Amperometric Oxygen Electrodes

Although the Winkler method is very precise when performed with strict control of pH and iodine concentration, it is still susceptible to errors. For example, ions such as Fe2+ and SO3-, suspended solids, and some organics can produce gross inaccuracies. There have been numerous modifications of the method over the years to compensate for its shortcomings (2). Besides the errors inherent in the Winkler method, it is extremely labor intensive and slow (even when automated). In situ measurements are practically impossible and no real-time monitoring can be done (3). Furthermore, the Winkler method is suitable only for aqueous solutions, not solvents or gases. Heyrovsky’s work with the dropping mercury electrode in the early 1920s was conducive to the acceptance of electrochemistry as a routine analytical tool (4). He identified the broad cathodic waves belonging to the reduction of dissolved oxygen at the dropping mercury electrode (DME) and established the direct relationship between dissolved oxygen concentration and limiting current. In the decades following, many oxygen detectors were developed using a variety of materials and configurations. While the DME was instrumental in the determination of many biologically significant substances, the most practical working electrodes for dissolved oxygen detectors were solid electrodes that could be easily put to use in biological samples such as blood, tissue, and cultures. The two-electrode detectors usually consisted of a gold or platinum working electrode and the Ag/AgCl counter or reference electrode. (Note: Although “polarography” is usually reserved for those experiments done with a DME, much of the literature uses the term to describe vol tammetric and amperometric oxygen detectors.) The basis of amperometric oxygen electrodes is fairly straightforward. By placing the two electrodes in solution and applying a potential between them that is sufficient to reduce dissolved oxygen at the working electrode (typically ~ -650 mV vs. Ag/AgCl), one can determine the partial pressure of oxygen (pO2) in an ideal solution because pO2 is proportional to the limiting current. This procedure would be sufficient if there were no other species in the solution that could reduce at Amperometric Oxygen Electrodes