Chemistry

Question

How to find the percentage of calcium carbonate in egg shells?
(2 methods)

Answer 1

determine through the calcium content
1. add hydrochloric acid to diggest the egg shells, heat gently to complete the reaction, neutrlized the excess HCl with ammonia solution and add Ammonia/Ammonium chloride buffer and titrate the mixture with standard EDTA solution use E-Black Chrome indicator. (but this method is complicated)
2.diggest the egg shells as method one, add sodium oxalate to produce calcium oxalate ppt, collect and dry the ppt (105 degree celcius)and weight......to confirm the results you can heat the calcium oxalate ppt (~500 degree celcius)and change it to calcium carbonate, wieght again.

P.S. I do not agree determine the content through gaseous mean, because large volume of gas is not easy to handle and the reactions may be not complete due to leak of the experiment system.(this is my personal opinion only.)

Answer 2

1. heat the egg shell strongly, so that calcium carbonate decomposes into calcium oxide and carbon dioxide.
CaCO3 → CaO + CO2
then, pass the gas produced into limewater, so that precipitate of calcium carbonate is formed.
CO2 + Ca(OH)2 → CaCO3 + H2O
filter, dry and weigh the precipitate. mass of calcium carbonate formed should be equal to mas of it in egg shell.

2. add excess hydrochloric acid to the shells, and collect the gas produced by a syringe.
CaCO3 + 2HCl → CaCl2 + CO2 + H2O
measure the volume of gas produced at room temp. and pressure. calculate the no. of mole of CO2 produced, as well as the mass of calcium carbonate reacted.