Up until as recently as 1986, canaries were used in coal mines to warn miners of the presence of harmful gases such as carbon monoxide or methane. Since the birds are more sensitive to these gases than humans, they are affected before the gases get to a level that is dangerous for humans. This use of a living organism to test for toxicity is called a bioassay. Though canaries have been replaced by electronic devices, bioassays with other biological organisms are used frequently in chemistry and environmental science. They are used to test for herbicide residue in soil, harmful chemicals in water, the effect of de-icing chemicals, and more.
The following procedure involves running a bioassay to determine what concentration of salt (NaCl) is toxic for seeds. Lettuce seeds work well because of their sensitivity, but other seeds (such as radish seeds) will work also. This project can be easily modified to test toxicity in your local environment. If you live in an area where there is lots of snow and ice, try testing different concentrations of a solution containing de-icing substances used in your city. If local refineries or factories release waste into a river, research what kinds of chemicals are in the waste and run a bioassay to determine what concentration of the chemical becomes toxic for local plant life. You can also research methods of performing bioassay tests on soil samples.
Question: At what concentration does salt (NaCl) become toxic to seeds? Is a certain amount of NaCl beneficial to seed germination/growth?
Observe/Gather Data: Research all you can about how large concentrations of salt (or whatever chemical you decide to test for) could get into local water or ground water, and find out the existing data about how salt affects plant germination and growth. The more detail you gather in your research, the more informed your hypothesis will be.
Hypothesis: Based on your research, write a detailed hypothesis predicting the answer to the question.
Experiment: To test your hypothesis, run a bioassay testing the effect of solutions with varying concentrations of NaCl. The following procedure uses three different concentration levels, but depending on your results you may want to test more levels to arrive at the most precise result (e.g., try a few solutions in between .1M and .2M, or a solution higher than .2M). The more test samples you have, the more accurate your results will be: try multiplying this experiment so you have several dishes of seeds for each concentration level.
What You Need:
- NaCl (table salt)
- distilled water
- 4 petri dishes
- filter paper
- 1000 ml beaker (or other 1 liter container)
- 100 ml graduated cylinder
The first part of the experiment involves making solutions with several different concentrations. The easiest way to do this is to make a high concentrate solution and then dilute it for your other solutions.
grams of chemical = (molarity of solution in mole/liter) x (MW of chemical in g/mole) x (ml of solution) ÷ 1000 ml/liter
In our case, 11.69 g = (.2) x (58.44) x (1000) ÷ 1000 ml/liter.
- Begin by making a liter of .2M NaCl solution. To do this, find the molecular weight (MW) of NaCl by adding the atomic masses of sodium (Na) and chloride (Cl). Since the atomic mass of Na is 22.99, and the atomic mass of Cl is 35.45, the molecular weight of NaCl is 58.44. Now that you know the molecular weight, plug it and the molarity of the concentration you want (.2M) into this formula to determine how many grams of chemical you need.
ml of .2M solution = (ml of solution) x (molarity of solution) ÷ .2
For a .1M solution, for example: 50ml = (100) x (.1) ÷ .2. This means you will mix 50ml of the .2M NaCl solution with enough distilled water to add up to 100ml. In this case, you will add 50ml of.2M solution to 50ml of distilled water.
- Use your balance to weigh 11.69 g of NaCl and dissolve it in 1000ml of distilled water.
- Now determine how many ml of your .2M solution you need to make 100 ml of each of your concentrations. Use the following formula:
- Repeat step three to make 100ml of a .075 concentration. Make sure you label each solution.
In the next part of the experiment set up and analyze your bioassay samples.
- Put a piece of filter paper in each of four petri dishes. Label each petri dish: Control, .2M, .1M, .075M.
- Use a pipet to add 2 ml of the appropriate solution to each petri dish. (For the control dish use plain distilled water.)
- Place eight seeds from the same packet in each dish, evenly spaced.
- Put the lids on the dishes and seal them all in a plastic bag to help keep them moist.
- Put the dishes in a dark place and keep them at room temperature for 5 days.
- Check them each day and record how many seeds have germinated in each dish.
- After 5 days, measure the radicle (embryonic root) length in mm of each seed that germinated. Look closely so you measure just the root, not the shoot as well.
Analyze Data/Form Conclusions
How many seeds sprouted in each dish? Did the seeds in the control dish sprout faster than the others? Did the seeds in the control dish grow longer roots than the others? Did a minimal concentration of salt actually aid growth? Can you determine what concentration impedes seed germination? What concentration is toxic enough that the seeds don’t germinate at all? If none of your concentrations seemed to affect germination negatively, you might try the experiment again with stronger solutions.
Did your results support your hypothesis? Why or why not? What do your results tell you about the effects of salt toxicity on seeds? How could you develop this project further to test the toxicity of herbicides or de-icing chemicals in your area?
If you decide to test chemicals that are more hazardous than table salt, you will need to take extra safety precautions.
Always carefully read the entire label before using a chemical. Be sure you understand the hazards involved, the proper safety equipment to wear, and what you will do in case of a spill or contact with your skin.
Basic safety equipment that you should wear include safety goggles (splash type), chemically resistant gloves, and a chemically resistant lab apron.
Work in a clean, well ventilated, uncluttered area where you can quickly wipe up spills.
Always keep chemical bottles tightly capped except for the short period of time you are measuring the chemical