Gravimetric Determination of Calcium

When a hot solution of a calcium salt in HCl is treated with ammonium oxalate, calcium oxalate monohydrate is precipitated. The small solubility of the precipitate in water is reduced in ammonium oxalate solutions (common ion effect). The precipitated calcium oxalate is washed, dried and weighed. However, because of its hygroscopic nature and the possibility of coprecipitation, it is preferable to convert it to the carbonate in a muffle furnace. 

Prepare a solution of 5 g purest calcium carbonate in dilute HCl. Heat to dissolve the powder and then boil to expel carbon dioxide, cool and make up to 250 cm . Pipette 25.0 cm of the solution into a 400 cm beaker, add 175 cm water and a couple of drops of methyl red indicator. Heat so that the solution boils gently and then add slowly a warm solution of 2 g purest ammonium oxalate in 50 cm solution. Ensure that the temperature of the suspension is 80°C and then add, dropwise and with constant stirring, a filtered 1 1 ammonia solution until the colour changes to yellow. Allow to cool, after covering with a watch glass and stand for an hour. When the precipitate settles, check for complete precipitation with a little oxalate solution. Filter through a prepared sintered glass crucible. Dry at 105°C to constant mass. 

Repeat using a sintered porcelain crucible instead of the glass crucible. Follow the procedure as above until the precipitate settles after standing for an hour. Decant the supernatant into the crucible. Transfer the precipitate with a jet of water and with the aid 

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