Determination of Calcium Carbonate in Eggshells
ed, by J. McMurry and R. Fay. Prentice Hall, 2004, Sections 3.10, 4.5
To measure the mass percent of calcium
carbonate in an eggshell.
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)
(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.
|You will be using a buret (~$75), brush, 10 mL
pipet and green pipet pump in this experiment.|
|Be sure to rinse all the glassware with water and
return it to the box when you are done.|
This part of the experiment is done as a team.
- For each group of two students, obtain one egg and
any necessary glassware.
- Break the egg into a beaker. Add water
to the egg and stir before pouring down the drain.
- 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
- 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.
- Grind the shell to a very fine powder in a
The rest of this experiment is done
- 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.
- Add several drops of ethanol to each
flask. This acts as a wetting agent and helps the hydrochloric acid dissolve the CaC03.
- 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.
- 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.
- Add 3-4 drops of phenolphthalein indicator to each flask.
- 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.
- 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.
- Repeat the titration for the other two
- Calculate the percent calcium carbonate in each
sample and the mean value. Calculate the standard deviation from the
results of your 3 trials.
- Wash the egg residue out of the conical flask with
hot soapy water and a test tube brush.
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
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
- 4. Use the stoichiometry of the reaction as
shown above to calculate the number of moles of CaCO3 in
- 5. Calculate the mass percent of CaCO3
in each sample.
- 6. Calculate the mean value and the standard deviation
from your results.
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
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 =
- 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.%
average percent calcium carbonate by mass in the eggshell and the relative standard deviation.
average percent calcium by mass in the eggshell and the relative standard
Are your results consistent with reported values? Be
sure to cite your references.