Weigh out accurately sufficient of
the sample to contain 0.2 g of calcium into a 500mL beaker covered with a
clockglass and provided with a stirring rod. Add 10 mL of water, followed by
about 15mL of dilute hydrochloric acid (1:1). Heat the mixture until the solid
has dissolved, and boil gently for several minutes in order to expel carbon
dioxide. Rinse down the sides of the beaker and the clockglass, and dilute to
200 mL: add 2 drops of methyl red indicator. Heat the solution to boiling, and
add very slowly a warm solution of 2.0 g of ammonium oxalate in 50 mL of water.
Add to the resultant hot solution (about 80 °C) filtered dilute ammonia
solution (1:1) dropwise and with stirring until the mixture is neutral or
faintly alkaline (colour change from red to yellow). Allow the solution to
stand without further heating for at least an hour. After the precipitate has
settled, test the solution for complete precipitation with a few drops of
ammonium oxalate solution.
Decant the clear supernatant liquid through a weighed silica or porcelain filtering crucible. Transfer the precipitate to the crucible with a jet of water from the wash bottle; any precipitate adhering to the beaker or to the stirring rod is transferred with the aid of a rubber-tipped rod. Wash the precipitate with a cold, very dilute, ammonium oxalate solution (0.1-0.2 per cent) at least five times, or until the washings give no test for chloride ion (add dilute nitric acid and a few drops of silver nitrate solution to 5 mL of the washings). Dry the precipitate in the steam oven or at 100-120 °C for 1 hour, and then transfer to an electrically heated muffle furnace, maintained at 500±25 °C for 2 hours. Cool the crucible and contents in a desiccator, and weigh. Further heating at 500 °C should not affect the weight. As a final precaution, moisten the precipitate with a few drops of saturated ammonium carbonate solution, dry at 110 °C, and weigh again. A gain in weight indicates that some oxide was present; this should not occur.
CaO + (NH4)2CO3 = CaCO3 + 2NH3 + H2O