Specifics of the Clark-Type O2-Electrodes Application in Biological Studies

This paper is focused on the specifics of amperometric oxygen (O₂) measurements in biological media (cultivation media, reaction mixtures, media designed for isolated cellular and subcellular structures) and other complex media using Clark-type electrodes. Specifically, the impact of the qualitative and quantitative composition of liquid media on the readings of Clark electrodes is considered. Using sucrose, sorbitol, NaCl and phosphate buffer as examples, it has been shown that, provided the content of dissolved O₂ is constant, an increase in the content of concomitant to O₂ substances in the test medium results in an increase of the signal of the Clark electrode located in this medium. On the basis of the obtained and published data, the effect of the qualitative and quantitative composition of media on the readings of Clark electrodes is evaluated. In its turn, the very effect of media composition on Clark electrodes readings can be explained by the fact that in reality Clark electrodes measure the activity of dissolved O₂, which usually increases with an increase in the content of concomitant to O₂ substances. Since Clark electrodes measure the activity rather than the true O₂ content in media and the ratio between the activity and true O₂ content is often unknown, the issue regarding the calibration of Clark electrodes has been also considered in this study. It is noted that the methods of adding known amount of O₂ to liquid media (the Hill reaction, catalase reaction, or injection of a micro portion of water saturated with pure O₂), which were applied in this work to demonstrate the effect of media composition on the Clark electrode readings, can be used to calibrate the Clark electrodes in units of the true O₂ content, and, thus, to take into account the influence of the media content on the readings of these O₂-sensors when it is necessary. It is suggested that the calibration based on the addition of an O₂-saturated micro-portion of water or, if necessary, O₂-saturated micro-portion of another suitable liquid to the medium with the Clark electrode is, due to its almost unlimited applicability and other advantages, the most satisfactory way of calibration. The introduction of this calibration method into scientific practice can make quantitative O₂-measurements with Clark electrodes more accurate in many difficult cases.