Determination of lead and cadmium in seawater using a vibrating silver amalgam microwire electrode.

Silver amalgamated electrodes are a good substrate to determine lead (Pb) and cadmium (Cd) in seawater because they have properties similar to mercury but without the free mercury (Hg). Here a silver amalgamated microwire (SAM) electrode is optimised for the determination of Pb and Cd in coastal waters and uncontaminated ocean waters. The SAM was vibrated during the deposition step to increase the sensitivity, and electroanalytical parameters were optimised. The Hg coating required plating from a relatively concentrated (millimolar) solution, much greater (500×) than used for instance to coat glassy carbon electrodes. However, the coating on the ex situ amalgamated electrode was found to be stable and could be used for up to a week to determine trace levels of Pb in seawater of natural pH. The limit of detection square-wave ASV (50 Hz) using the pre-plated SAM electrode was 8 pM Pb using a 1-min plating time at pH 4.5. The limit of detection in pH 2 seawater was 4 pM using a 5-min plating time, and it was 12 pM using a 10-min plating time at natural pH in the presence of air, using a square-wave frequency of 700 Hz. The vibrating SAM electrode was tested on the determination of Pb in reference seawater samples from the open Atlantic (at the 20 pM level), Pacific, and used for a study of Pb in samples collected over 24 h in Liverpool Bay (Irish Sea).

Bi Z ，Salaün P ，van den Berg CM 《-》

Determination of chromium in estuarine waters by catalytic cathodic stripping voltammetry using a vibrating silver amalgam microwire electrode.

Chromium (Cr(VI)) in water can be determined by adsorptive catalytic cathodic stripping voltammetry in the presence of diethylenetriaminepentaacetic acid (DTPA) and nitrate on the hanging mercury drop electrode (HMDE). Predominately Cr(VI) is detected and the water is UV-digested to convert all Cr to Cr(VI) prior to analysis. We develop here an alternative to the HMDE by using a silver amalgam electrode based on a vibrating microwire. The microwire electrodes were 12.5 μm in diameter and electrochemically coated with mercury, and were stable for a week. Conditions were re-optimised, and we used a DTPA concentration of 5mM, 30 mM acetate pH buffer (pH 5.5 in seawater and pH 5.8 in pure water), and 1.5M nitrate solution. The microwire was reactivated prior to each scan by applying a negative potential (-3V) for 2s which removed all deposited Cr. The detection limit for chromium in pH buffer was found to be 0.2 nM Cr(VI) and in seawater 0.3 nM Cr(VI) at a deposition time of 30s. The response increased linearly with the concentration of Cr(VI) up to 60 nM Cr(VI) in seawater. The limit of detection is less good than using the HMDE, but the linear range is good and the microwire electrode could form the basis of apparatus for flow-analysis. The method was successfully tested on water samples from the estuary of the river Mersey (Liverpool Bay) giving chromium concentrations between 1.48 nM and 2.29 nM.

Espada-Bellido E ，Bi Z ，van den Berg CM 《-》

Determination of iodide and total iodine in estuarine waters by cathodic stripping voltammetry using a vibrating silver amalgam microwire electrode.

Iodide in natural waters is an important nutrient to aquatic organisms and its determination is of relevance to marine aquaculture. For this reason it is of interest to have a simple analytical method for determination of iodide in water samples. Iodide in seawater can be determined electrochemically by cathodic stripping voltammetry (CSV) with a mercury drop electrode which has environmental drawbacks. In an attempt to minimise the use of mercury in voltammetry, a vibrating silver amalgam microwire electrode is used here for the determination by CSV of iodide speciation in natural waters including seawater. Microwire electrodes were made from silver wires (diameter: 12.5µm) and electrochemically coated with mercury. The electrode surface was stable for extended periods of analyses (at least one week) and was then replaced. The optimised conditions include a pH 8, a frequency of 500Hz and a deposition time of 60s, among others. The microwire was reactivated between scans using a conditioning potential at -3 V for 1s. The detection limit for iodide in seawater was found to be 0.7nM I- at a deposition time of 60s. The response increased linearly with the concentration of iodide in seawater up to 100nM I-. The method was successfully applied to various samples from the estuary of the river Mersey (Liverpool Bay). An existing procedure for iodine speciation was modified to enable determination of iodate and total iodine as well as iodide in estuarine waters.

Espada-Bellido E ，Bi Z ，Salaün P ，van den Berg CMG ... -  《-》

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

The oxidation potentials of As(0)/As(III) and Sb(0)/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 As(0) 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. The methods were tested in samples from the Irish Sea (Liverpool Bay). As(III) was determined on-board ship immediately after sampling. As(III) concentrations were found to range from 0.44 to 1.56 nM and were higher near the coast. Sb(III) was below the detection limit (<0.1 nM Sb(III)), iAs was comprised between 8 and 25 nM while iSb varied from 0.5 to 1.7 nM.

Salaün P ，Gibbon-Walsh KB ，Alves GM ，Soares HM ，van den Berg CM ... -  《-》

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 As(0) 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 As(0) 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.

Gibbon-Walsh K ，Salaün P ，van den Berg CM 《-》