Ammonium oxalate is added to a calcium carbonate solution to precipitate calcium oxalate, which is insoluble in water. This is a common method for removing calcium ions from a solution. The reaction between ammonium oxalate and calcium carbonate can be represented by the following equation:

(NH4)2C2O4 + CaCO3 → CaC2O4 + 2NH4+ + CO2

The calcium oxalate precipitate can then be filtered out of the solution, leaving behind a calcium-free solution. This process is often used in water treatment plants to remove calcium hardness from water.