DETERMINATION OF CALCIUM CARBONATE IN EGGSHELLS

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DETERMINATION OF CALCIUM CARBONATE IN EGGSHELLS

BY ACID/BASE TITRATION

OBJECTIVE

To determine the mass percent of calcium carbonate (CaCO3) in an eggshell.

INTRODUCTION

During this lab, the percentage of CaCO3 in an eggshell is determined by reacting the eggshell with hydrochloric acid. The equation for this reaction is:

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

This reaction cannot be used directly to titrate the CaCO3. Instead, an excess of hydrochloric acid is added to dissolve the eggshell, and the remaining acid is titrated with NaOH solution to determine the amount of acid that did not react with the eggshell. The equation used to determine the amount of leftover acid is:

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

In order to help the hydrochloric acid dissolve the CaCO3, ethyl alcohol is added to the eggshell as a wetting agent. Wetting agents are chemicals that increase the spreading and penetrating properties of a liquid by lowering its surface tension-that is, the tendency of its molecules to adhere to each other.
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Although it is now banned in the United States, the pesticide DDT has caused significant damage to the environment and its wildlife. Birds are especially affected because the DDT weakens the shells of their eggs, which would break before hatching. This caused certain bird species to become endangered (i.e. the American bald eagle). One method of monitoring the strength of the egg is by determining the percent calcium carbonate in the eggshell.1

The percentages calculated from the experiment will be compared against a reported value of 95% CaCO3 in a dry eggshell.2

MATERIALS and PROCEDURE

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