Determination of Calcium Carbonate in Eggshells

Introduction
The major component of eggshells is calcium carbonate (CaCO3). This analysis will be done volumetrically by using a characteristic reaction of carbonate compounds, namely their reaction with acids. Calcium carbonate (limestone) is very insoluble in pure water but will readily dissolve in acid according to the reaction:

2HCl (aq) + CaCO3(s) Ca2+ (aq) + CO2(g) + H2O + 2Cl- (aq)



This reaction cannot be used directly to titrate the CaCO3 because it is very slow when the reaction is close to completion (endpoint). Instead, the determination is achieved by adding an excess of acid to dissolve all of the CaCO3 and then titrating the remaining excess H3O+ with NaOH solution to determine the amount of acid that did not react with the calcium carbonate. The difference between amount of the acid (HCl) initially added and the amount left over after the reaction is equal to the amount that is used by the CaCO3. From this, the amount of CaCO in the sample can be calculated.

The reaction used to determine the leftover acid is

 HCl (aq) + NaOH (aq) H2O + Na+(aq) + Cl- (aq)

Stockroom: Things to borrow and return on the same day.

bulletBuret (~$75) and brush 

Procedure

  1. Each group of two students should obtain one egg and the necessary glassware from the cart.
  2. Break the egg into a beaker. Add water to the egg and stir before pouring down the drain.
  3. Wash the shell with deionized water and peel off all of the membranes from the inside of the shell.  If you leave the membranes, the proteins in the membrane will react with the sodium hydroxide and give poor results.  Dry your eggshell with a paper towel and put into a labeled beaker.  Wash your hands with soap and water.
  4. Dry the shell for about 10 minutes in the oven.
  5. Grind the shell to a fine powder with a mortar and pestle.
 

The rest of this experiment is done individually.  Each person does their own analyses.

  1. Accurately weigh between 0.450 and 0.550 g of dried shell into each of 3 labeled 250mL conical flasks. Be certain you record the exact mass of shell in each flask, in your notebook.
  2. Add several drops of ethanol to each flask. This acts as a wetting agent and helps the HCl dissolve the CaCO3.
  3. Pour  about 40 mL of 1.0M HCl into a beaker.  Using a pipet to transfer10.0 mL of the 1.0M HCl  to each 250mL flask containing eggshell. Swirl the flasks to wet all of the solid. Any HCl that you do not use should be disposed of in the sink and diluted with plenty of water.
  4. Heat the solutions in the 250 mL flasks on a hotplate until they just begin to boil and then allow them to cool.  Do not let them boil dry! Rinse the walls of the flask with water from your wash bottle.
  5. Add 3-4 drops of phenolphthalein indicator to each flask.
  6. Fill a beaker with about 100 mL of 0.100 M sodium hydroxide.  Using a funnel, a few mL of this solution to a clean buret  to rinse it. Empty the buret into the sink. Fill the buret with the NaOH solution to just above the top mark. . Run some solution out to remove all bubbles from the tip. Replenish the solution in the buret if necessary.  Read and record the initial volume to 0.01 mL.
  7. Titrate one sample to the first persistent pink color. When you are close to the endpoint the color will fade slowly. Add the remaining NaOH dropwise (or by half drops) until the color remains for at least 30 sec. Read and record the final volume to 0.01 mL. The volume added is the difference between the initial and final readings (to 0.01 mL)
  8. Repeat the titration for the other two samples.
  9. In your Results section, calculate the percent calcium carbonate in each sample and the average value to report in your Conclusion. Be sure you show all of your calculations, with units!
  10. Wash the egg residue out of the conical flask with hot soapy water and a test tube brush.


Results

Put your data and results in a neat table.  Show one complete calculation for the % calcium carbonate.  Show the calculation for your average.  

Calculations  (Show each step clearly using units and keeping only the significant figures)

1. Calculate the number of moles of HCl added to each shell sample. This is given by the expression:

moles HCl added = (0.0100 L HCl)*(1.00 moles HCl/liter) = 0.01000 moles HCl added

.
2. Calculate the moles of HCl left in each sample after the reaction with CaC03.
moles HCl left = (L NaOH you used)*(0.100 moles NaOH/liter)*(1 mole HCl/1 mole NaOH)

3. For each sample determine the number of moles of HCl that  reacted with CaCO3 by taking the difference between the moles of HCl added and the moles of HCl remaining after the reaction is complete.
moles HCl reacted = moles HCl added - moles HCl left
 
4. The moles of CaCO3 in each sample is calculated by:
moles CaCO3 = (moles HCl reacted)*(1 mole CaCO3 / 2 moles HCl)
 
5. Calculate the percent CaCO3 in each sample by using
% CaCO3 = [(moles CaCO3) *(100.09 g CaCO3/mole CaCO3)*(100)] / grams of sample
 
6. Calculate the mean value


Sample Calculation

Problem:

In an experiment, 0.500 g of eggshell is dissolved in 10.00 mL of 1.00 M HCl. The volume of 0.100 M NaOH required to neutralize the leftover HCl is 29.70 mL. What is the percent CaCO3 in the eggshell?

 Solution:

Initial moles HCl = VHCl MHCl= 0.0100 L x 1.00 mole/L = 0.0100 mol
.

Moles HCl left = VNaOH MNaOH =  0.02970 L x 0.100 mole/L = 0.00297 mol.

Moles CaCO3 = moles HCl reacted x (1 mol CaCO3 / 2 mol HCl )
.
Moles HCl reacted = 0.0100 - 0.00297 = 0.0070 moles
.
Moles CaCO3 = 0.0070 moles HCl x (1 mol CaCO3 / 2 mol HCl ) = 0.0035 moles
 
% CaCO3 = ( 0.0035 mols x 100.1 g/mol ) / 0.500 g = 70.0%  (report your answer to three significant figures.)

Conclusion

Using complete sentences, in paragraph form, restate the average percent calcium carbonate in the eggshell and the average deviation.  What were the sources of error in this experiment?  Why did you put the eggshells in the oven?  Are eggshells  a good source of calcium?  

lasted edited by jim byrd on 01/20/2006