VKC Notes · Analytical chemistry · Geo-Scientist P-III
17.8 Thermal Methods: Thermogravimetry (TG), DTG and DTA, with Ca and Mg Analysis
★★★ 2021–2025: 10 parts, 65 marks, all in Geo-Scientist Paper III, two parts every year. One is a 5-mark theory part (thermobalance, calibration, factors); the other is usually a 5–10-mark mass-loss numerical on a calcium or magnesium compound.
Syllabus line. Geo-Scientist Paper III (A4 Thermal methods of analysis): “Theory of thermogravimetry (TG), basic principle of instrumentation, techniques for quantitative analysis of Ca and Mg compounds.” DTG, DTA and DSC are added here because the questions on furnace atmosphere and endothermic steps need them. The gravimetric side (factors, BaSO₄, Ni–DMG) is in 17.4.
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CSE 2024 Q7(a) = asked there; tap to open the solved answerSN = line kept in the short notes
17.8.1 Thermal methods at a glance ★★
- Thermal analysis: a property of the sample is recorded while its temperature is changed by a fixed programme. In thermogravimetry (TG) the property is mass; the plot of mass against temperature (or time) is the thermogram.SN
- A horizontal plateau means a compound stable over that range; a step means a reaction that gives off a gas. Ti is the lowest temperature at which a mass change is detected, Tf the temperature at which it is complete; Tf − Ti is the reaction interval.SN
| Method | Records | Gives |
|---|---|---|
| TG | mass against T or t | composition from step heights; stability ranges; drying and ignition temperatures |
| DTG | dm/dT against T | one peak per step at the fastest loss; separates overlapping steps |
| DTA | ΔT = Tsample − Treference | every heat effect, with or without mass change (melting, phase change, crystallisation) |
| DSC | heat flow dH/dt | enthalpies and heat capacities, quantitatively |
17.8.2 The thermobalance; scanning and isothermal TG ★★★ asked 2 times
Draw it in 60 s: box “balance” at top; a wire down into a tall rectangle “furnace” with the crucible inside and a thermocouple just under it; gas arrow in at the side; “programmer” box feeding the furnace; “recorder” box from the balance.
- Recording microbalance (null-point electrobalance): a change in mass tilts the beam; a photocell senses it and a current through a coil restores the beam. The restoring current is proportional to the mass change. Sensitivity about 1 µg; sample usually 5–20 mg.SN
- Furnace, ambient to 1000–1600 °C, wound non-inductively (no magnetic pull on the sample) and thermally shielded from the balance; sample holder (Pt, alumina or quartz crucible) hung inside it.SN
- Temperature programmer with a thermocouple close to the sample, giving a linear rate β = dT/dt of 0.5–50 °C min⁻¹ or a fixed temperature; purge-gas control (N₂, Ar, air, O₂; static or flowing); data system plotting TG and DTG Geo 2022 Q1(e) 5m.SN
| Scanning (dynamic) TG | Isothermal (static) TG | |
|---|---|---|
| Temperature | raised linearly at β | held constant |
| Plot | mass against temperature | mass against time |
| Used for | decomposition temperatures, stable intermediates, composition in one run | rates and kinetics, drying to constant mass, long-term stability |
A third mode, quasi-isothermal TG, stops heating whenever mass begins to change and so separates close steps Geo 2021 Q4(c) 5m.SN
17.8.3 Calibration of temperature and mass ★★
- Why: the thermocouple sits near the sample, not in it, so the recorded temperature lags (or leads) the true sample temperature, more so at fast heating Geo 2023 Q1(e) 5m.SN
- Curie-point method: a ferromagnetic standard is weighed with a small magnet outside the furnace, which adds an apparent mass. At its Curie temperature it becomes paramagnetic and a sharp apparent mass step appears at a known temperature: alumel (≈ 163 °C), nickel (354 °C), perkalloy (596 °C), iron (780 °C). Several standards give a correction curve.SN
- Fusible-link method: a small weight hung on a wire of pure metal (In, Sn, Pb, Zn, Al, Ag, Au) drops when the wire melts. Calibration is run at the same β, gas and crucible as the analysis.
- Mass: standard weights; a blank run with an empty crucible gives the buoyancy correction (the hot gas is less dense, so an empty pan seems to gain mass). CaC₂O₄·H₂O, with three known steps, checks the whole instrument.SN
17.8.4 Factors that change the TG curve ★★★ asked 2 times
| Factor | Effect on the curve |
|---|---|
| Heating rate ↑ | Ti and Tf move up, the step widens, close steps merge (the sample lags the furnace) |
| Furnace atmosphere | product gas in the atmosphere pushes a reversible step up (CaCO₃ starts near 600–650 °C in N₂, near 900 °C in CO₂); a reactive gas (O₂, H₂) adds new steps or a mass gain; a flowing purge sweeps products away (earlier, cleaner steps) |
| Sample mass, particle size, packing ↑ | slower heat transfer and slower escape of gas: step starts later and ends higher |
| Heat of reaction | endothermic: sample cools itself, recorded T too high; exothermic: the reverse |
| Crucible, buoyancy, convection | shape and material change heat transfer; apparent mass change with temperature (blank run corrects it) |
Draw it in 60 s: two S-shaped falls to the same 56 % line; the faster-rate curve sits to the right and is wider.
Solved PYQ Geo 2025 Q1(e) 5m
- Higher heating rate: the step moves to higher temperature and spreads over a wider range; in Fig. 17.8.2 it shifts from about 550–760 °C to about 618–869 °C; consecutive steps overlap.SN
- Larger particles: less surface, slower heat transfer and slower escape of the gas: decomposition starts later and ends higher.SN
- Endothermic reaction: the sample absorbs heat and lags behind the furnace temperature, so the recorded decomposition temperature is higher than the true one.SN
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