1999B07 Describe how the partial pressure of oxygen in a blood sample is measured using a Clark electrode.




·  Silver anode

·  Platinum cathode

·  KCl salt bridge

·  0.6V power source

·  Ammeter


·  Anode: 4 x (Ag + Cl -> AgCl + e-)

·  Electrolyte: 4 x (KCl + OH- -> KOH + Cl-)

·  Cathode: O2 + 2H2O + 4e- -> 4OH-

How does it work

·  O2 diffuses down concentration gradient across the plastic membrane into the salt bath

·  O2 is consumed at the cathode

·  Hence PaO2 current

·  Accuracy ±2mmHg. Delay in analysis may cause reduction in pO2.

Sources of error

·  Non-linear relationship between pO2 and potential difference

o But almost linear at 0.6V

·  Non-linear relationship between potential difference and current

o But almost linear at 0.6V

·  ↑temp -> ↑rate of reactions -> false ↑

o Hence temp control to within 0.1°C

·  Inaccuracy with time and use

o Hence regular two point calibration with pO2 660mmHg and 160mmHg

·  Blockage of the membrane with blood components -> falsely low result

o Hence keep clean

·  Delay in sampling -> consumption -> ↓pO2

o Hence cool with ice, process immediately


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