Kids, have you ever heard the term “ppm” or parts per million? Sometimes a scientist will have to discuss what is in water or air at very low levels, and they use the term ppm because the amounts are so small. Even parts per billion (ppb) is used sometimes. One ppm means that one part of something exists in one million parts of the liquid, gas, or solid that it is found in. But just because these numbers sound so small does not mean that they are not important. For example, a fish (such as bass) needs at least 4 ppm dissolved oxygen in their water, and the air quality standard for sulfur dioxide (SO2, a pollutant) is 30 ppb. Chemists can detect to even parts per trillion levels for some materials using the right instrumentation.
Please note: All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.
This column will make use of more typical laboratory equipment than we usually require, but you can use ounces (oz) instead of milliliters (mL) and use common kitchen measuring tools as well. We will give measurements in both units. We are going to make a series of solutions of progressively higher dilution, and then test for the presence of a substance. This will demonstrate that something can be present at ppm levels even though we cannot see them with our eyes. You will need 8 small clear containers, milk, a small flashlight or laser pointer, a dark surface, and 100-mL and 10-mL graduated cylinders (or measuring cups to measure 1 and 10 ounces).
Measure and pour 100 mL (or 10 ounces) of milk into one of the containers, and do the same for pure water into another container. Mark these as #1 and #2, which is 1/1 or one part per one. Then measure 90 mL (or 9 oz) of water into each of the other six containers, marking them as #3-8. Pour 10 mL (or 1 oz) of milk from #1 (using the smaller graduated cylinder or measuring cup) into container #3. This concentration is 1/10, or one part per ten. Continue this serial dilution taking 10 mL (or 1 oz) from #3 and adding it to #4, and so on. They will progressively decrease in concentration as 1/100, 1/1000, 1/10,000, 1/100,000, and 1/1,000,000 (one ppm). What do you observe? Now place them on a dark tabletop and turn off the lights. Shine a flashlight through the side of the container and through the liquid. (Alternatively, have an adult partner use a laser pointer. But take care never to shine this in anyone’s eyes, including your own).
Look down on top of the liquid surface from above. What do you observe? It is hard to see any light pass through the pure milk because it is so thick. Next shine the light through the container of pure water, look down, and you shouldn’t see anything except the light on the other side. Containers 3-8 are another story, however. Shining the light through them at the side and looking over the top, you should see the beam of light right in the liquid as it passes through. Even though the last three containers (#6-8) look clear to the eye, there is enough milk present to scatter the light, albeit more weakly as the solutions get more dilute. The light is visible here due to what is called the Tyndall effect, a light scattering phenomenon. The light is scattering from colloids (proteins and other very small particles) in the milk.
Provided by K. A. Carrado and J. Sullivan, Argonne National Laboratory
--------------
Kathleen Carrado Gregar, PhD, Argonne National Labs
[email protected]
October 2004
----------------
References:
http://members.cox.net/bennowak/environment/envla/ppmlab.htm
