Analytical Instrumentation & Environmental Monitoring


Ross Cell technology

August 2002 Analytical Instrumentation & Environmental Monitoring

The three-electrode, polargraphic Ross Cell for monitoring dissolved oxygen (DO) consists of a platinum working electrode (cathode), a platinum counter electrode (anode) and a silver reference electrode. The reaction cell is isolated from the process solution by an oxygen permeable membrane. The oxygen that is dissolved in the process diffuses through the membrane and is reduced at the working electrode. The reverse of this reaction takes place at the counter electrode.

Since the cell generates as much oxygen as it consumes, flow dependency and frequency of maintenance are reduced. The Ross Cell sensor also has a longer lifetime than a Clark Cell sensor because its electrodes are not consumed by the electrochemical reaction as in the Clark Cell.

The Ross Cell remains at a constant potential even during changing electrode conditions. The reference electrode is connected to an internal amplifier circuit that essentially draws no current. If the voltage sensed by the reference electrode is less than the 'normal' supply voltage provided by the internal amplifier, a feedback control circuit provides a corrective voltage, changing the supply voltage to compensate for the change in cell or electrolyte resistance. This provides greater sensor accuracy and stability due to better long-term polarisation stability.

Steve Herbst, PREI Instrumentation, 011 448 2172



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