Duckweed Bioassays: Compounds and Concentrations
You can use bioassays to measure the sensitivity of duckweed to a wide variety of compounds, or different concentrations of the same compound. Below is information on how to create a range of concentrations (serial dilutions) of a compound you would like to test, as well as examples of different compounds you may be interested in using.

Creating serial dilutions
The first step in carrying out a dose/response experiment is to create a wide range of concentrations of your test solution. Initially, you probably will have no idea what concentrations will kill your test organisms and what concentrations will cause them no harm. Therefore, it is a good idea to start by testing a very wide range of concentrations. You can do this by creating a serial dilution - a series of solutions, each of which is ten times more dilute than the one from which it is made.

The highest concentration in your serial dilution will be labeled, "100% Concentration". This can be whatever concentration you choose - just remember to write down what chemical you are using and what concentration (in mg/L) you have selected to be the 100% concentration. The table below has suggestions for possible 100% concentrations.

Materials

• Balance
• Six 250 mL beakers, flasks, or cups
• One 100 ml graduated cylinder
• One 10 ml pipette or graduated cylinder
• Pipette bulb (if using pipettes)
• Tape or wax pencil for labeling flasks
• Parafilm or plastic wrap
• Test chemical (use the table below or consult with a teacher)
• Goggles
• 2 L spring water

Procedure

1. Label the six beakers with your name, the date, the compound you will be testing, and the the following percent concentrations: 100%, 10%, 1%, 0.1%, 0.01%, and 0.001%. Make sure to record the concentration of your 100% solution (in mg/L).
2. Weigh out the appropriate amount of your chemical and place it in the beaker labeled 100%. Add 100 ml water and swirl gently until your compound is dissolved.
3. Using a 10 ml pipette or graduated cylinder, transfer 10 ml of your 100% solution to the beaker labeled "10%". Add 90 ml water to the beaker and gently swirl. Thoroughly rinse the pipette when finished.
4. Transfer 10 ml of your 10% solution to the beaker labeled "1%". Add 90 ml water to the beaker and gently swirl.
5. Repeat these steps until you have made all five dilutions. Unless you will be using them immediately, cover them tightly with plastic wrap to prevent water loss through evaporation.