In Situ Control of Local pH Using a Boron Doped Diamond Ring Disk Electrode: Optimizing Heavy Metal (Mercury) Detection

Abstract

A novel electrochemical approach to modifying aqueous solution pH in the vicinity of a detector electrode in order to optimize the electrochemical measurement signal is described. A ring disk electrode was employed where electrochemical decomposition of water on the ring was used to generate a flux of protons which adjusts the local pH controllably and quantifiably at the disk. Boron doped diamond (BDD) functioned as the electrode material given the stability of this electrode surface especially when applying high potentials (to electrolyze water) for significant periods of time. A pH sensitive iridium oxide electrode electrodeposited on the disk electrode demonstrated that applied positive currents on the BDD ring, up to +50 μA, resulted in a local pH decrease of over 4 orders of magnitude, which remained stable over the measurement time of 600 s. pH generation experiments were found to be in close agreement with finite element simulations. The dual electrode arrangement was used to significantly improve the stripping peak signature for Hg in close to neutral conditions by the generation of pH = 2.0, locally. With the ability to create a localized pH change electrochemically in the vicinity of the detector electrode, this system could provide a simple method for optimized analysis at the source, e.g., river and sea waters.

Assessment of Boron Doped Diamond Electrode Quality and Application to <em>In Situ</em> Modification of Local pH by Water Electrolysis

“…Furthermore, if the BDD contains significant NDC at grain boundaries it is also possible that upon application of the extreme anodic/cathodic potentials advocated in this article for pH generation, etching could occur at these weaker points. This would cause the film to corrode and for thin films, eventually delaminate, manifesting itself in an unstable pH generation profile, as seen with thin film Au and Pt electrodes 17 . Hence a stringent protocol for assessing the quality of the electrode prior to use is adopted to assess NDC content as discussed in ), it will be charge depleted, at potentials negative of the flatband potential, resulting in a decrease in electrochemical performance 7,42 .…”

“…This process takes advantage of the surface resilience of NDC-free BDD towards corrosion or dissolution under application of extreme applied potentials (or currents) for long periods of time. In particular the use of a BDD electrode to generate stable proton (H + ) or hydroxide (OH -) fluxes due to electrolysis (oxidation or reduction respectively) of water in close proximity to a second (sensor) 16,17 is described herein.…”

“…river, lake, sea and the pH of the system is not optimal for the electrochemical measurement of interest. Two recent examples include: (i) generation of a localized low pH, in a pH neutral solution, for the electrodeposition and stripping of mercury 17 ; note BDD is a favored material for electrodeposition of metals due to the extended cathodic window 9,18,19 .…”

Interfacial kinetics in a model emulsion polymerisation system using microelectrochemical measurements at expanding droplets (MEMED) and time lapse microscopy

“…Electrochemical characterisation of Au electrodes after construction was carried out using 500 µM Aqueous solutions of jeffamine D230 with a known pH were used to calibrate the iridiumoxide coated Au microelectrodes. The iridium oxide solution used for the electrodeposition was prepared as described in the literature [33] and stored at 4 C between uses. All reagents were used as received, and all solutions and the aqueous receptor phases were prepared with deionised water (Milli-Q, Millipore, 18.2 M cm resistivity at 25 C).…”

An electronic approach to minimising moisture-associated skin damage in ostomy patients

“…2) which can actively measure the pH and respond accordingly. The electrochemical control of processes that involve either the production of protons (H + ) or their consumption (and hence enable pH manipulation) have been successfully demonstrated in a number of electroanalytical [29,30] and electroporation systems [31][32][33][34] but the underpinning reactions are now being extensively researched in solar conversion and hydrogen fuel cell technologies [35][36][37][38][39][40][41]. Our contention is that these processes which, in principle, can lead to changes in pH profile at the electrode, could be harnessed to control the pH at the skin surface.…”