Activity #4
Optimum pH of Nutrient Solutions

Grade Level: 9-12

Module: Hydroponics

Overview

In this activity students monitor plants growing at various pH concentrations to determine the optimum pH concentration for hydroponic plant growth.

Key Question

• What is the optimum pH for growing plants hydroponically?

Time Frame:

Materials

• 8 small plant containers (yogurt containers or film canisters)
• 8 lettuce seedlings
• 4 containers for nutrient solution delivery-plastic shoe boxes or two liter bottles*
• 8 liters of nutrient solution
• fluorescent light
• 0.1 molar potassium hydroxide
• 0.1 molar nitric acid
• balance (if needed for measuring dry chemicals in making chemical solutions)
• pH meter or pH paper
• burette or pipette
• graduated cylinders
• graph paper
• ruler
• safety goggles, gloves, and aprons, for handling nitric acid and potassium hydroxide solutions
• vented fume hoods, chemical storage and disposal facilities *If using plastic shoe boxes, use boxes large enough to hold several liters of nutrient solution.

Getting Ready

2. Students will need materials for hydroponic growth containers set up prior to beginning the activity. Refer to "Materials" section of "Activity 3: Hydroponic Systems" for more detail on how to set up a hydroponic system.

3. Prepare approximately 8 liters of stock nutrient solution prior to beginning the activity. Contact your local hydroponics supply store (check your local Yellow Pages) and follow instructions from the supplier. Another good source of information on hydroponics is the Hydroponic Society of America, P.O. Box 3075, 2819 Crow Canyon Rd. Suite 218, San Ramon, CA 94583; phone: 510-743-9605; fax: 510-743- 9302.

4. Solutions of potassium hydroxide and nitric acid need to be purchased or made available prior to beginning the activity. You can mix your own or use standard stock solutions available from commercial science supply houses. The 0.1 molar solutions can be prepared from 1 molar stock by diluting 1:10 (one part stock to nine parts water).

CAUTION:
If you or your students handle chemicals (potassium hydroxide and/or nitric acid solutions), make sure proper precautions are taken, such as always adding acid to water and mixing chemicals very slowly. Diluting acid is an exothermic reaction (giving off heat), and by adding the acid slowly to the water you achieve the greatest dilution (for safe handling) with the least risk of excessive heat production. Proper safety precautions must be taken with the stock (1 M) and working (0.1 M) solutions of acid and base. This includes use goggles, gloves, aprons, and vented fume hoods. Proper storage and waste disposal of the chemicals used must also be arranged. Check with your own school district for more detailed instructions.

A simpler alternative is to use vinegar and baking soda solutions in place of nitric acid and potassium hydroxide.

Classroom Activity

1. Place equal amounts of nutrient solution in each nutrient solution delivery container. Mark the water level after adding 1.5 liters. You will need more nutrient solution to replenish the containers during the course of the experiment.

2. Choose the pH concentrations to be tested. Plain nutrient solution is typically pH 5.5 - 5.6. This can act as a control. Choose three other concentrations in the ranges of pH 3, 8, and 10.

3. Adjust the pH by adding 0.1 molar nitric acid (H2NO3) or 0.1 molar potassium hydroxide (KOH) in milliliters until the pH reaches the desired level. Use a pH meter or pH paper to test the pH.

4. Monitor the pH concentrations daily and adjust the value by one of the methods below:

Method A for pH Adjustment

a. Take an initial pH reading.
b. Using a graduated cylinder, refill the nutrient solution delivery container to the 1.5 liter mark with nutrient solution.
c. Remove 20 ml of sample solution from the container and titrate (add small volumes of the 0.1 molar acid or base solutions) with a burette or pipette until the desired pH level is obtained.
d. Calculate the amount of acid or base needed to adjust the remaining 1.48 L solution.

a. Take an initial pH reading.
b. Monitoring with a pH meter or pH paper, add acid or base one milliliter at a time from the 0.1 molar solutions, mixing between each addition, to the nutrient solution until the desired pH is reached.
c. Record the total volume added for future reference.

5. Every other day make observations of the plants (height and number of leaves).

6. At the end of two weeks prepare a summary of the observations and have the students draw conclusions about the relationship between pH and plant growth.

Plant Growth Record

pH	DAY 0	DAY 2	DAY 4	DAY 6	DAY 8	DAY 10	DAY 12	DAY 14
pH=3
pH=5
pH=8
pH=10

Plant Growth Record

Height of Plant (in centimeters)

pH	DAY 0	DAY 2	DAY 4	DAY 6	DAY 8	DAY 10	DAY 12	DAY 14
pH=3
pH=5
pH=8
pH=10

Wrap-up Session

Have students write their findings in a science journal.

More Activity Ideas

2. Have student do experiments to determine effects of nutrient deficiency.

Background for Teachers

At their roots, plants take in nutrients in the form of anions and cations. Anions typically include nitrates (NO3-), sulfates (SO4-2) and phosphates (H2PO4-). Cations needed for plant growth may include calcium (Ca+2), magnesium (Mg+2), and potassium (K+). A plant generally absorbs more negatively charged than positively charged particles. In order to maintain equilibrium, the plant excretes hydroxyl ions (OH-) into the nutrient solution. The hydroxyl ions combine with the hydrogen ions (H+). When the H+ ions are removed, the pH of the solution is increased causing it to become more basic.

Plants have a narrow range of pH tolerance. They typically do best in pH solutions of 5 to 6.5. If a solution is too basic, this may impact the plants' ability to absorb iron. Iron, an essential element for plants, is no longer soluble beyond a pH of about 8.

Prerequisites:

• Facility with computation of pH
• Knowledge of anions and cations

Vocabulary:

• pH - the relative acid or base content of a liquid
• Acid - a chemical that tends to give up protons
• Base - a chemical that tends to take up protons
• Anion - a negatively charged ion in solution
• Cation - a positively charged ion in solution
• Titration - a method of ascertaining the quantity of a given constituent in a solution by accurately measuring the volume of a liquid reagent of known strength necessary to convert the constituent into another form, the close of the reaction being marked by some definite phenomenon such as change of color

Skills:

• Adjusting the titration of a liquid
• Computing strengths of solutions

Concepts:

• The relationship between plant growth and pH, and pH of solutions.

Keywords: Hydroponics, pH, Solutions, Titration