SERIAL DILUTION
During this activity you are going to perform a serial dilution using a solution of Congo Red (a
natural stain). The Congo Red solution was created by adding 10 grams of Congo Red to 90 ml of
water thus creating a one-part-per-ten solution of Congo Red. This is equivalent to 100,000 parts per
million (ppm) because 10 x 100,000 is equal to one million. It is also equal to 100,000,000 parts per
billion (ppb) or 100,000,000,000 parts per trillion (ppt).
Obtain a multi-well tray, a dropper, a beaker of water and a container of the one-part-per-ten
solution of Congo Red. Put three or four drops of the Congo Red solution into well one. Put
nine drops of water into each of the other wells.
Now take the dropper and carefully transfer one drop of the Congo Red solution from well
one into well two. Use a separate stirring rod (not the tip of the dropper!) to stir up well
two. Observe what you see. Now you need to write some information down on your
worksheet: What is the new ratio of Congo Red to water? (Remember, the ratio in the first
well was 1:10), how many parts per million, billion, or trillion does this represent? and
finally, what is the color density? In other words, what color of red or pink do you see? Or do
you see any color at all?
After you are done with well two carefully, wash the glass dropper. Now transfer a drop from
well two into well three. Answer the same questions for well three as you did for well two.
Continue the serial dilution all the way through to the last well making sure to carefully wash the
dropper and stirring rod before making a transfer to the next well
When you are finished, thoroughly wash the dropper, the stirring rod, and the well tray.
Answer the following questions in your lab notebook.
1.
How many parts per billion were in your last well?
2.
How many parts per trillion were in your last well?
3.
What was the last well in which you could see any trace of the Congo Red dye?
4.
Were there any Congo Red molecules in the last well?
5.
How could you prove your answer to question four?
6.
From what you have learned in this activity, do think it is possible that clean, clear
drinking water could be contaminated with chemical toxins? Explain.
7.
How effective are the human senses of smell, taste, and sight at monitoring water quality?
Support your answer.
8.
One way of dealing with water contaminants is through dilution with more water. As the saying
goes "dilution is the solution to pollution." Do you think that dilution is an effective way of
dealing with water pollution?
9.
An Olympic-sized swimming pool holds about 660,000 gallons of water. How many gallons
of your starting solution would be needed to achieve a concentration of 1 ppm?
10.
The threshold dose-response model says that a toxic chemical must reach a certain level before it
starts to do any harm to an organism whereas the non-threshold dose-response model says that
harm begins with the first molecule of the substance in the organisms body. If the chemical in
the activity was one that bioacculmulates (like mercury), which model would you chose to
follow? Why?
11. Research and identify two ways that the EPA uses to detect very small amounts of toxic
contaminants in our air or water.
Well #
%
Concentration
Concentration
(parts per
thousand)
http://chs.carlsbadusd.k12.ca.us/DeCino/Webpage/APES/ppmlab.htm
Concentration
(parts per
million)
Observations
(intensity of color
present)