Chemistry of Pollutants

This exercise examines some aspects of the chemistry of pollutants. There are potentially lots of things that could be done here, and I would love to add more. What I have so far are exercises that look at octanol/water partitioning, coagulation for particulate removal, and effects of carbon dioxide and sulfur dioxide on water pH.

Students are told that the relative polarity of a substance's molecules determines how rapidly it may be washed away from the site of a pill, and that its partition between octanol and water is used as an indicator of molecular polarity. They prepare six test tubes containing 1 mL each of water and 1-octanol.  To separate tubes they add one drop of each of the following solutions: 0.05% malachite green, 0.1% methylene blue, 0.1% reactive red 120, and 0.1% reactive yellow 86 in water; and 0.1% solvent red 27 and 0.1% sudan black B in ethanol. (There isn't anything special about these six choices. I wanted a balance of colors and relative solubilities and these are dyes I had on hand.) After mixing well and allowing phases to separate, each dye preferentially (if not totally) partitions into either the octanol or water layer.

I have some really nasty clay underlying the topsoil around my house, of which I make a 1% suspension in water and let the larger particles settle out for an hour or so.  Students first observe the aluminum hydroxide precipitate formed by mixing one drop of 20% aluminum chloride and 6 M ammonia in 5 mL of water.  They then set up four tubes containing 5 mL of the clay suspension. They let one tube stand to see how well it settles (it essentially doesn't) and they filter another through filter paper (fast flow for coarse precipitates, either Whatman 4 or Fisher P8; amazingly, a significant amount of clay passes through). In the other two tubes they prepare the aluminum hydroxide precipitate and observe settling and filtration.  The difference between tubes with and without the aluminum hydroxide coagulant is profound.

To study the effects of carbon dioxide and sulfur dioxide on water pH, I use a universal indicator such as Bogen's. One drop of indicator is added to 50 mL of distilled water in a beaker and the pH is noted (5.5~6). The water is then brought to a boil and the new pH noted (~7). Finally, carbon dioxide is bubbled into the water and the final pH is noted (<4). A small sample of sulfur is ignited in a deflagrating spoon and lowered into a 125-mL flask containing 20 mL water with a drop of indicator. When the sulfur has been consumed, the contents of the flask are shaken and the low pH is noted.

Questions at the end of the exercise ask them (1) to identify, given the molecular structures of solvent red 27 and reactive red 120, which is which based on their observations of octanol/water partitioning, (2) to describe the aluminum hydroxide precipitate and its effects on settling and filtration, (3) to identify the acid formed when carbon dioxide dissolves in water and explain the change in pH on boiling, and (4) to identify the acids formed from sulfur oxides.

Revised 9/22/06