Marble is just CaCO3 Gravimetric Determination of Ca GOAL OF EXPERIMENT: To determine the % calcium in antacid tablets (CaCO3) METHOD: Gravimetric Analysis. Isolate calcium as calcium oxalate precipitate and compare mass of calcium in precipitate (known formula CaC2O4•H2O) with original mass of sample.

SCHEDULE: Today Demonstration of cleaning crucibles Students check-out materials from stockroom and clean crucibles and place in oven Demonstration of reaction and filtering procedures Obtain weight of clean, dry, cool crucibles Prepare Samples Grind antacid tablet Dissolve samples in acid Add ammonium oxalate precipitating agent, indicator, and urea Next Week: Digest sample Filter sample Dry product Weigh resulting precipitate Clean Crucibles Calculate % Ca in samples (individuals, partners, and class)

The Chemistry of this experiment dissolution//acid/base CaCO3 (s) ⇄ Ca2+(aq) + CO32-(aq); CO32-(aq) + 2H+(aq) ⇄ H2CO3(aq) → CO2(g) + H2O(l) dissolution//acid/base base oxalic acid Urea enzyme hydrolysis Very slow reaction; imperceptible at room temperature and neutral pH What do we do to increase rate? Heat and go to acidic conditions (low pH) What does your body do? enzyme: increases rate by 1014 !!!! oxalate acid/base Slow production of NH3 makes reaction slow so ppt is slow in order to make good crystals!!! precipitation

PROCEDURE: Obtain mass of clean, dry crucibles (Change in procedure) Grind tablet Weigh out samples (0.35-0.38 g) to nearest 0.1 mg Dissolve in 100 mL H2O, 6 mL 6M HCl, adjust pH if necessary Add pH indicator Add (NH4)2C2O4 precipitating agent (20 mL sat. sol. + 1 mL 6 M HCl) Add 15 g urea _________________________________________________________________________________ Heat at near boiling until color changes to yellow and precipitate forms Filter hot solutions into crucibles Wash and dry precipitate Weigh precipitate Clean Crucibles Do calculations and turn in report EXPERIMENTAL PROCEDURE 1. Cleaning of Crucibles. (Steps 1 and 2 will be completed in the previous week.) Check out two crucibles and one crucible holder from the stockroom. If your crucibles do not have identifying marks, file one small nick in the rim of one crucible and two nicks in the rim of the other. Oils from your fingers can affect the weight of the crucible. After cleaning, therefore, handle crucibles with tongs or strips of paper towel only. Set up the filtration apparatus as shown in Figure 1. Connect your filter flask to the vacuum line with heavy-walled rubber tubing. Fit the flask with a crucible holder and place one of your crucibles in it. With the vacuum off, fill the crucible with 6 M HCl. Let stand for about 5 minutes. Pull the HCl through the frit with vacuum. Repeat this procedure once more and finish by rinsing the crucible several times under vacuum with de-ionized water. If either of your crucibles appears not to be clean, see your instructor. DO NOT use any other cleaning agent without checking with your instructor.