determination of calcium as oxalate

8/14/2019 Determination of Calcium as Oxalate

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Determination of Calcium asOxalate.

By gravimetric method


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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 diluteammonium oxalate solution


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Weighed in one of the following forms

1. As CaC2O4.H2O by drying at 100-105OC

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. Theresults are usually 0.5-1 per cent high.


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

In an electric muffle furnace. This is the most satisfactory method, since

calcium carbonate is non-hygroscopic.

CaC2O4(s) CaCO3(s) + CO(g)


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3. As CaO by igniting at 1200 OC.

This method is widely used, but theresulting calcium oxide has a

comparatively small relative molecular

mass and is hygroscopic; precautionsmust therefore be taken to prevent

absorption of moisture (and of carbon

dioxide).

CaCO3(s) CaO(s) + CO2(g)


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Calcium oxalate monohydrate has a

solubility of 0.0067 g and 0.0140 g/L at25 O and 95 OC respectively.

The solubility is less in neutral solutions

containing moderate concentrations ofammonium oxalate owing to the common-

ion effect hence a dilute solution of

ammonium oxalate is employed as

thewash liquid in the gravimetric

determination.


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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).


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Heat the mixture until the solid has

dissolved, and boil gently for severalminutes 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


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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.


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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.


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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 arubber-tipped rod ('policeman').


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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.


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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 at500 OC should not affect the weight.


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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)


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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 centammonium oxalate solution until free from

chloride.


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Transfer the moist precipitate to a

previously ignited and weighed 10-15

minutes with a Meker or Fisher high-temperature burner until two successive

weighings do not differ by more than

0.0003 g.


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The covered crucible and contents are

placed in a desiccator containing pure

concentrated sulphuric acid or

phosphorus(V) pentoxide (but not calciumchloride), and weighed as soon as cold.

determination of calcium as oxalate

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