Mixing Ratios or Parts per Million, Billion

Certain gases such as ozone occur in the atmosphere in very tiny amounts. In the stratosphere, for instance, you may find only one to ten ozone molecules for every one million molecules of other gases. This amount is called one to ten parts per million (ppm). Measurements such as parts per million can be expressed in terms of volume or mass. With gases in the atmosphere, we usually think in terms of volume and may express this as parts per million by volume (ppmv). You can also use the units parts per billion by volume (ppbv), and parts per trillion by volume (pptv). Measurements such as ppmv, ppbv, and pptv are called mixing ratios.

In this activity, you will gain experience with these very small amounts using the technique of serial dilution.

Materials (per team)

• 10 ml graduated cylinder
  
  
• Test tube rack
  
  
• 10 test tubes
  
  
• Masking tape
  
  
• Markers
  
  
• Pipette or eyedropper
  
  
• Water
  
  
• A deeply colored liquid. Note, you should be very careful not to spill any on your skin or clothes. Many chemicals that produce intense color can make permanent stains on clothes, and very long-lasting stains on skin.
  

Procedure

1. According to your teacher's directions, form teams of 2-4 students. Gather the materials above.
   
   
2. Using masking tape and markers, label the test tubes 1 through 10.
   
   
3. Using the 10 ml graduated cylinder, put 9 ml of water in test tubes 2 through 10.
   
   
4. Using the 10 ml graduated cylinder, put 10 ml colored liquid in test tube 1.
   
   
5. Draw 1 ml of water into the pipette or eyedropper and mark the level using a marker pen. After marking the water level, you may empty the pipette or eyedropper.
   
   
   
   
6. Using the pipette or eyedropper with the measure for 1 ml, draw 1 ml of the colored liquid from test tube 1 into the pipette or eyedropper and transfer it to test tube 2. Shake the test tube to mix the colored liquid and the water.
   
   
7. Draw 1 ml of the liquid in test tube 2 into the pipette or eyedropper and transfer it to test tube 3. Shake the test tube to mix the colored liquid and the water.
   
   
8. Continue this process with all test tubes.
   
   
9. Next fill out the mixing ratio in the chart provided. Test tube 1 contains pure color, so its mixing ratio is one part in one = 1/1 = 1. Write this down for the mixing ratio in the parts by volume column in Table 1.
   

   Table 1. Dilution Chart

   Container number	Parts by volume	Parts per million by volume (ppmv)	Parts per billion by volume (ppbv)
   1
   2
   3
   4
   5
   6
   7
   8
   9
   10



10. Test tube 2 has one part coloring for ten parts liquid. What is this mixing ratio? (1/10 or 10-1) Write it on your chart and translate it into exponential notation. A mixing ratio of 1/10 is written , 1/100 is , etc. Continue this process for all ten containers.
    
    
11. Now convert into parts per million by volume by multiplying the parts by volume (the second column) by . This will tell you how many parts per million by volume you have in each test tube.
    
    
12. Convert to ppbv in the same way.
    

Observations and Questions

Answer in your lab book or on a separate sheet of paper.

1. Which containers have the highest concentration? Which have the lowest concentration?
   
   
2. Which container has the highest mixing ratio? Which has the lowest mixing ratio?
   
   
3. What happens to the color of the liquid as the mixing ratio decreases? Why does this happen?
   
   
4. Does the liquid ever become colorless? If so, at what mixing ratios is the liquid colorless? Why do you think it is colorless?
   
   
5. Which test tube contains one ppmv of coloring? Which test tube contains one ppbv of coloring?
   
   
6. Ozone in the stratosphere has a mixing ratio in the range of one to ten ppmv. Which containers represent one and ten ppmv?
   
   
7. A typical mixing ratio for ozone in the troposphere is 10 to 100 ppbv. Which test tubes represent this range of mixing ratios?
   
   
8. Your teacher will provide you with a chart showing the major gases that make up the atmosphere, along with their mixing ratios in the atmosphere. Note that some of the mixing ratios are given in ppmv, others in ppbv. Look at the table and at your dilution chart. See if you can find gases on the table that have mixing ratios similar to those of the 10 tubes in the dilution chart. Write the name of the gas next to the dilution chart values.
   

When you're finished with the activity, click on Back to Teacher Guide at the top of the page.