Polarographic electrodes differ from the typical pH and ion specific cells in that a polarographic cell is run at a constant potential in order to force two electrochemical reactions to take place. The current which flows in the cell is determined by and is proportional to the limiting concentration of one reactant. A Clark cell for the measurement of the partial pressure of oxygen dissolved in water, blood, or of oxygen in the air is illustrated below:
The Clark cell has four parts:
- A platinum or gold cathode.
- An Ag/AgCl anode.
- An electrolyte solution (the electrolyte is generally a buffered, saturated KCl solution.
- Plastic membrane (polypropylene, Teflon or Mylar) which is permeable to O2 gas.
At the noble metal cathode, a reduction reaction (i.e. a loss of electrons) takes place, which can be described as:
O2(g) + 2H2O + 4e– + 4KCl(s) → 4KOH(s) +4Cl–
At the Ag/AgCl anode, the oxidation reaction (i.e. an increase in electrons) takes place and can be described as:
4Cl– + 4Ag(s) → 4AgCl(s) + 4e–
It is established experimentally that, when the anode is held at 0.7 V positive with respect to the cathode, a current flows, that can be described the following equation:
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I = I0 + β[pO2]
Where β is typically about 10 nA/mm Hg pO2, both I0 and β are increasing functions of temperature.
The response time of the Clark electrode to step changes in pO2 is limited by the ease of O2 diffusion through the membrane to the cathode surface. Nonetheless, it is important to note that, the settling time for Clark cell is proportional to the pO2 that is being measured.
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Shortcoming of Clark Cell in Partial Pressure of Oxygen (pO2) Measurement
The effect of temperature causes major sources of error in measuring pO2 using the Clark cell, therefore, the Clark cell must be calibrated and operated at a controlled, fixed, temperature which varies no more than ±0.1 °C, or if we know its temperature compensation, we can measure its temperature and calculate the compensation.
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