 We're going to analyze the sample of eggshells for the percent calcium carbonate in the eggshells using a back titration technique. We've masked about one gram of dried powdered eggshell. We're going to add that to a 125 milliliter Erlenmeyer flask. We're going to pipette 25 milliliters of a standard solution of approximately one mole or hydrochloric acid to the eggshells. The calcium carbonate in the eggshells will react with the hydrochloric acid to produce water, carbon dioxide, and calcium chloride. There will be an excess of hydrochloric acid at the end of the reaction. We will need to heat the sample in order to get the reaction to go to completion. We will be left with some protein at the end which held the calcium carbonate together. We've heated the sample to drive off the carbon dioxide and to get the sample to completely react. We have a small amount of protein left floating in the flask. We're going to add a few drops of bromphenol blue indicator to the reaction mixture. Bromphenol blue is yellow in acidic solution and blue in basic solution. We're now going to titrate the excess HCl with a standardized solution of sodium hydroxide. We've taken an initial burette reading. We're going to slowly add the sodium hydroxide solution until the reaction mixture turns blue. At that point we'll have the same number of moles of sodium hydroxide added as moles of excess HCl in the reaction mixture. We'll rinse down the flask periodically to make sure that all the reaction mixture has a chance to react. The flush has now reached a pale blue endpoint. We'll take a final burette reading.