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Question Respirometer help

8 months 3 weeks ago #41680 by thedoctor
Can anyone suggest why this simple respirometer is not drawing up the fluid from the beaker. There are no air locks in the pipette and I have sealed the joins with Vaseline.

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8 months 3 weeks ago #41681 by TecHKnow
First of all what are you using as your Live specimen?
We use peas or mung beans. These need to be germinated for about 24 hours before hand. Simply sprinkle some seeds onto wet paper. We use a tray. After 24 hours set up your respirometer. Its not easy to clearly see what way you have set this up. Into your respirometer place the respiring peas in the basket.You are supposed to use some KOH or NaoH to absorb the CO2

Then repeat the practical with dead peas, ie., boil them for about 5 - 10 minutes.

If your practical still does not work at this point, then you may have to consider the live speciment is dead. This happens if you have peas/mung beans stored in wrong conditions for a long period of time. Go out and buy a box of batchelors dried peas should solve that issue quickly or alternatively go hunt down some woodlice to use as Live speciments and anything like rolled up paper balls to show the dead specimen.

Hope this helps

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8 months 3 weeks ago #41683 by thedoctor
I was using mealworms which I think have become a bit sluggish so I bought a fresh supply today. Woodlice are difficult to find around our school grounds! I’ll definitely give the peas a go though. Thank you so much for your advice.

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8 months 3 weeks ago #41684 by AndyG
Is that a standard tube or a narrow capilliary - can't tell from the photo?

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8 months 3 weeks ago #41685 by thedoctor
It is a narrow capillary.
I bought a fresh supply of mealworms and both respirometers worked perfectly.
Fingers crossed I can repeat it for the other classes!
Thank you so much

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8 months 3 weeks ago - 8 months 3 weeks ago #41686 by Baldilocks
My experience of this is that it is a nightmare to get right for several reasons.
1) The apparatus is a very effective thermometer/barometer. A large volume of air together with a narrow capillary just enhances its sensitivity at weather forecasting.
2) Ultimately, the gas reaction is O2 in -> CO2 out, mole for mole so the volume doesn't change. To see the effect your method of adsorbing the CO2 has to be highly optimised. The soda lime needs to be close to the mung/larvae, above and below and separated by something minimal - a bit of net curtain.
3) A second control apparatus alongside the first but without the mung/larvae would show the effects of temp/pressure
4) Setting up the apparatus involves a lot of handling (= warming) and breathing (= extra CO2). You need to wait a good while to let it all settle down.

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8 months 3 weeks ago #41687 by AndyG
I believe these should always be set up slightly warm then they "work" really well as they cool to lab temperature ...

It's not fiddling, just making sure the practical demonstrates the theory :-)

There's no way to avoid the 'manometer' problems unless you use a second sealed compensating tube on the opposite end of a u-tube manometer. The theory being that the pressure on both ends of the manometer varies equally with air pressure and temperature, the differential pressure is provided by the experimental subject. However even that doesn't always work as the mealworms warm up ...
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