Field of the Invention

This invention relates to an electrode assembly and its use in electrochemical and electroanalytical methods.

Background to the Invention

Hydrodynamic methods are so widely applied within electrochemistry and electroanalysis that rotating discs (Riddiford, Adv. Electrochem. Eng. 1966, 4, 47; and Pleskov et al, Studies in Soviet Science: The Rotating Disk Electrode; Consultants Bureau: New York, USA, 1976), channels (Cooper et al, Electroanalysis 1998, 10, 141; Compton et al, Prog. React. Kinetics, 1995, 20, 245; and Lee et al, Electrochem. Commun. 1999, 1, 190), wall-jets and tubes (Macpherson et al, Anal. Chem. 1994, 66, 2175; and Gunasingham et al, Anal. Chem. 1983, 55, 1409) are familiar electroanalytical tools.

More recently, the use of insonation (Banks et al, Analyst 2004, 129, 678) and the application of microwave radiation (Tsai et al, J. Am. Chem. Soc. 2002, 124, 9784; and Compton et al, Chem. Commun 1998, 23, 2595) have been used to promote mass transport in the vicinity of solid electrodes, thereby enhancing analytical performance. In particular, the extreme increases in rates of mass transport associated with the latter two approaches have enabled macroelectrodes to be conferred with the dynamical properties of microelectrodes, that is to say a diffusion layer of the order of microns can be routinely employed.

Whilst the benefits of insonation or microwave heating are well documented, the use of conventional electrodes such as the rotating disc is attractive because of the relative ease of application. However, such electrode systems are limited by, for example, low rotation speeds and high electrochemical noise.

Summary of the Invention

The present invention is based on a realisation that the limitations of conventional rotating electrode systems can be overcome by using an assembly in which rotation of an electrode is driven by a fluid under pressure.

According to the present invention there is provided an electrode assembly comprising:

(a) an electrode;

(b) means for supporting the electrode and allowing rotation of the electrode relative thereto; and (c) means for feeding fluid under pressure to said support means to effect rotation of the electrode.

The present invention also provides an electrochemical or electroanalytical method in which a working electrode is rotated by means of fluid under pressure.

The use of a fluid-driven rotating electrode, as compared with conventional mechanical devices, may enable relatively high rotation speeds to be achieved and a considerable reduction in electrochemical noise.

Brief Description of the Drawings

Fig. 1 is a schematic representation of an electrode assembly of the present invention.

Fig. 2 is a graph showing the voltammetric oxidation of 1.47 mM ferrocyanide in 0.1 M KCI at a gold electrode in stationary (B) and rotating (A) mode, both recorded at 15 mVs ^'1 vs. SCE and with only the forward scan being shown for clarity.

Fig. 3 is a graph showing linear sweep voltammograms of 1 μM arsenic (III) in 0.1 M nitric acid solution for quiescent (thick line) and rotating mode (dotted line) and showing also the response in the absence of arsenic, the parameters being - 0.5 V for 60 seconds followed by a potential sweep at 50 mVs ^"1 (vs. SCE).

Description of Various Embodiments

Various embodiments of the present invention will now be described with reference to the accompanying drawings. These embodiments are provided for the purpose of illustrating the invention and should not be construed as limiting.

The invention involves the use of an electrode which is rotated by fluid under pressure. The electrode may be any appropriate form of rotating electrode, examples of which include rotating disc (or rotating disk), rotating ring, rotating ring disc and rotating cylinder electrodes. A preferred electrode is a rotating disc electrode.

The fluid may be a gas or a liquid. An example of a suitable gas is nitrogen.

The electrode is preferably rotated at a speed sufficient to generate either laminar or turbulent flow. Normally, the electrode is rotated at a speed in excess of 100Hz, preferably more than 200Hz, more preferably greater than 300Hz and most preferably greater than 400Hz. It may be particularly preferred to rotate the electrode is rotated at a speed in excess of 500 Hz, more particularly preferred at a speed in excess of 600 Hz, e.g. the electrodes may rotate at a speed of about 656 Hz. At such a high speed, the quantified mass transfer coefficient has been found to correspond to a diffusion layer thickness of ca. 2 μm or below.

Preferably, the assembly also includes means for varying the speed of rotation of the electrode.

Referring to the embodiment shown in Fig. 1, an electrode assembly 1 in accordance with the present invention comprises a working electrode 3 which is a rotating disc electrode and which is mounted at its upper end within an electrode support shown generally at 5.

The electrode support 5 includes lower housing member 7 and upper housing member 9 within which is mounted a rotatable wheel 11. Wheel 11 is provided with a bearing 13 which locates on a bearing surface (not shown) within lower support member 7. Extending downwardly from bearing 13 is a lower locating spigot 15.

Above bearing 13 is provided an enlarged portion 17 of the wheel 11 , which is shaped to provide a plurality of spaced apart circumferential vanes 19, each vane having a forward radial surface and a rearward surface which is inclined relative thereto.

Above portion 17 of wheel 11 there is provided a further locating spigot 21.

Upper support member 9 has extending therefrom an electrical lead 23 providing electrical contact between the working electrode 3 and an external location.

Lower support member 7 has, extending radially therefrom, gas feed means 25 comprising an outer tube 27 and an inner tube 29. The latter provides the inlet for high pressure gas fed to the electrode support in such a way that it impinges on the vanes 17 of wheel 11 causing the wheel to rotate on its bearing 13.

An assembly of the invention may be employed in electrochemical or electroanalytical methods. Such methods include voltammetry experiments, for example cathodic stripping voltammetry, adsorptive stripping voltammetry or AC/DC voltammetry, including square wave and pulse voltammetries.

The electrode assembly described above was used in the following Examples, which are given for the purpose of illustrating the invention.

In the Examples, all chemicals used were of analytical grade and used as received without any further purification. These were: potassium ferrocyanide, (99+ %, Aldrich) sodium (meta) arsenite (III) oxide, (Fluka, + 99 %) and nitric acid (Aldrich, 70%, double distilled PPB grade with any trace metal impurities no more than parts per trillion as determined by ICP-MS). All solutions were prepared with deionised water of resistivity not less than 18.2 MΩ cm (Vivendi Water Systems).

Voltammetric measurements were carried out using a μ-Autolab Il (ECO-Chemie) potentiostat. All measurements were conducted using a three-electrode configuration. A gold electrode (0.075 cm diameter, area 0.00441 cm ² ) was fabricated in-house by sealing gold wire into PTFE. The counter electrode was a bright platinum wire, with a saturated calomel electrode (Radiometer) completing the

circuit. The working electrode was polished with decreasing alumina sizes on a soft lapping pad.

The high speed rotating disc main casing was obtained from W&H UK Limited. The manufacturers quote a rotation speed of 5800 Hz in air without any shaft connected to the main compartment. A schematic diagram of the high speed-rotating disc is depicted in Fig. 1. The distance from the base of the working electrode to the top was 6 cm while the length of the body was 7 cm. The electrode operates by passing nitrogen (BOC, 99%) through the main compartment of the electrode (see Fig. 1) at a constant feed pressure of 1 bar of nitrogen which causes the vaned wheel to rotate which in turn makes the shaft of the working electrode spin.

Example 1 : Voltammetric detection of ferrocyanide solution

The voltammetric response of a gold macroelectrode in a 1.47 mM ferrocyanide solution containing 0.1 M KCI was explored using the above described electrode assembly. Cyclic voltammograms were recorded with the electrode stationary over a range of scan rates with a formal potential of 0.18 (0.01) V which compare well with literature reports of 0.17 V (Moore et al, Anal. Chem. 2004, 76, 2677). Analysis of the peak currents using the Randles-Sevcik equation yielded a diffusion coefficient of 7 x 10 ^"6 cm ² s ^"1 . Next cyclic voltammograms were recorded with the high speed rotating disc 'on' during the voltammetric scan. Fig. 2 shows the voltammetric response recorded at 15 mVs ^"1 , which corresponds effectively to steady-state voltammetry. A characteristic hydrodynamic voltammogram is seen with a well defined transport limited current.

The heterogeneous rate constant, Zc ₀ , for the electrochemical oxidation of ferrocyanide was estimated by first measuring the experimental difference between the mid-point of the cyclic voltammetric response, recorded at 15 mVs ^'1 to the half wave potential obtained from the hydrodynamic voltammogram (both shown in Fig. 3). Digisim ^® was used to simulate the hydrodynamic response using the rotating disc option for a range of heterogeneous rate constants (1 x 10 ^~3 to 1.0 cm s ^"1 ) using the rotation speed found experimentally (see below) and a literature value for the diffusion coefficient (Adams, Electrochemistry at Solid Electrodes, Monographs in Electroanalytical Chemistry and Electrochemistry, Dekker, New York, 1969). The

reversible limit for the cyclic voltammetric response was then used to calculate the difference in the simulated voltammetry with the changing half wave potential from varying the Zc ₀ value. Comparison of the differences between simulation and experimental provided an estimation of the steady-state current yielding a heterogeneous rate constant, k ₀ value of 0.01 cm s ^"1 , which is in agreement with a reported value of 0.02 cm s ^'1 in 0.1 M aqueous KCI (Montenegro, Applications of Microelectrode in Kinetics; Elsiver Science, B.V., 1994).

The limiting current, I _L and diffusion layer thickness, δ, at a rotating disc operating under laminar conditions are described by the following equations:

I _L = 0.62nFAD ^2/ W ^/2 υ- ^υ6 C _bu!k (1)

δ = L6lD ^m ω- ^m υ ^m (2)

where I _L is the limiting current, F is the Faraday constant, n is the number of electrons transferred, D is the diffusion coefficient of the eletroactive species, ⁽ D is the rotation speed (rad s ^"1 ), U is the kinematic viscosity, δ is the diffusion layer thickness and C _bulk is the bulk concentration of the electroactive species. Using the limiting current from Fig. 2, and a kinematic viscosity of 8.8 x10 ^"3 cmV ¹ (Bard et al, Electrochemical Methods: Fundamentals and Applications.; Wiley, New York, N. Y., 1980) and a literature value for the diffusion coefficient, the rotation speed was found to correspond to 4120 (± 200) rad s ^"1 or 656 (± 20) Hz. Using equation (2) the diffusion layer thickness was found to be 2.1 (± 0.2) μm.

Next, the Reynolds number was calculated to see if the high-speed rotating disc electrode operated under laminar or turbulent conditions, noting that equations (1) and (2) presume laminar conditions. The Reynolds number was estimated via the following:

ωr

Re = (3) υ

The critical Reynolds number separating laminar from turbulent flow has been described as being Re _c = 3 x 10 ⁵ (Hanna et al, Chem. Eng. Sci 1988, 43, 1407; and Deslouis et al, Electrochim. Acta, 1980, 25, 1027). Using equation 3, the Reynolds number was found to be 658 (± 32) which is below the f?e _c number confirming that the rotating disc electrode operates under a laminar flow regime.

Next, the mass transport coefficient, k, was calculated for the high speed rotating disc electrode which is described via the following equation:

k 7, ₍₄ )

Using equation 4, the mass transport coefficient was found to be 3.5 x 10 ^'2 cm s ^"1 . For comparison, a hand-held infra-sonotrode (Simm et al, Anal. Chem. 2004, 76, 5051) showed a value of 0.49 x 10 ^"2 cm s ^'1 with a diffusion layer thickness of 15.2 μm recorded using ferricyanide. In contrast, power ultrasound applied from a sonic horn in a face on arrangement produces a mass transport coefficient of 0.13 cm s ^"1 with the smallest corresponding diffusion layer ever recorded under steady-state conditions at a macro-electrode ( 0.7 μm, Ru(NH ₃ ) ₆ ³⁺ ; Banks et al, Electroanalysis 2003, 15, 243). It is clear that the electrode is on a par with insonation as a means of achieving high rates of mass transport for kinetic and/or mechanistic studies, as well as for enhancing electroanalysis.

Example 2: Detection of Arsenic

The analytical sensing opportunities offered by the electrode were then considered. A solution containing 1 μM As (III) in 0.1 M nitric acid was first prepared. Using linear sweep voltammetry with a deposition time of 60 seconds at - 0.5 V (vs. SCE) a stripping signal corresponding to the oxidation of zero-valent arsenic to arsenic (III) is observed at ca. + 0.16 V (see Fig. 3), which is in agreement with literature values of 0.15 V on gold (Simm et al, Anal. Chem., 2004, 76, 5051). This was then repeated with the high speed rotating disc electrode On' during the accumulation period. The charge under the stripping peak was found to be 1.42 x 10 ^"7 C under quiescent conditions while for the electrode 'on' during the accumulation step the

charge under the peak corresponds to 2.26 x 10 ^"6 C; an increase of a factor of ca. 16. Clearly there is a significant enhancement in the magnitude of the arsenic stripping peak using the electrode, where a small signal is transformed to a large and easily quantifiable signature.

Accordingly, the high speed rotating disc electrode assembly of the invention allows the achievement of very thin diffusion layers at the micron scale under sustained steady state conditions. Moreover, is it evident from Fig. 3 that the use of gas to drive the electrode produces an output much less noisy than alterative mechanical constructions, such as those of Pleskov et al; Editors, S. Studies in Soviet Science: The Rotating Disk Electrode; Consultants Bureau: New York, USA, 1976; and Albery et al, Ring-Disk Electrodes (Oxford Science Research Papers). Oxford Univ. Press, Fair Lawn, N. J., 1971.