Clarke Oxygen Microelectrode

Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712-0165, USA 

Any inert aqueous electrolyte solution is suitable for use in a Clarke electrode (4-6). There may be some benefits to using a buffered electrolyte solution because the product of the four-electron reduction of oxygen is the hydroxide ion, which causes a localized shift in pH at the electrode surface during use. However, satisfactory behavior is obtained using unbuffered solutions. 

The electrolyte layer should be reduced to the smallest possible thickness, dictated by the roughness of the tip surface and the stretching of the membrane. However, the layer should not be too thin, as it is essential to maintain good electrochemical contact between the working and reference electrodes and also to allow the efficient removal of the hydroxide ion products from the surface of the cathode. The thickness of the electrolyte layer may be estimated by adding a redox active species to the electrolyte layer and performing SECM approach curves (see Chapter 12) to an insulator. An estimate of electrolyte layer thickness of 20-60 am was obtained for a typical Clarke UME using this technique (7). 

The response time, %, of the electrode is the time taken for 99% of the current change on alteration of oxygen concentration and can be calculated according to the equation (5)  

Several unique complications are encountered when nsing a Clarke UME as a SECM tip (7). Increased response time of the membrane UME and convection in the gas phase mean that conventional approach cnrves cannot be obtained. In addition, care must be taken during imaging not to touch the substrate surface with the membrane, as changes to the tip current are then observed, dne to deformation of the membrane and electrolyte layer. 

---

Figure 17-3 Clark oxygen microelectrode used to measure dissolved O2 in marine sediment in Box 6-1. The tip of the cathode is plated with Au, which is less prone than Pt to fouling by adsorption of species from the test solution. [Adapted from N. P. Revsbech, Limnol. Oceanogr. 1989,34, 474.]...

Electrochemical methods for NO determination offer several features that are not available with spectroscopic approaches. Perhaps the most important is the capability of microelectrodes to directly measure NO in single cells in situ, in close proximity to the source of NO generation. Figure 2 shows sensors that have been developed for the electrochemical measurement of NO. One is based on the electrochemical oxidation of NO on a platinum electrode (the classical Clark probe for detection of oxygen) and operates in the amperometric mode [17]. The other is based on the electrochemical oxidation of NO on conductive polymeric porphyrin (porphyrinic sensor) [24]. The Clark probe uses a platinum wire as a working electrode (anode) and a silver wire serves as the counterelectrode (cathode). The electrodes are mounted in a capillary tube filled with a sodium chlo-ride/hydrochloric acid solution separated from the analyte by a gas-permeable membrane. A constant potential of 0.9 V is applied, and direct current (analytical signal) is measured from the electrochemical oxidation of NO on the platinum anode. In the porphyrinic sensor, NO is catalytically oxidized on a polymeric metalloporphyrin... 

---