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Previous Product ID: 26100-24

POS theory

  • Question: In my quest for understanding the O2k, I wanted to learn more about the POS - "clark type polarographic oxygen sensor". - And of course now I have a question:So the cathode is gold -the anode is Ag/AgCl2 - and the electrolyte is KCl 3 M... After cleaning the electrode in 25% ammonium don't you have to anodization the anode again - or is this achived just by applying the voltage of +?? with respect to the cathode; and the reason that the POS needs to equlilbrate 1 day before reused? And how are you sure that ammonium will not complex silver and dissolve the AgCl? - I guess it does not because you would loose your signal if the anode is KAPUT.
  1. Answer: Dear Stine, first let my congratulate you for being the first person ever to write an E-Mail about the principal working of the POS without having run in problems already! I think the quest to understand the working of the O2k will save you a lot of work and problems later. Your two questions are basically identical: Re-anodization of the AgCl anode and re-growing of the AgCl layer is the same thing.

NH3 is far better in dissolving (complexing) AgO (the black stuff we want to get get rid off) than in dissolving AgCl. However, you are right, a small amount of AgCl will be dissolved each time you do a sensor service. It is nearly impossible to remove all the AgCl with NH3 (we tried) but in any way you are correct that the AgCl layer grows again each time you use the oxygraph (apply the polarization voltage). And, yes that may indeed be one reason that running the O2k over night after a sensor service is usually a good idea! Any further question, that help you to understand what is going on, are highly welcome! Fasching Mario 12:14, 8 March 2013 (CET)

Over-night ammonia cleaning in perspex vial

  • Question: For an over night cleaning with ammonia a POS membrane has to be applied on the sensor to prevent the ammonia from evaporating. Can I seal the sensor (with ammonia) in the perspex vial used for transporting the sensor instead?
  1. Answer: Experience from Therkelsen_AIb
    1. Sensor-tip immersed in NH3-solution (1 ml)
    2. Closed perspex vial + 25 % Ammonia, 24 hour. No problemo. (in regard to the perspex vial)
    3. Comment: I am still worried that ammonia gas may reach the inner part of the electrode (that is usually srewed onto the POS-connector) and causes corrosion there. At least, make sure that the sensor is screwed tightly onto the o-ring of the perspex vial. Fasching Mario 16:26, 11 March 2014 (CET)

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