Determination of Calcium Carbonate in Eggshells

References:

 Chemistry, 4th ed, by J. McMurry and R. Fay. Prentice Hall, 2004, Sections 3.10, 4.5

Purpose:

To measure the mass percent of calcium carbonate in an eggshell.

Method

The major component of eggshells is calcium carbonate.  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  readily dissolves in acid according to the reaction

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



      This reaction cannot be used directly to titrate the CaCO3 because it is very slow when the reaction is close to the endpoint.  Instead the determination is achieved by adding an excess of acid to dissolve all of the CaCO3 and then titrating the remaining H3O+ with NaOH solution to determine the amount of acid which has not reacted 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 used by the CaCO3. The reaction used to determine the leftover acid is:

     

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

 

Special Equipment: Things to use and return on the same day.

bulletYou will be using a buret (~$75), brush, 10 mL pipet and green pipet pump in this experiment.
bulletBe sure to rinse all the glassware with water and return it to the box when you are done.

Procedure:

This part of the experiment is done as a team.

  1. For each group of two students,  obtain one egg and any necessary glassware.
  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. There are two membranes, one that is easy to see and one that you can find by rubbing your finger on the inside of the shell.  Dry it with a paper towel and put into a labeled beaker.
  4. Dry the shell for about 20 minutes in the oven.  It is important that the shell be dry in order to get the best results.
  5. Grind the shell to a very fine powder in a mortar.

The rest of this experiment is done individually.

  1. Accurately weigh between 0.450 and 0.550 g of dried shell into each of 3 labeled 125 or 250 mL conical flasks. Be certain you record the mass of shell for each flask in your notebook.
  2. Add several drops of ethanol to each flask. This acts as a wetting agent and helps the hydrochloric acid dissolve the CaC03.
  3. Slowly, and carefully, pipet 10.00 mL of 1.0 M HCl solution into each flask. Swirl the flasks to wet all of the solid. Any excess HCl should be disposed of in the sink by diluting with water.
  4. Heat the solutions in the flasks until they begin to boil and the solid egg shell dissolves.  It is important that all of the eggshell dissolves because this contains the material you are analyzing for.  Eggshell is dense and will settle on the bottom of the flask.  A white proteinacious substance may form, but it will be suspended in the solution.  Allow the flasks to cool. Rinse the walls of the flasks with water from your wash bottle.
  5. Add 3-4 drops of phenolphthalein indicator to each flask.
  6. Using a funnel, partly fill a clean buret with your standardized sodium hydroxide solution to rinse it. Empty the buret into the sink. Fill the buret with the NaOH solution. Run some solution out to remove all air 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 until the color remains for at least 30 sec. Read and record the final volume to ± 0.01 mL.
  8. Repeat the titration for the other two samples.
  9. Calculate the percent calcium carbonate in each sample and the mean value. Calculate the standard deviation from the results of your 3 trials.
  10. Wash the egg residue out of the conical flask with hot soapy water and a test tube brush.


Calculations:

Show all of the calculations for each individual trial, for the data in your Table.  As always, pay close attention to significant figures in your calculations.

1. Calculate the number of moles of hydrochloric acid added to each shell sample. 

2.  For each titration, calculate the number of moles of hydrochloric acid left in each sample after the reaction with CaC03.

3.  For each sample determine the number of moles of hydrochloric acid that has reacted with CaCO3 by taking the difference between the moles of hydrochloric acid added and the moles of hydrochloric acid remaining after the reaction is complete.

4.  Use the stoichiometry of the reaction as shown above to calculate the number of moles of CaCO3 in each sample.

5.  Calculate the mass percent of CaCO3 in each sample.

6.  Calculate the mean value and the standard deviation from your results.



 Example Calculation:

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?

Initial moles HCl = VHCl MHCl
            = 0.0100 L x 1.00 mole/L = 1.00x10-2 mol

moles HCl left = VNaOH MNaOH.
            = 0.02970 L x 0.100 mole/L = 2.97x10-3 mol

moles HCl reacted = 0.0100 - 0.00297 = 0.0070 moles
 
moles CaCO3 = moles HCl reacted x (1 mol CaCO3 / 2 mol HCl )
 
moles CaCO3 = 0.0070 mols HCl x (1 mol CaCO3 / 2 mol HCl ) = 0.0035 mols
% CaCO3 = ( 0.0035 mols x 100.1 g/mol ) / 0.500 g = 70.%  
 

Conclusion:

Report the average percent calcium carbonate by mass in the eggshell and the relative standard deviation. 

Report the average percent calcium by mass in the eggshell and the relative standard deviation. 

Are your results consistent with reported values?  Be sure to cite your references.