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 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 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 《-》

Advantages of using a mercury coated, micro-wire, electrode in adsorptive cathodic stripping voltammetry.

Gun J ，Salaün P ，van den Berg CM 《-》

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)(3)(0), 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 microM 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.

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