## Investigating Parts Per Million (PPM)

### From the Summer 1996 sciencelines

Submitted by Pat Franzen, Madison Junior High School, District 203, Naperville, IL.

BACKGROUND

How much toxin can destroy a population? What is a lethal dose/lethal concentration?

This investigation is designed to help the student conceptualize one part of a million - a concept boggling to most adults! The exercise will allow students to visualize succesive dilutions of a 10% solution of food coloring or dye until 1 ppm is established.

From that point, continued dilutions will be completed to simulate the conditions which would be toxic enough to kill at least 50% of a given population over a period of time. The relative concentrations derived will be corresponded with current EPA standards for common heavy metals often produced as by-products of industry. NaCL (table salt), an ingredient in ice-melting crystals used on our wintery roads and often found in "run off" that enters our waterways will be considered as well. (Lab Sheet 1.)

Lab Sheet 2 includes EPA figures representing "lethal concentrations" of various substances. A lethal concentration is the amount considered capable of killing 50% of a population over a period of time. The term "concentration" is used rather than "dose" because we are considering the concentrations of a liquid solution. "Dose" is typically used to refer to solids.

The EPA concentrations represent mg/l. A milligram is 1/1000 of a gram, and there are 1000 ml in a liter. Since water is the solvent we are considering, and one liter of water has a mass of 1000 grams (density of water is 1 g/ml), we can calculate 1 mg/l to equal 1 ppm. (See Lab Sheet 2.)

Grade Level: Junior High - High School

Objectives: · Students will develop operational definitions of "ppm" and "lethal concentrations."

• Students will understand the concept of successive dilutions.
• Students will refine appropriate lab procedures and techniques.
• Students will relate information presented to local and global pollution concerns.

Materials: For each group of two students you will need:

• Powdered dye such as Rit or liquid food coloring of approximately a 10% solution
• Water in 500 ml beaker (or similar-sized container.)
• Medicine dropper - pre-calibrated by teacher (see above).
• 10 small clear pre-calibrated medicine dispensing cups or plastic beverage cups (pre- calibrated).
• 10 ml graduated cylinder (or calibrated cups)
• Lab sheets.

Teacher Preparation:

1. Make a 10% powdered dye and water solution by mass. 1 part solute (dye) to 9 parts water. Alternative - commercial food coloring can be used. I have found that the concentration is approximately 10%, but brands vary. A colorless liquid should be reported in cups E, F, or G. (Student error is to be expected!)

2. Measure 1 ml of water in an eye dropper and use a permanent marker to indicate the appropriate level on all the droppers.

3. If you do not have access to a 10 ml graduated cylinder per pair of students, the plastic cups can also be marked at the 10 ml level.

4. Instruct students to follow the directions presented on lab sheet.