CHEMISTRY 221 LABORATORY—QUANTITATIVE ANALYSIS
EXPERIMENT: Gravimetric Determination of Calcium as Calcium Oxalate
Soluble calcium can be quantitatively analyzed by precipitation as calcium
oxalate monohydrate CaC2O4•H2O. Before
you come to class, calculate the formula weight of calcium oxalate monohydrate
CaC2O4•H2O. The analytical reaction is
Ca2+ + C2O42- + H2O CaC2O4•H2O
Note that oxalate, C2O42- is the salt of the hydrolysis of the weak second proton of oxalic acid, H2C2O4.
H2C2O4 H+ + HC2O4- pKa1 = 1.25
HC2O4- H+ + C2O42- pKa2 = 4.27
Very little precipitate will form in acidic solutions since the equilibrium would favor the formation of oxalic acid. Large, easily filtered, relatively pure crystals can be obtained if the precipitation is performed homogeneously by slowly raising the pH. This is accomplished by adding urea to the solution and slowly decomposing the urea by heating the solution on a hot plate.
(H2N)2CO + 3 H2O + heat CO2 + 2 NH4+ + 2 OH-
|Reagents:||ammonium oxalate solution||0.1 M HCl|
|methyl red indicator||urea|
Summary Procedure: Make sure you read sections 19-1 through 19-3 of your text before coming to lab. Read the detailed experimental procedure starting on page 389 in the Harris text.
Perform the analysis in duplicate.
Dry two medium-porosity, sintered-glass funnels for 1-2 hours at 105 °C; cool them in a desiccator for 15 minutes and weigh them. Repeat the procedure with 30-minute heating periods until successive weighings of a funnel agree to within ±0.3 mg.
Accurately weigh out approximately 0.35-0.38 g of your unknown into a 400 mL beaker.
Dissolve the unknown in a minimal amount of 0.1 M HCl, avoiding splattering. Start with about 50 mL. All of the calcium unknown should dissolve with effervescence. If necessary, add 10-20 mL of 0.1 M HCl to complete the dissolution of the calcium carbonate. Some samples may have a trace of fine insoluble silica (sand) that appears transparent upon close observation.
After dissolution add enough distilled water so that you have a total solution volume of approximately 100 mL.
Add 5 drops of methyl red indicator to each beaker.
Add 25 mL of the ammonium oxalate solution to each sample while stirring with a glass rod. Add 15 g of solid urea to each sample, cover with a watch glass, and boil gently for approximately 30 minutes or until the indicator turns yellow (the yellow color may be hard to see but the pink color will have disappeared.)
Filter each hot solution through a weighed funnel at the vacuum station (be sure to empty the filtrate in the filter flask before it overflows into the vacuum system.) Rinse each beaker with a few mL of cold distilled water (place your wash bottle in a tray full of ice to obtain the cold rinse water.) Make sure that you quantitatively transfer your precipitate into the funnel (any losses will be reflected in the accuracy of your result.)
Dry the precipitate, first by drawing air through the precipitate for about 1 minute, and then in an oven at 105°C for at least an hour (you may wish to dry the sample overnight.)
After obtaining your final weights, clean out the precipitate and put your filter crucibles in the acid soak in the hood so that they will be available for the next laboratory section.
Report the result in terms of the percentage of calcium oxide (%CaO) in your unknown. The range on unknown values should be between 35% to 60%.
Calculate the percentage of calcium oxide in your unknown at this point:
(note that "F" on the label indicates a "fine" porosity frit
SAVE YOUR UNKNOWN FOR THE NEXT EXPERIMENT
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