F12iii: Measurement of Oxygen Tension in Blood

  • O2 possesses 2 identical atoms:
  • Unsuitable for infrared absorption spectrometry
  • But does have 2 outer-shell electrons which are highly reactive & ∴can be used to measure partial pressure

Paramagnetic Oxygen Analyser

  • O2 = paramagnetic gas → attracted into a magnetic field
  • All other gases, except NO, are diamagnetic = repelled by magnetic field
    • 2 glass spheres shaped like a dumbbell are filled with nitrogen & suspended in a magnetic field
    • They are suspended by a wire which keeps tension
    • O2 entering chamber displaces dumbbell which displaces the N2 containing spheres
    • The detection system can be via:

DEFLECTION: position of reflected light beam is detected by photodiodes → calibrated to give % oxygen

NULL: an opposing current is applied to restore dumbbell to original position → current required directly proportional to pO2 → v. accurate within 0.1%

Clarke Electrode: See 14b22

Fuel Cell

  • Fuel cell is like Clarke electrode
  • 2 half cells + salt bridge
  • But does not require power
  • Is itself a power source!
    • Lead anode
    • Gold mesh cathode (separated from sample by O2 permeable membrane)
    • KOH solution
    • Thermostat (for temp compensation)
  • Reaction at Gold Cathode

O2 + 4e– + 2H2O → 4(OH)–

  • Reaction at Lead Anode

Pb + 2(OH)– → PbO2 + H2O + 2e–

  • Current registered → proportional to PO2 present in sample
  • Fuel cell = small, light, portable
  • Like a battery, will expire → the more O2 it is exposed to, the faster this will occur