The calcium is precipitated as calcium oxalate CaC2O4,H2O by treating a hot hydrochloric acid solution with ammonium oxalate, and slowly neutralising with aqueous ammonia solution:
Ca2+ +C2O42- +H2O = CaC2O4,H2O
The precipitate is washed with dilute ammonium oxalate solution and then weighed in one of the following forms:
- As CaC2O4,H2O by drying at 100-105 °C for 1-2 hours. This method is not recommended for accurate work, because of the hygroscopic nature of the oxalate and the difficulty of removing the co-precipitated ammonium oxalate at this low temperature. The results are usually 0.5-1 per cent high.
- As CaCO3 by heating at 475-525 °C in an electric muffle furnace. This is the most satisfactory method, since calcium carbonate is non-hygroscopic.
CaC2O4 = CaCO3 + CO
- As CaO by igniting at 1200 °C. This method is widely used, but the resulting calcium oxide has a comparatively small relative molecular mass and is hygroscopic; precautions must therefore be taken to prevent absorption of moisture (and of carbon dioxide).
CaCO3 = CaO + CO2
Calcium oxalate monohydrate has a solubility of 0.0067 g and 0.0140g L-1 at 25° and 95 °C respectively. The solubility is less in neutral solutions containing moderate concentrations of ammonium oxalate owing to the common-ion effect; hence a dilute solution of ammonium oxalate is employed as the wash liquid in the gravimetric determination. Procedure
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...
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