Abstract
Oxygen tension in aqueous media of the order of 10^β6^ to 10^β9^ molar is measured amperometrically by means of a stationary inert metal electrode. The electrolytic cell is connected into a high resistance bridge circuit, which is energized from a low resistance d.c. source of voltage adjusted to give a potential difference across the cell corresponding to a position on the diffusion current plateau of the first oxygen wave. This cell potential is monitored by means of a vacuum tube millivoltmeter and maintained constant. A vacuum tube millivoltmeter measures the bridge output. The bridge is initially balanced with the medium deoxygenated; subsequent introduction of low concentrations of oxygen dissolved in the medium unbalance the bridge. Either the resulting bridge output voltage, as read on the millivoltmeter, or the resistance change in the bridge arm opposite the cell, is related to the oxygen tension by means of a previous calibration. Both quantities are directly proportional to oxygen concentration, at least over limited ranges of such concentration changes. The method is also applicable to the estimation of other substances which can be discharged under amperometric conditions at a stationary electrode.