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 Whatman No. 40 or 540 filter paper, transfer the precipitate to the filter, and wash with a cold 0.1 -0.2 per cent ammonium oxalate solution until free from chloride. Transfer the moist precipitate to a previously ignited and weighed platinum crucible, and ignite gently at first over a Bunsen flame and finally for 10-15 minutes with a Meker or Fisher high-temperature burner until two successive weighings do not differ by more than 0.0003 g. The covered crucible and contents are placed in a desiccator containing pure concentrated sulphuric acid or phosphorus(V) pentoxide (but not calcium chloride), and weighed as soon as cold.