Mixture of calcium and magnesium oxalates:

Calcium oxalate monohydrate is unsatisfactory weighing form for determining calcium as oxalate because its tendency to retain excess moisture even at 110 °C, co-precipitated ammonium oxalate leftovers un-decomposed. A anhydrous calcium oxalate is hygroscopic other than CaCO₃   is excellent weighing form if heated to 500 ± 25 °C as can be seen in TG curve of CaC₂O₄.H₂O. Above 635 ⁰C the decomposition of CaCO₃ commences to become significant under usual laboratory conditions and completely converted to CaO at 900 °C. Therefore, calcium oxalate monohydrate decomposes within three steps, first dehydration, second removal of CO and formation of CaCO₃ then third step CaCO₃ decomposes to CaO.

CaC₂O₄.H₂O → CaC₂O₄ + H₂O       at about 135-220 °C

CaC₂O₄ → CaCO₃+CO        at about 400-480 °C

CaCO₃ →CaO+ CO₂            at about 635-900 °C

But magnesium oxalate dihydrate decomposes in two steps alternatively of three steps. Initial, dehydration then removal of CO and CO₂ concurrently and forming MgO, there is no horizontal corresponding to MgCO₃ as it is thermally unstable at this temperature.

MgC₂O₄.2H₂O → MgC₂O₄+2H₂O              at about 135-220 °C

MgC₂O₄ → MgCO₃+CO→MgO+CO₂       at about 500 °C

The TG curve for the combination of CaC₂O₄.H₂O and MgC₂O₄.2H₂O display two mass losses up to 500 °C, first at about 200 °C because of loss of water and the second mass loss occurs in the 390-500 °C range that is because of decomposition of both calcium and magnesium oxalates. Therefore, at 500°C, the composition of the combination will be CaCO₃ and MgO. Third mass loss after 500 °C is because of only the decomposition of CaCO₃. If m₁ and m₂ are the mass of the combination at 500 °C and 900 °C then same to the previous instance we could calculate the amount of calcium and magnesium in the original sample.

Figure: DTG/TG curve of calcium oxalate