Gravimetric Determination of Calcium as CaC2O4.H2O
SCHA 265 Modified 1/3/2007
Since the determination of weight is one of the most accurate and precise measurements
one can carry out in an analytical lab, gravimetric analysis is an excellent option in the
assay of a variety of analytes. One takes a sample through a series of quantitative
chemical reactions and hopefully ends up with a pure final product with exactly known
composition. From this final mass one can determine the number of moles of a final
analyte and thus know the number of moles started with. From this percent composition
can be determined.
Calcium ion can be analyzed by precipitation by with oxalate in a basic solution to form
CaC2O4.H2O. The calcium is soluble in acidic solution because the oxalate anion is a
weak base (and therefore protonated at acidic pH). Large, easily filtered, relatively pure
crystals of product will be obtained if the precipitation is carried out slowly. Urea is
added to the mixture and the solution is made less acidic slowly by heating of the
solution. Urea decomposes to produce OH- which then deprotonates the oxalate. This
ionic form of the ligand then combines to the calcium ion to form the precipitate. (See
Reaction 27-2 and related information in the textbook).
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. (This will be prepared
for you).
Unknown Preparation: Prepare 100.0 mL of solution of your unknown. Place 1.2 to 1.5
grams of your CaCO3 unknown into ~75 mL of distilled water and add about 3.8 mL of
12 M HCl. Boil your solution gently (hot plate) to complete the dissolution (there might
be some insoluble matter left) until bubbles no longer form. Cool the solution, filter into
a 100.0 mL volumetric and then fill the flask to the mark.
PROCEDURE
1. Dry three medium-porosity, sintered-glass funnels for 1–2 h at 105° C; cool them in a
desiccator for 30 min and weigh them. Repeat the procedure with 30-min heating periods
until successive weighings agree to within 0.3 mg.
2. 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 of unknown to each of three 400-mL beakers, and dilute each with ~75 mL of 0.1 M
HCl. Add 5 drops of methyl red indicator solution (Table 11-4 in the textbook) to each
beaker. This indicator is red below pH 4.8 and yellow above pH 6.0.
3. 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 watchglass, and boil gently for ~30 min until the indicator turns yellow.
4. Filter each hot solution through a weighed funnel, using suction (Figure 2-15 in the
textbook). 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 washings over the
precipitate.
5. Dry the precipitate, first with aspirator suction for 1 min, then in an oven at 105° C for
1-2 h. Bring each filter to constant mass. The product is somewhat hygroscopic, so only
one filter at a time should be removed from the desiccator, and weighings should be done
rapidly. The water of crystallization is not lost.
6. Calculate the weight percent of CaO in the unknown solid. Report the standard
deviation and relative standard deviation (RSD) (s / x = standard deviation/average).
Questions to address in the report. (Answer as a separate section in your report)
1.
2.
3.
Why are big crystals important?
What is the name of the process of generating the precipitation reagent in a
chemical reaction?
Why is it important that you know that the final product is a monohydrate?
1. C. H. Hendrickson and P. R. Robinson, J. Chem. Ed. 1979, 56, 341.
2. Thorn Smith Inc., 7755 Narrow Gauge Road, Beulah, MI 49617. Phone: 231-882-4672;
e-mail: www.thornsmithlabs.com.
3. R. Q. Thompson and M. Ghadiali, J. Chem. Ed. 1993, 70, 170.