Biomedical Analysis & Related Instrumentation

Polarographic Clark Electrodes

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:

Figure 1.0: A cross section through a Clark, oxygen sensing, polarographic electrode.

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:

Find out more about: Fingertip Pulse Oximeter Blood Oxygen Saturation Monitor

4Cl^– + 4Ag(s) → 4AgCl(s) + 4e^–

Figure 1.1: A basic circuit used with a Clark O₂ probe. Note that, temperature compensation isn’t shown in this circuit.

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:

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.

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.

Related Articles:

John Mulindi

John Mulindi has a background in Instrumentation, and he writes on various topics ranging from Technical, Business to Internet marketing fields. He likes reading, watching football, writing and taking on adventure walks in free time.