic Using A Novel Metal Side Disk Rotating Electrode

A special type of metal side disk rotating electrode has been demonstrated for the direct estimation of total arsenic [As(III) and As(V)] at low ppb (μg/L) level using anodic stripping differential pulse voltammetry (ASDPV). A conventional three electrodes cell is used equipped with side disk rotating gold electrode as working, graphite/platinum electrode as auxiliary and Ag/AgCl/3M KCl as reference electrodes. Arsenic is estimated in various acidified samples without any digestion, containing a trace amount of copper at low ppb level. The major problems associated with ASDPV of high acidic condition (acid hazards), irreproducible results due to the interference of hydrogen bubbles, evolved at the cathode during the deposition of arsenic in acidified samples and poor detection level are overcome with the help of the specially designed gold side disk rotating electrode and modified electrolyte. The presence of a trace amount of copper(II) salt in the electrolyte is found to enhance the sensitivity of the technique. The shape and position of the metal disk at the electrode, rotation speed of the electrode and electrolyte are optimized to have less hydrogen gas bubbles interference and high reproducibility in the detection of arsenic up to 2 ppb±15% level in various samples. The electrode has been found very stable and reproducible even for more than 200 estimations.     

Optimization of a flow injection system with amperometric detection for arsenic determination.

A selective and sensitive analytical procedure for rapid arsenic determination by gas-diffusion flow injection analysis with amperometric detection was developed. The method is based on the arsenite reduction by NaBH(4). Derived arsine diffuses through a PTF membrane into the acceptor flow stream and is amperometrically determined on a platinum working electrode. The limit of detection (3 sigma) at room temperature was 5 microg/dm(3) of As(III). The relative standard deviation for a 1 mg/dm(3) As(III) standard was 1.96% for six repetitive injections. Arsenic(V) was determined after its prereduction with potassium iodide. Arsenic determination was not interferred with by 1 mg/dm(3) Sb(III), 5 mg/dm(3) Sn(II), 10 mg/dm(3) Se(IV), 1 mg/dm(3) As(V), 1 mg/dm(3) hydrasine, 1 mg/dm(3) Fe(II) or 0.5 mg/dm(3) Fe(III) solution. The throughput of this method was 60 analyses per hour. This method was successfully applied to arsenic determination in some power plant waste water samples.     

Construction and Evaluation of a Gold Tubular Electrode for Flow Analysis: Application to Speciation of Antimony in Water Samples

A tubular gold electrode (TGE) is described for the first time by summarizing the important aspects of its construction and evaluation. Applicability of the TGE is evaluated in the speciation of Sb(III) and Sb(V) using anodic stripping voltammetry in a single flow manifold. Studies with surface active interferences and metallic cations were performed. The proposed conditions for antimony determination showed good tolerance towards cationic, anionic and nonionic surface active substances. A linear response for antimony was obtained for solutions containing significant amounts of several metallic cations. Linear calibration curves for Sb(III) were obtained in the range 1–10 ppb with a detection limit of 0.19 ppb (CV=2.91%, n=5, [Sb(III)]=5 ppb). For Sb(V), linear calibration curves were in the range 1–15 ppb with a detection limit of 0.32 ppb (CV=1.41%, n=5, [Sb(V)]=5 ppb). The figures of merit achieved sustain for the good applicability of the proposed method as it allows the determination of antimony at levels below maximum values permitted in consuming waters. Results of antimony concentration determined in water samples were validated against the ICP‐MS reference procedure or compared with reference water samples.     

Cathodic stripping voltammetric analysis of arsenic species in environmental water samples

A simple, fast and sensitive arsenic speciation method has been developed for environmental water analysis by using differential pulse cathodic stripping voltammetry (CSV) performed on a hanging mercury drop electrode (HMDE). Electroactive As(III) is determined by direct CSV analysis. As(V) is converted to As(III) species first and is subsequently quantified by the concentration difference between total inorganic arsenic and As(III). A new batch-mode As(V) reduction procedure by l-cysteine was developed in this study. The optimized parameters for quantitative As(V) reduction include treatment with 20 mM l-cysteine and 0.03 M HCl for 6 min at 70 °C. Organic arsenic, including monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA), can be decomposed to As(V) through UV photooxidation with peroxydisulfate and quantified through subtracting total inorganic arsenic from the total arsenic. At optimum condition, the detection limits for As(III), As(V), and organic arsenic (MMA and DMA) were all 0.3 μg/L and with the linear range from 2.5 to 190 μg/L. Interference from ions common in natural water (Mn, Fe, Cr, Cd, Ca, Zn, Mg, and phosphate) is minimal. The method was validated by analyzing the NIST 1640 natural water standard reference material and by recovery tests on spiked tap water and groundwater. When applied to on-site analysis of sediment pore water and stream water, the CSV results agree well with those obtained by inductively coupled plasma–mass spectrometry (ICP–MS) and graphite furnace atomic absorption spectrometry (GFAAS) methods.