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Pesticides and Eggshell Thinning

David Tucker



Type of entry:

  • Class Lab Activity

Type of activity:

  • Hands-on
  • Inquiry Lab
  • Group/Cooperative Learning
  • Community outreach/off-site
  • Integrated Biology and Chemistry

Target Audience:

  • Environmental Studies (most applicable)
  • Life Science
  • Biology
  • Advanced/AP
  • Integrated Science
  • Advanced/AP Biology


Background information:

Question to answer: How do some pesticides promote eggshell thinning in birds?

Notes for Teacher:
This laboratory investigation promotes discussion of harmful effects of pesticides on biological systems. This activity fits in a unit on pesticides or toxic substances, or enzymes. The teacher must understand concepts related to acid/base chemistry, including titrations, neutralization, preparation of a primary acid. Gather reagent chemicals a month ahead of time, organize acid/base conceptual work to fit individual teaching and learning styles. Give lot of time to make standard acid and base solutions the first time through. Specific notes are included within the experiment itself.

Required of students:
Students must be able to use an analytical balance, demonstrate lab skills like pipetting, volumetric transfer, and titrating acid/base solutions.

Preparation Time needed:
4-6 hours first time through.

Class time needed:
Extensive lab time (1 week, but can be intermittent)


Abstract of Activity

This activity is a wonderful way to introduce toxic substances like pesticides and their effects on biological systems. When studying pesticides, most activities use only paper and pencil; so, this activity represents a true lab investigation that can be related easily to eggshell thinning in birds. There are many off shoots and extensions to this activity.

The activity starts with an introduction of how birds sequester calcium to make an egg. Students are asked to bring eggshells from home from different kinds of birds, if possible. These shells are prepared for analysis. Student learn how to prepare a primary acid, then use this acid to neutralize a base which is used to make a standard working acid used to determine the % of CaCO3 in shell material.

These results lead to a discussion of how Ca moves through this biochemical system and how a pesticide can prevent Ca from building eggshell. Scientific extensions include enzymatic action, polar and nonpolar substances, pesticide chemistry, etc.

Students are then encouraged to research the issue of chlorinated hydrocarbon pesticides and how they work. Students can explore the issues of banning pesticides, pesticide usage throughout the world and the pesticide "double standard" in this country.


PROCEDURE #1: PREPARING THE EGGSHELLS

1. Bring 2 or 3 eggshells from home. Boil them to remove a protein membrane on inside of shell. Cool and peel membrane away. Use only those shells that have had membrane removed.

2. Grind the shells in a mortar to a very fine powder. (1.0 gram of shell needed for experiment.)

3. Place in a drying oven overnight. [See Note 1 at end of this experiment].

PROCEDURE #2: PREPARING A STANDARD ACID

1. Clean four narrow-mouth containers or flasks. They do not have to be dried. LABEL them.

2. ADJUST analytical balance.

3. WEIGH out 4 samples of reagent-grade KHP, potassium acid phthalate, each having a weight of about 0.61 grams. [See Note 2 and Note 3]

4. Place one sample in each flask. Dissolve EACH OF THE FOUR KHP samples in 25 ml of distilled water.

5. ADD three drops of phenolphthalein to each flask. The solution will remain colorless. [See Note 4.]

PROCEDURE #3: PREPARING A WORKING BASE

1. Clean a 1000 ml volumetric flask to hold 1 liter of an NaOH solution.

2. PREPARE 1 Liter of a 0.1 M NaOH solution. [Students may need refresher instruction to do this.]

3. Dissolve the NaOH with distilled water and perform a quantitative TRANSFER of all of the solution to your clean NaOH container. Add water to 1000 ml mark.

4. Cap this solution tightly. Keep the cap on all of the time. [See Note 5.]

PROCEDURE #4: TITRATING THE BASE

1. PREPARE a burette for a titration.

2. FILL your burette to the top mark with NaOH solution (remember to cap the NaOH bottle).

3. Titrate 1 of your 4 KHP acid samples with this base solution fairly rapidly to determine where the end point. [See Note 6].

4. READ the volume of NaOH on the burette that was required to neutralize the acid. Record.

5. TITRATE the remaining KHP acid samples SLOWLY. Record the volume of acid needed each time.

6. CALCULATE an exact concentration of your NaOH solution using each sample. [See NOTE 7]

PROCEDURE #5: MAKING AN ACID TO REACT WITH EGGSHELL

1. Make a dilute hydrochloric acid solution to react with the egg shell; that is about 500 ml of a 0.2 M HCl solution. Use dilution law. Clean suitable container. [See NOTE 8.]

2. Measure acid out carefully. [See NOTE 9].

PROCEDURE #6: DETERMINING CONCENTRATION OF YOUR ACID

1. Titrate this acid with your standardized NaOH solution to determine the acid concentration to 4 sig figs.

2. Prepare PIPETTE and transfer four 25 ml samples of your reaction HCl solution into 125 ml flasks.

3. Add a few drops of phenolphthalein indicator to the acid solution.

4. Fill a burette with your standardized NaOH solution and titrate the acid samples. [See NOTE 10].

5. Titrate at least 4 samples for accuracy. Record volume of base each trial.

6. Calculate the concentration of the HCl reaction acid. [See NOTE 11].

PROCEDURE #7: Determining calcium carbonate content of shells

1. Weigh accurately about 0.15 gram of shell into a clean and dry 125 ml Erlenmeyer flask. Record.

2. Add 5 ml of ethanol to the flask.

3. Pipette 25 ml of your acid solution into the flask to dissolve your egg shell.

4. Allow the acid to dissolve the eggshell by swirling the flask gently and washing down the sides of the flask with distilled water until the volume is about 50 ml. [See NOTE 12].

5. Gently boil the solution using a hot plate for 5-10 minutes until all of the shell material is dissolved. Wash the walls of the flask with distilled water to ensure that the volume remains at about 50 ml. Do not boil over the shell material. Let cool.

6. Add 4 drops of phenolphthalein to each flask and titrate with your standard base solution to a pink color. Record volume of base.

7. Perform 4 trials for accuracy.

PROCEDURE #8: Calculating the % of calcium carbonate in shells.

1. Construct a data and calculation table to provide for the following items:

a. EXACT weight of shell (in MILLIGRAMS)
b. VOLUME of HCl added to flask with shell
c. VOLUME OF NaOH added during titration of acid and shell
d. MMOLES of HCl added to dissolve shell
e. MMOLES of NaOH added to neutralize excess acid
f. MMOLES of HCl equivalent to MMOLES of base (1 mmol base will neutralize 1 mmol acid)
g. MMOLES of HCl consumed in the reaction ( total mmol - excess mmol = mmol reacted)
h. MMOL of CaCO3 (there is 1 mmol of CaCO3 that reacts for every 2 mmol of HCl)
i. MOLAR MASS OF CaCO3 (this is found on periodic table)
j. MILLIGRAMS of CaCO3 reacted ( this is found from (h) and (i) above)
k. % of CaCO3 in eggshell

Teacher notes

Note 1. This experiment should be done individually. It gives teacher a good chance to monitor student lab skills. The entire process can be evaluated using performance criteria. Encourage students to use different kinds of eggshells. Use reagent grade primary standard KHP (potassium acid phthalate). Dry the KHP for one hour at 110o and store in a desiccator. Weigh 4 samples of the solid acid into four 125 ml Erlenmeyer flasks. Each sample should contain enough acid to react with 30 ml of 0.1 M NaOH. [See calculation below] :

30 ml NaOH X (0.1 mmol NaOH/ml NaOH) X (1 mmol KHP/mmol NaOH) X 

(0.2042 mg KHP/1 mmol KHP)  =   0.61 grams of KHP

Note 2. Results will improve if you dry the KHP ahead of time and student know how to use an analytical balance quickly.

Note 3. The amount of KNP only needs to be approximately 0.61 grams; however, you must accurately know how much was weighed out. Create good data table.

Note 4. The phenolphthalein must not be forgotten. A lot of lab work will have been accomplished for nothing if it is forgotten.

Note 5. Again, this solution needs only to be approximate since its concentration will be determined accurately in the titration with the KHP.

Note 6. Instruct students in proper titration technique.

Note 7. Remember, since each trial had a different result, each trial will have to be calculated separately. DO NOT average the volumes. Calculate each separately, then AVERAGE the concentration numbers. Calculation is...

(a) Weight of KHP (mg) X 1 mmol KHP/0.2042 mg KHP = mmol of KHP
(b) mmol KHP = mmol NaOH
(c) mmol NaOH / ml NaOH = Molarity (M) of NaOH

Note 8. Concentrated HCl has a molar concentration of 11.7. Use the dilution law... Ma . Va = Mb . Vb where M1 is molarity of strong acid (11.7 and V1 is volume of this acid to be added, M2 is molarity of new weaker acid and V2 is volume of weak acid chosen.

Note 9. Use precaution measuring out acid. Fume hood. Acid to water.

Note 10. Before you do the titration think about how much of the base will be needed to neutralize the acid. The base solution is about 0.1 M and the acid is about 0.2 M.

Note 11. The neutralization equation... Ma . Va = Mb . Vb where (a) stands for acid and (b) stands for base.

Note 12. The shell reaction is...

SHELL  + ACID  ---->  CARBON DIOXIDE  +  WATER  +  CALCIUM CHLORIDE

CaCO3    +   2Hcl   ------>   CO2   +   H2O   +   CaCl2

Notice there will be two moles of acid required to react with one mole of shell material.


Evaluation:

Students must demonstrate proficiency in all lab techniques, all calculations and arrive at reasonable value for %CaCO3 in shell material. Must demonstrate understanding of acid/base chemistry and chemistry of eggshells.


Discussion:

Results are discussed. %CaCO3 range generally from 70-95%. Prelab discussion on lab techniques and performance assessment of them. Calculations. Pesticide action and interference.


Extensions:

Students research pesticide action, type of pesticides, perform polar and nonpolar investigations, pesticide issues, local pesticide problems, and pesticide laws.


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