KOH strength for Respirometer / Manometer please

Discussion in 'Supporting Biology' started by Heather Evans, Feb 11, 2019.

  1. Hi - we usually use a simple respirometer, with soda lime, & coloured water in capillary tube. This used to work, but I cannot get a seal since acquiring plastic 3-way taps, even after dispensing with the darned things.....So teachers have decided to use the 'U' tubed manometers in a repeat attempt on this Core A-level prac tomorrow!

    The respirometer / manometer print out I have uses KOH to absorb the CO2, rather than soda lime, but fails to give any indication of solution strength.
    A simple prep. morning is now very stressful.
    Help please.

    ps - what's the difference between R & M? - any??
  2. Hello Heather, I have used 50% KOH in the past but prefer using soda lime now - easier and safer to handle.
    I think the manometer is simply the capillary U tube part of the respirometer.
  3. Thank you Kiki - I did wonder.

    One of my teachers who failed last week, clutching at straws, was blaming the soda lime....so, as the sheet for the one with M said KOH, I thought that might resolve the doubt over the chemical by using the KOH.
    I'll discuss. Soda lime does seem best....

  4. All the Soda lime I have is self indication so changes colour when used up.
  5. The Nuffield version uses:

    Potassium hydroxide solution, 15% (= 2.7 M), about 15 cm3, Note 3
    Use a funnel to pour 5 cm3 of potassium hydroxide solution (Note 3) into each respirometer vessel. Make sure none of the potassium hydroxide touches the sides of the vessels.
    This seems a lot of KOH but just soaking cotton wool in it will suffice.
  6. Hi Paul - is this blue to pink(ish)? I have found one tub of this, although most tubs I have are larger white granules.
  7. white to violet (c1987)
  8. My soda lime is white to pale blue when used up - It never seemed to change to pale blue but I tested it with CO2 from the Kipps apparatus and it went pale blue so works
  9. Ah - thanks Paul - I won't use the THAT tub then! Glad I asked!!!

    Thanks all - feeling a bit less stressed now.


  10. karen b

    karen b COMMITTEE

    This sounds like silica gel with cobalt chloride as the indicator (don't know if this is still available, but used it years ago)
  11. No, silica gel with cobalt chloride is a drying agent not a CO2 absorber:

    The main components of soda lime are

    • Calcium Hydroxide, Ca(OH)2 (about 75%)
    • Water, H2O (about 20%)
    • Sodium Hydroxide, NaOH (about 3%)
    • Potassium Hydroxide, KOH (about 1%).
    The indicator is ethyl violet which is clear and goes purple when the pH falls due to increased CO2 absorption.
    Last edited: Feb 12, 2019
  12. karen b

    karen b COMMITTEE

    I know - I was just suggesting what Heather might have in the tub of stuff that is blue going to pink
  13. fair point - sorry
  14. That sounds more like self-indicating silica gel, for absorbing moisture. Put a small quantity of the substance in a warmish (over 100degrees) oven and see if it turns vivid blue. That pretty much confirms it. (Sorry Karen, just spotted your post)