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Thermogravimetry of calcium oxalate

Heat calcium oxalate monohydrate on a thermobalance, identify the gases from its three mass-loss steps, and use peak temperatures at several heating rates to find the activation energy of each step.

Difficulty ●●○ ≈ 30 min

Aim Establish the decomposition products of calcium oxalate monohydrate from the step heights, and obtain activation energies from DTG peak temperatures at several heating rates with the Kissinger equation.
TG / DTG computed from stoichiometry and kinetics · checked against NETZSCH measurements

Thermobalance: heating calcium oxalate

3.235 mg1100 °C

Furnace 1100 °C · mass 3.235 mg · 107.5 min elapsed

Thermogravimetric curve (TG)
2004006008001000Temperature / °C406080100Mass / %
Derivative thermogravimetry (DTG): rate of mass change
2004006008001000Temperature / °C−5−4−3−2−10dm/dt / (% min⁻¹)
  • model
  • NETZSCH measured (10 K/min)
Step analysis
StepDTG peakLossLost per moleGas
dehydration171 °C12.33 % · 1.039 mg18.0 gHX2O\ce{H2O}
loss of CO504 °C19.17 % · 1.616 mg28.0 gCO\ce{CO}
loss of CO₂740 °C30.12 % · 2.539 mg44.0 gCOX2\ce{CO2}
Kissinger plot: ln(β/Tₚ²) against 1000/Tₚ
0.811.21.41.61.822.21000 K / Tₚ−11−10−9−8−7ln(β / Tₚ²)
  • dehydration: E = 75 kJ/mol
  • loss of CO: E = 291 kJ/mol
  • loss of CO₂: E = 197 kJ/mol
  • NETZSCH · your recorded runs
Step heights come from stoichiometry (H₂O 12.33 %, CO 19.17 %, CO₂ 30.12 %, leaving CaO at 38.38 %). Each step is integrated as a first-order reaction dα/dt = A e^(−E/RT)(1 − α), with E and A from the Kissinger equation applied to the DTG peak temperatures measured at 10 and 200 K/min in NETZSCH Application Note 016, so the peaks at those two rates match the measurements; other rates are the model’s interpolation and extrapolation. First-order peaks are broader than those of the real solid-state reactions: at 10 K/min the dehydration peak is about 0.65 times as high as measured. The furnace colour is computed as black-body radiation and is only illustrative.

Procedure

  1. Set the heating rate to 10 K/min and the sample mass to about 8 mg (calcium oxalate monohydrate is the standard substance for checking thermobalances).
  2. Press “Start heating” and follow the mass and the DTG curve as the furnace climbs to 1100 °C.
  3. Read each step’s mass loss, convert it to mass lost per mole of sample, and identify the three gases and the final residue.
  4. Set the heating rate to 3, 30, 100 and 200 K/min in turn, press “Show the whole run” and then “Record this run’s peak temperatures” each time.
  5. On the Kissinger plot, compare your points with the two NETZSCH measurements (hollow circles) and read each step’s activation energy from the slope of its line.

What to notice

  • At 10 K/min the three DTG peaks sit near 171, 504 and 740 °C, losing 12.3, 19.2 and 30.1 %, and 38.4 % remains as CaO.
  • The faster the heating, the higher the steps move — to 239, 557 and 875 °C at 200 K/min — while their heights stay the same.
  • On the Kissinger plot each step’s points fall on a straight line: about 75 kJ/mol for dehydration, 291 kJ/mol for the loss of CO and 197 kJ/mol for the carbonate.

In a real lab

  • Use a few milligrams spread thinly in an alumina crucible, and run the empty crucible first as a blank to subtract the apparent mass gain caused by the changing buoyancy of the purge gas.
  • Temperature is calibrated with Curie-point standards: ferromagnetic metals lose their magnetism at known temperatures, and held in a magnetic field their apparent mass jumps there.
  • The second step releases carbon monoxide, which is toxic: the instrument’s exhaust must be vented.

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