With the exception of the information on CBLs, the information on this page comes from the Global Lab Curriculum.  Find out more about this program by going to the GL Home Page.

CO2 is a tasteless, odorless, colorless gas.  CO2, like many other chemicals, has special properties which cause it to react with other materials creating some type of noticeable change.  For example, CO2 specifically absorbs infrared radiation.  Scientists use a spectrometer to beam infrared light through a mixture of gases and measure how much of the light is absorbed, which indicates the amount of CO2 in the mixture.  Another way to measure CO2 is to use chemicals that specifically react with CO2 by changing color.  One of these chemicals is called hydrazine hydrate.  This chemical reacts with CO2 to produce a blue-violet color which serves as an indicator of the presence of CO2.  Higher concentrations of CO2 cause increased reactions and more color change.

Three ways to measure CO2 will be discussed here:

1. Using Calculator Based Labs (CBLs)  to Measure CO2
2. Detecting CO2 Using Grab Sampling Tubes
3. Detecting CO2 Using Limewater

Using Calculator Based Labs to Measure CO2

Calculator Based Labs (CBLs)  could be used to measure CO2 in two ways.  First you could use the pH probe to monitor changes in pH which would give you an indirect measure of the amount of dissolved CO2.  This might prove difficult with automobile emissions, however, you can find a sample activity on Cellular Respiration at this site - http://www.ti.com/calc/docs/act/vern3.htm.

The CBLs also have a CO2 probe.  You would need to capture a sample of the emissions and then connect the probe to get a reading.  Information on obtaining these items is available at http://www.ti.com/calc/docs/calcover.htm.

Detecting CO2 Using Grab Sampling Tubes
The gas sampling tube functions quite simply.  Each tube is filled with small granules of a special chemical that has a very high ability to absorb the gas that is being measured.  This chemical is called a sorbent and in the CO2 sampling tubes the sorbent is hydrazine hydrate. You can see the sorbent by looking at the tube through a microscope.  When the sorbent absorbs CO2, it reacts chemically with it.  The product of this reaction has a blue color.
 
 

As a measured volume of air is pulled through the tube, air molecules flow through the sorbent. The sorbent absorbs the CO2 molecules in the air and changes its color.  As the air flows up the tube, the air contains less and less CO2.  As a result, the sorbent along the length of the tube is exposed to less CO2 and it changes color only a little bit, or not at all.  By viewing how much of the sorbent changes color along the length of the tube, we can determine the concentration of CO2 in the measured volume of air.  The gas sampling tubes are designed to measure the amount of a particular gas in a 1,000-cubic centimeter air sample.
Materials:

• air pump (to build pump, see directions below)
• gas sampling tubes
• safety glasses
• pliers

Procedures:

1. Put on safety glasses.  Carefully break both ends of the glass gas sampling tubes.  Use a pair of pliers to carefully snip only the tips of each end.
2. Insert one end of the glass sampling tube into the flared open end of the air pump.  Make sure the direction of the arrow on the gas sampling tube points toward the air pump.  Remember, air is pulled through the gas sampling tube by the air pump and exhausted into the atmosphere through the rubber one-way valve on the air pump.  The arrow points in the direction of the airflow.
3. Pull 1 liter of air through the CO2 tube with the plastic air pump. Since the pump can draw a maximum volume of 140 milliliters, you must use 10 strokes of 100 milliliters each to draw a total of 1 liter of air.  Pull each stroke with a slow, deliberate, even pressure on the plunger of the pump so that it reaches the 100-ml mark in 10 seconds.  Hold the plunger there for approximately 10 more seconds, at which point the plunger should no longer be pulling back against you.  At this point, all the air for that stroke has been pulled through, and the plunger is ready for the next stroke.
4. After pumping 1 liter of air through the gas sampling tube, disconnect the tube from the air pump and determine how far up the tube the color change extends.  Read the value of CO2 on the scale at the color transition.

Building an Air Pump
Materials:
140 ml plastic syringe
1 section of plastic tubing with one end flared
2 sections of plastic tubing with neither end flared
one-way air valve
T-fitting

Procedures:

1. Fit the two pieces of plastic tubing without the flared ends onto the two ends of the T-valve so that they are at a right angle to each other.
2. Fit the other piece of plastic tubing to the T-valve so that the flared end is open.  Later the gas sampling tube will be inserted here.
3. Attach the plastic syringe to one of the free nonflared tubing ends.
4. Fit the one-way valve onto the other free nonflared tubing end.  If you are unsure of which direction air can move through the valve, try blowing through one way and then the other.  Attach the valve so that air cannot be forced into the tubing assembly through the one-way air valve.

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Materials: 

• limewater (Ca(OH)2)
• 2 or more containers (one with a lid)
• straw or tubing
• air pump

Procedures:

1. Decant an appropriate amount of limewater into another container.
2. Slowly bubble gas(es) through the limewater using a straw, tubing, or an air pump.
3. Observe the results.  If there is CO2 in the gas(es), the limewater will turn cloudy.

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