Sulfide determination in hydrothermal seawater samples using a vibrating gold micro-wire electrode in conjunction with stripping chronopotentiometry

A rapid electrochemical stripping chronopotentiometric procedure to determined sulfide in unaltered hydrothermal seawater samples is presented. Sulfide is deposited at −0.25 V (vs Ag/AgCl, KCl 3 M) at a vibrating gold microwire and then stripped through the application of a reductive constant current (typically −2 μA). The hydrodynamic conditions are modulated by vibration allowing a short deposition step, which is shown here to be necessary to minimize H₂S volatilization. The limit of detection (LOD) is 30 nM after a deposition step of 7 s. This LOD is in the same range as the most sensitive cathodic voltammetric technique using a mercury drop electrode and is well below those reported previously for other electrodes capable of being implemented in situ.     

Determination of arsenate in natural pH seawater using a manganese-coated gold microwire electrode

Direct electrochemical determination of arsenate (As^V) in neutral pH waters is considered impossible due to electro-inactivity of As^V. As^III on the other hand is readily plated as As⁰ on a gold electrode and quantified by anodic stripping voltammetry (ASV). We found that the reduction of As^V to As^III was mediated by elemental Mn on the electrode surface in a novel redox couple in which 2 electrons are exchanged causing the Mn to be oxidised to Mn^II. Advantage is taken of this redox couple to enable for the first time the electrochemical determination of As^V in natural waters of neutral pH including seawater by ASV using a manganese-coated gold microwire electrode. Thereto Mn is added to excess (∼1 μM Mn) to the water leading to a Mn coating during the deposition of As on the electrode at a deposition potential of −1.3 V. Deposition of As⁰ from dissolved As^V caused elemental Mn to be re-oxidised to Mn^II in a 1:1 molar ratio providing evidence for the reaction mechanism. The deposited As^V is subsequently quantified using an ASV scan. As^III interferes and should be quantified separately at a more positive deposition potential of −0.9 V. Combined inorganic As is quantified after oxidation of As^III to As^V using hypochlorite. The microwire electrode was vibrated during the deposition step to improve the sensitivity. The detection limit was 0.2 nM As^V using a deposition time of 180 s.     

Determination of arsenic and antimony in seawater by voltammetric and chronopotentiometric stripping using a vibrated gold microwire electrode

The oxidation potentials of As⁰/As^III and Sb⁰/Sb^III on the gold electrode are very close to each other due to their similar chemistry. Arsenic concentration in seawater is low (10–20 nM), Sb occurring at ∼0.1 time that of As. Methods are shown here for the electroanalytical speciation of inorganic arsenic and inorganic antimony in seawater using a solid gold microwire electrode. Anodic stripping voltammetry (ASV) and chronopotentiometry (ASC) are used at pH ≤ 2 and pH 8, using a vibrating gold microwire electrode. Under vibrations, the diffusion layer size at a 5 μm diameter wire is 0.7 μm. The detection limits for the As^III and Sb^III are below 0.1 nM using 2 min and 10 min deposition times respectively. As^III and Sb^III can be determined in acidic conditions (after addition of hydrazine) or at neutral pH. In the latter case, oxidation of As⁰ to As^III was found to proceed through a transient As^III species. Adsorption of this species on the gold electrode at potentials where Sb^III diffused away is used for selective deposition of As^III. Addition of EDTA removes the interfering effect of manganese when analysing As^III. Imposition of a desorption step for Sb^III analysis is required. Total inorganic arsenic (iAs = As^V + As^III) can be determined without interference from Sb nor mono-methyl arsenious acid (MMA) at 1.6 < pH < 2 using E_dep = −1 V. Total inorganic antimony (iSb = Sb^V + Sb^III) is determined at pH 1 using E_dep = −1.8 V without interference by As.     

Pseudopolarography of copper complexes in seawater using a vibrating gold microwire electrode

Copper (Cu) in seawater can be determined by anodic stripping voltammetry using a vibrating gold microwire electrode (VGME) with a much lower limit of detection than using a mercury electrode, enabling detection of labile Cu at trace level. The possibility of pseudopolarography of Cu using the VGME is investigated here and is calibrated against known chelating agents. The sensitivity much (15-fold) improved by application of a desorption step to remove adsorbed organic substances and excess anions. The notorious tendency of solid electrodes to be affected by memory effects was overcome by a conditioning interval between measurements that stabilized the electrode response. Model ligands, including EDTA, humic substances (HS), and glutathione (examples of natural ligands) were analyzed to calibrate the half-wave shift to complex stability. The half-wave shift on the VGME is much greater (~2×) than that on the mercury drop electrode which is attributed to several parameters including a much (5-fold) thinner diffusion layer on the VGME. Experiments showed that the same procedure is suitable for pseudopolarography of zinc. Application of the new method to samples from the Irish Sea showed Cu occurring in several complexes, all strongly bound, and some occurring in the electrochemically reversible region of the pseudopolarogram. The humic substance complex of Cu was also found to occur in the reversible region of the pseudopolarogram. The pseudopolarograms of Cu in seawater were unaffected by sample filtration and did not require purging to remove dissolved oxygen, suggesting that this method can be readily used as part of an in situ measuring system.     

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.     

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.