Determination Of Calcium As Oxalate

Determination of Calcium as Oxalate. By gravimetric method

Discussion.  The

calcium is precipitated as calcium oxalate CaC2O4.H2O by treating a hot hydrochloric acid solution with ammonium oxalate, and slowly neutralizing with aqueous ammonia solution. The precipitate is washed with dilute ammonium oxalate solution

Weighed in one of the following forms 1. As CaC2O4.H2O by drying at 100-105 OC 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.

2. As CaCO3, by heating at 475-525 OC  In

an electric muffle furnace. • This is the most satisfactory method, since calcium carbonate is non-hygroscopic. CaC2O4(s) CaCO3(s) + CO(g)

3. As CaO by igniting at 1200 OC. • 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(s) CaO(s) + CO2(g)

 Calcium

oxalate monohydrate has a solubility of 0.0067 g and 0.0140 g/L at 25 O and 95 OC respectively.  The solubility is less in neutral solutions containing moderate concentrations of ammonium oxalate owing to the commonion effect hence a dilute solution of ammonium oxalate is employed as thewash 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 clock glass and provided with a stirring rod.  Add l0 mL of water, followed by about 15 mL 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 olution of 2.0 g of ammonium oxalate in 50 mL of water

 Add

to the resultant solution (about 80 OC) filtered dilute ammonia solution (1 : 1) drop wise and with stirring until the mixture is neutral or faintly alkaline (color 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. The subsequent procedure will depend on whether the calcium oxalate is to be weighed as the carbonate or as the oxide.

Weighing as calcium carbonate  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 ('policeman').

 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.  Testing the chloride  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 OC for 1 hour, and then transfer to an electrically heated muffle furnace, maintained at 500 OC for 2 hours.  Cool the crucible and contents in desiccator, and weigh. Further heating at 500 OC should not affect the weight.

 As

a final precaution, moisten the precipitate with a few drops of saturated ammonium carbonate solution, dry at  110 OC, and weigh again.  A gain in weight indicates that some oxide was present; this should not occur. CaO(s) + (NH4)2CO3(aq)

CaCO3(s) + 2NH3(g) + H2O(l)

Weighing as calcium oxide  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 10-15 minutes with a Meker or Fisher hightemperature 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.