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Investigating Parts Per Million (PPM)

From the Summer 1996 sciencelines

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


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."

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

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.

Think About It! (For Teachers)

The student section on Lab Sheet 2 includes a "Think About It!" section. Here's some input, about the responses for teacher background:

1. Does a dissolved toxin ever disappear? Completely? (No, the toxin may not be visible, but since matter can not be created or destroyed, it will always be there. Only the state of matter may change.)

2. If 50% of the population of the organism you studied was killed, how might that affect the ecosystem? (Answers will vary.)

3. When the color disappeared from the cups, was there any colored solution still in the cup? (Yes. The particles are still present. They are in small concentrations that are not visible to the unaided human eye.)

4. Is dilution the solution to pollution? Be specific. If we just keep adding water to toxins, will the organisms be safe?