Experiment 8
Gravimetric Determination of Calcium as CaC2O4·H2O
Objective
To precipitate Ca as CaC2O4·H2O.
Introduction
In this experiment you will measure the calcium content of an unknown using a gravimetric
method of analysis that involves a homogeneous precipitation. Calcium ion can be analyzed by
precipitation with oxalate in basic solution to form CaC2O4·H2O. The precipitate is soluble in
acidic solution because the oxalate anion is a weak base. Large, easily filtered, relatively pure
crystals of product will be obtained if the precipitation is carried out slowly. This can be done by
dissolving Ca2+ and C2O42- in acidic solution and gradually raising the pH by thermal
decomposition of urea.
Reagents
Ammonium oxalate solution: Make 1 L of solution containing 40 g of (NH4)2C2O4 plus 25 mL
of 12 M HCl. Each student will need 80 mL of this solution.
Unknowns: The unknown will be in the form of a solid as limestone, or calcium carbonate
(CaCO3).
Procedure
1. Dry three medium-porosity, sintered-glass funnels for 1–2 h at 105°C; cool them in a
desiccator for 20 min and weigh them. Repeat the procedure with 30-min heating period until
successive weighing agree to within 3 mg. Use a paper towel, not your fingers, to handle the
funnels. An alternative method of drying the crucible and precipitate is with a microwave
oven.
2. You will be using solid unknown. Accordingly, you will need to transfer a known amount
(1.2 to 1.5 g) of your dried unknown (place in a 105oC oven for 1 hour) to a 250 mL beaker
and dissolve in an acid solution. Do this by adding roughly 30 mL of deionized water to the
beaker, followed by 12 ml of 3 M HCl. Stir with a glass stirring rod for several minutes. If
the material does not dissolve, heat gently (avoid splattering). If some material still does not
dissolve (most likely silicates), filter sample through a #2 filter paper and washing 3 times
each with ~10 ml of hot water. Once all the solid has dissolved (and filtered out, if
necessary), let the solution cool off to room temperature before quantitatively transferring it
to a 100 mL volumetric flask A quantitative transfer involves a minimum of three washing of
the beaker with deionized water, so be sure you allow room in your flask to fill to the
graduation mark.
3. Use a few small portions of unknown to rinse a 25-mL transfer pipet, and discard the
washings. Use a rubber bulb, not your mouth, to provide suction. Transfer exactly 25.00 mL
(or the volume that you calibrated your pipette to be) of unknown to each of three 250- to
400-mL beakers, and dilute each with ~75 mL of 0.1 M HCl. Add 5 drops of methyl red
indicator solution to each beaker. This indicator is red below pH 4.8 and yellow above pH
6.0.
2. 4
3.
4. Add ~25 mL of ammonium oxalate solution to each beaker while stirring with a glass rod.
Remove the rod and rinse it into the beaker. Add ~15 g of solid urea to each sample, cover it
with a watch-glass, and boil gently for ~30 min until the indicator turns yellow. Place your
wash bottle in ice for the next step.
Note: This is a good place to stop if you don’t have enough time to proceed to the end of the lab
(~2.5-3 hrs). You have to be able to dry and weigh your precipitate the same day.
4. 5Filter each hot solution through a weighed funnel, using suction. Add ~3 mL of ice-cold
water to the beaker, and use a rubber policeman to help transfer the remaining solid to the
funnel. Repeat this procedure with small portions of ice-cold water until all of the precipitate
has been transferred. Finally, use two 10-mL portions of ice-cold water to rinse each beaker,
and pour the washing over the precipitate.
5. 6Dry the precipitate, first with aspirator suction for 1 min, then in an oven at 105°C for 1 h.
Bring each filter to constant mass, as done in (1) above. The product is somewhat
hygroscopic, so only one filter at a time should be removed from the desiccator, and
weighing should be done rapidly. Alternatively, the precipitate can be dried in a microwave
oven once for 4 min, followed by several 2-min periods, with cooling for 15 min before
weighing. The water of crystallization is not lost.
Data Analysis
Calculate the weight percent of Ca in the unknown solid for each of the three trials. Remember to
use your calibrated values for your 25-ml pipette and your 100-ml volumetric flask in your
calculations. Also, be sure to take into account that the formula of the precipitate is
CaC2O4•H2O, i.e., each "molecule" of calcium oxalate has one molecule of water associated with
it. For all three weigh percent values, report the mean, the standard deviation and the relative
standard deviation using manual and Microsoft Excel. Discuss the relative standard deviation
obtained.
