Water Quality - Unit 4

Measuring pH (Hydrogen Ion Concentration)

Teacherís Instructions

Purpose: The purpose of this experiment is for the students to gain experience in altering the pH of water and for them to see how goldfish respond to different pH levels in their environment.

Note: Be sure to read the directions of both the pH test kit and the pH adjustment chemicals before using them. When selecting chemicals (pH Adjuster and pH Down distributed by Aquarium Pharmaceuticals, Inc.) it is important to select chemicals that are safe and easy for the students to use.  pH Adjuster and pH Down are available at most pet stores.  Also, make sure the students know the 'control group' concept (in this case, the aquarium with normal pH). The control group is how they will determine if the fish in the high pH and low pH containers behave differently than the norm. They will not make predictions on what happens to the control group.

Suggestion: When designing this experiment we had in mind that you, the teachers, would divide the class into groups. This experiment is designed to be flexible given each teacherís available supply of beakers, fish, etc. So, you may modify our setup given your number of students and available resources.

Materials for the entire class:

    • 40 liter aquarium (to house goldfish before the experiment)
    • masking tape
    • marker

Materials for each group:

    • 3- goldfish
    • 3- 1000 mL beakers 
    • pH measuring kit (test kit)
    • pH Adjuster - for increasing the pH 
    • pH Down - for decreasing the pH

Teacher Setup:

1.) A few days before the activity, begin adjusting the pH of the aquarium. It needs a have a neutral pH (as close to 7.0 as possible). The pH will need to be changed in small increments to ensure that you do not shock the goldfish. You will need to add a specific amount (specified in the directions of the adjustment chemical) and then wait for the solution to mix thoroughly in the aquarium. You will need to check the pH each day and make adjustments as necessary.

Student Setup:

2.) When beginning the activity, have the students fill each beaker with 1000 mL of the neutral water from the aquarium. Also, have them label one beaker Low pH, one High pH and one Control.

3.) In the Low pH beaker, the water needs to be adjusted to a pH between 5.5-6.0. Instruct the students to add the indicated amount of pH Down. Next, they need to stir the water and allow it to sit for 30 seconds. Then have them measure the pH. If the pH of the Low pH solution is not at the target range, have them add more pH down and measure again. Repeat this process until target pH is reached.

4.) In the High pH beaker, the water needs to be adjusted up to a pH between 8.0-8.5. Instruct the students to add the indicated amount of pH Adjuster. Next, they need to stir the water and allow it to sit for 30 seconds. Then have them measure the pH. If the pH of the High pH solution is not at the target range, have them add more pH up and measure again. Repeat this process until target pH is reached.

5.) Do not change the pH of the water in the Control beaker, it should still be 7.0.

Predictions:

6.) Have the students make predictions about what will happen to the fish and record these predictions in the appropriate box in the student journal activity for this experiment.

        a. What is the predicted behavior of the fish in Low pH?

        b. What is the predicted behavior of the fish in High pH?

Observations of goldfish behavior:

7.) Have each group of students remove 3 goldfish from the aquarium and place one each in the Low pH, High pH and Control beakers.

8.) The fish need to be left undisturbed in the beakers for 5 minutes to let them acclimate to the different pH of their environment.

9.)  Select one of the following physical movements to monitor in every goldfish:

    • swishing of tail
    • flexing of mouth
    • flexing of gills
Each of these movements are normally performed by fish.  

10.) After the 5 minute acclimation time has passed, make observations on your fish.

Count the number of times each goldfish makes the selected movement during a 1 minute observation period.

Also, record any differences in the speed that the fish move, physical appearance, or behavior.

Have the students record their observations in the Student Journal Activity sheet.
 
Note: Counting the number of times that the fish swishes its tail is probably the easiest measurement to make.

Discussion:

11.) Have students compare their results with what they predicted. The results will be based on any physical changes observed. Are the results the same as their predictions or are they different? Is the behavior of the experimental groups different from the control group?

12.) As a class, discuss the results. If any effect was (or wasn't) seen discuss what happened and how the fish were affected. If there was no marked effect, discuss why none was seen and what could be done next time so that an effect could be seen (e.g. more extreme pH or more sensitive organisms).