GRAVIMETRIC DETERMINATION OF CALCIUM
The purpose of this experiment is to determine the concentration of Ca2+ ions in a solution of unknown composition.
Calcium ions precipitate with oxalate ions in basic solution to form the hydrate CaC2O4•H2O:
Ca2+(aq) + C2O42-(aq) CaC2O4•H2O
To assure that large, easily filtered crystals form, the precipitation is carried out slowly by dissolving the calcium sample and the oxalate source in acidic solution, then gradually increasing the pH by thermal decomposition of urea, which produces hydroxide ions:
Procedure
Each individual will do one sample of the same unknown solution, and we will combine class results. Put a wash bottle of deionized water in an ice bath to prepare it for step 3.
1. Use a 10 mL volumetric pipet to transfer 20.00 mL of the solution containing an unknown amount of calcium ions into a 250 or 400 mL beaker. Dilute the sample with about 75 mL of 0.1 M HCl, then add 5 drops of methyl red indicator. Methyl red is red below pH 5 and yellow above pH 6.
2. Add about 25 mL of (NH4)2C2O4 solution to the beaker while stirring with a glass rod. Remove the rod, rinsing it so no solution is lost from the beaker, then add about 15 g of solid urea. Cover the beaker with a watch glass and boil gently for about 30 minutes, until the indicator has turned yellow.
3. Filter the hot solution through a weighed filter paper. Use small aliquots of ice-cold water to assist in transferring all of the solid into the filter. After all the precipitate has been transferred, rinse the beaker with two 10–mL aliquots of ice-cold water, adding these rinsings to the precipitate.
4. When the filter has finished draining, place it on a clean dry watch glass and dry it in an oven to constant mass. The product is somewhat hygroscopic (picks up moisture from the air), so at the end of the drying period, transfer it from the oven to a desiccator so that it will not pick up moisture as it cools. When the product has cooled to room temperature, weigh it.
Analysis and Discussion
1. From the mass of CaC2O4•H2O collected, calculate [Ca2+] in the unknown solution and add your answer to the class results. Put a copy of the class results in your discussion.
2. Determine the class average [Ca2+] and the average deviation from the mean (as you did in the first experiment), and decide how many sigfigs are justified by the class results. As your conclusion, report the average [Ca2+] in the unknown with this number of sigfigs.
3. Calculate the % error in your own value, using the class average as the accepted value, and discuss sources of error.