The precipitation method is the one used for the determination of the amount of calcium in water. Using this method, an excess of oxalic acid, HCO, is added to a measured, known volume of water. By adding a reagent, here ammonium oxalate, the calcium will precipitate as calcium oxalate. The proper reagent, when added to aqueous solution, will produce highly insoluble precipitates from the positive and negative ions that would otherwise be soluble with their counterparts (equation 1).
The reaction is:
Formation of calcium oxalate:
Ca + CO → CaCO
The precipitate is collected, dried and ignited to high (red) heat which converts it entirely to calcium oxide.
The reaction is pure calcium oxide formed
CaCO → CaO + CO+ CO
The pure precipitate is cooled, then measured by weighing, and the difference in weights before and after reveals the mass of analyte lost, in this case calcium oxide. That number can then be used to calculate the amount, or the percent concentration, of it in the original mix.