How to measure atmospheric composition?

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Offline tjandrews20

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How to measure atmospheric composition?
« on: 13/12/2015 01:28:05 »
I am trying to do an experiment for my school's science fair where I, in part, measure how different atmospheric compositions effect plant growth, but I might of hit a minor roadblock and I was hoping someone could help me out. I was wondering if anyone knew of a device or method to measure the percent composition of different components of air, specifically oxygen and carbon dioxide, in a closed environment. So for example I would want to manipulate the environment so it was say, 5% CO2, but I need a device to measure when it gets to this 5% level. Any help would be greatly appreciated.


Offline evan_au

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Re: How to measure atmospheric composition?
« Reply #1 on: 13/12/2015 02:42:30 »
A quick search on Google and eBay identified devices from $US90 to $US2000.
One of these may be able to meet your needs.
You may be able to persuade your school to buy one?

Carbon dioxide normally has a fairly low concentration (around 1 part in 2500), so you need a sensitive device to measure it accurately. However, for your experiment (1 part in 20), you don't need quite so much resolution.

However, 5% CO2 is a pretty extreme condition, equivalent to the air that we breathe out.

If the the atmosphere generally reached these levels, this would be enough to cause:
  • A feeling of asphyxiation, due to excess CO2 in the air we breathe in
  • Severe acidification of the oceans, resulting in major marine extinctions
  • An increase to 5% CO2 corresponds to a reduction in atmospheric O2 levels from about 20% to 15%. This reduction in oxygen is enough to cause
    Hypoxia in susceptible individuals

However, if this were part of a carbon capture project, eg growing tomatoes adjacent to a power station, CO2 concentrations could well reach these levels.

Be aware that plants absorb CO2 as part of photosynthesis, pumping out O2. So you will need to monitor and adjust the CO2 levels periodically throughout the day.

You can use chemical means to sample the air and measure the CO2 concentration - I am sure chiralSPO could advise a school-level method.


Offline chiralSPO

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Re: How to measure atmospheric composition?
« Reply #2 on: 13/12/2015 17:36:52 »
If you need to measure carbon dioxide and/or oxygen with precision at multi-ppm to a few % range, you will a detector like what evan pointed out.

I don't know what type of project you had in mind, but perhaps it would be easier to to grow the plants in an air-tight environment starting with different amounts of added (or subtracted) gases.

For instance, let's just imagine you are growing bean sprouts in 3-L soda bottles. Once each bottle is filled with the same amount of dirt, water and beans, and the atmosphere within each bottle is the same as the surrounding air, you can carefully add a vial of something that will modify the atmosphere before sealing the bottle.

For instance:

Adding known fractions of an alkaseltzer tablet to a vial of water will produce easily calculable amounts of CO2 (do the calculations before trying the experiment--generating 12 liters of CO2 in a sealed 3 L bottle isn't a good idea)

Adding a solution of sodium hydroxide in the vial will absorb CO2, depleting the air inside of it.

Calcium hydroxide (slaked lime) in the vial will absorb CO2 and precipitate out calcium carbonate (which in theory, you could weigh to see how much CO2 would be absorbed, but this would be very small for just 3 L of air).

Hydrogen peroxide (use <5% solutions) will generate oxygen when you add a speck of iron chloride or potassium iodide. Again, you can calculate what volume of peroxide is needed.

You can also probably come up with a few ways to remove oxygen from the atmosphere...

It might be easier (and safer) to seal the bottles with balloons in any experiments that generate gas. This way the volumes are different, but the pressures are roughly the same.

Of course, don't spill any of the contents of the vials onto the dirt/beans, as this will introduce confounding variables (a few mL of 10% NaOH will kill your bean pretty quickly!)

Good luck!