Calcium carbonate in eggshells - Back Titration

A series of free IGCSE Chemistry Activities and Experiments (Cambridge IGCSE Chemistry).

Calcium carbonate in eggshells - Back Titration
The major component of eggshells is calcium carbonate (CaCO₃). This analysis is done by reacting the calcium with acids. Calcium carbonate is insoluble in pure water but will dissolve in acid.

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

Back Titration Technique
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 HCl 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 CaCO₃ in the sample can be calculated. The reaction used to determine the leftover acid is
HCl (aq) + NaOH (aq) → NaCl₂(aq) + H₂O(l)

Method

1. Wash the egg 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.
   
2. Dry the shell for about 10 minutes in the oven and grind the shell to a fine powder.
   
3. Weigh 1 g of the dried shell and place into flask.
   
4. Pipet 25 mL of 1.02 M HCl to the flask containing eggshell.
   
5. Heat the solution on a hotplate until it just begin to boil and then allow it to cool.
   
6. Add a few drops of bromophenol blue indicator to the flask. (Bromophenol blue indicator is yellow in acidic solution and blue in basic solution).
   
7. Titrate the solution with a standardised solution of sodium hydroxide. The end point is 42.10 mL of NaOH.

Calculation Steps:

1. Calculate the number of moles of HCl added to the shell sample.
   
2. Calculate the moles of HCl left in the sample after the reaction with CaCO₃.
   
3. 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.
   
4. Calculate the moles of CaCO₃ in the sample.
   
5. Calculate the percent of CaCO₃ in the sample.

Questions

1. What are the most likely sources of error in this experiment?
   
2. How could you try to get a more accurate result?
   
3. What assumption have you made about the composition of the egg shell that could have a serious effect on your result if the assumption is not true?

Answers

• Show Answers
  1. The eggshell powder did not dissolve completely. The eggshell was not fully dried. Variation in visual judgment at the end point. Instrumental errors of the electronic balance. Some droplets of solution may still adhere on the beaker and the glass rod which lead to the reduction in number of moles of excess HCl.
  2. Repeat the experiment several times and obtain the average.
  3. There are no other chemicals in the egg shell that will react with the HCl or NaOH.

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