Date: January 2008
I teach Biology at Henry Ford community college. I have
an Honor student that would like to do a directed study to test the
effects of elevated carbon dioxide levels on plant development. Is
there a simple way to build a CO2 chamber? My secondy question is ,
if dry ice is used, is there a way to measure the amount of CO2
that is formed by the dry ice evaporating?
I did an experiment like this as a high school student. I set
up a slow-drip system where a non-caustic acid (it might have
been vinegar) was slowly dripped onto a large dish containing
baking soda. The plant pot was on the dish with baking soda
surrounding the base of the pot. The space was confined but
not closed (you don;t want pressure to build up in a closed space!).
This presumably elevated the CO2 concentration, because I did see some effects
(although this was so long ago that I might not be remembering things correctly).
Anyway, this might be a way to generate CO2 more safely than dry ice.
Mass is mass, so if you measure the mass of dry ice evaporated (mass before
minus mass after), you can determine how much CO2 was released. That said,
it is much more challenging to measure the concentration of CO2 in the
chamber, and it's the concentration that matters, not the overall quantity.
To measure the concentration, I would recommend a CO2 sensor. You can buy
one (numerous different models and types exists), but most biology labs
should have them readily accessible. If your school doesn't have this, and
if you can't afford to buy one (< $1000 for really fancy ones even), it's
just a matter of getting permission to use one at a local university such as
UM, UDM, Oakland, etc. Alternatively, you can make a sensor using various
chemical methods (essentially using CO2 as an acid to react with some basic
species), although the accuracy would leave much to be desired. Instructions
abound on the internet for these chemical sensors.
As for your chamber, the design of your experiment will govern how to build
your chamber. Keeping a chamber at a constant CO2 level for periods of time
long enough to evaluate plant growth would be exceedingly difficult. The
control schemes would be very costly. You can buy such systems, but I am
assuming your resources are more limited than that. I would instead focus on
simply measuring CO2, and then correlating growth with CO2 after the fact.
Although this is not as controlled, it may be your best compromise. If you
place some dry ice in an air-tight container, and force it to travel through
a long tube (allowing it to reach ambient temperature), and then have than
stream meet your container, you can elevate the CO2 levels near the plant.
You can heat the dry ice to increase the flow and increase the CO2.
Hope this helps,
To answer your second question first: the mass of CO2 in the dry ice
will not change if the ice melts. Weigh the solid dry ice and that will
tell you how many moles of the gas you will get. From there you can
calculate the CO2 concentration in your "chamber".
As for the "chamber", had you investigated using large clear plastic
bags? They should hold your modified "atmosphere" if you do not
pressure the bag up i.e., don't blow them up like balloons.
Equipment design here is very important, and must be planned carefully.
Before I address your question directly you need to address the following:
1. Measure and control CO2 and air flow rates. 2. Temperature and humidity
must be kept reasonably constant. This almost requires a "closed" system.
Fairly large plastic hemispheres used to make terrariums (spelling??) at a
reasonable price can be found in pet and tropical fish stores. Temperature
and humidity can be monitored simultaneously with a "weather station"
available from several suppliers, such as LaCrosse (FYI this isn't an
endorsement) -- you can find suppliers on the Internet. 3. If you use soil.
You need to formulate a "standard" soil, not something you dig up in the
back yard. You might consider hydroponics as a growth medium. There are a
lot of sites on the Internet showing how to set one of these up.
(Incidentally, this will set the relative humidity at ~100%.). 4. Since CO2
reacts readily with water forming carbonic acid, you need a way to control
the pH. Otherwise, over time the pH will drop and may confound the
measurements you wish to make. Acidic pH may even kill the plants. 5. What
plants are you considering as test subjects? Research their botany before
making a selection to make sure their growth rate/conditions are compatible
with your setup.
Now to your question. You need to monitor both air flow and CO2 flow.
Air flow can be pumped using a simple fish tank pump available at any fish
supply store. CO2 will flow on its own due to sublimation -- you may even
need an "escape" hose because the dry ice may evaporate much faster than you
want. The volume flow rate of CO2 and air can be measured using a device
called a "rotameter". Basically it is a floating ball in a calibrated
tapered tube. How high the ball "floats" in the gas, the greater the flow
rate. A rotameter + valve (necessary) costs about $150 each. You will need
two -- one for each gas. I'd also recommend that you adjust the relative
volumes and "premix" the gases before they enter the chamber. This is not
difficult. Just use a large ballast jar packed loosely with glass beads,
fiberglass or even surgical gauze and/or cotton. This ensures that the gas
entering the plant area is mixed. It also prevents "surges" in the flow
rates of the gases.
Do a websearch for other experimental setups that others have used to
make sure all the important variables mentioned here are "covered".
Check out the cost of a cylinder of CO2. These contain both CO2 gas and
liquid and would be easier to control than "dry ice". In the long run that
may be a better alternative source of CO2.
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