Volatilisation Gravimetry and Thermogravimetry

Introduction

Mass can reveal a volatile component even when no precipitate is collected. Heating a hydrated material may release water, and decomposition can release CO₂. Thermogravimetric analysis, or TGA, continuously records mass while temperature changes, exposing steps that a single before–after weighing would hide. Interpreting each loss requires a chemical equation and controlled atmosphere, not just a curve.

Core explanation

In direct mass-loss gravimetry, weigh a sample, heat it under specified conditions and weigh the residue. Loss = initial mass − final mass. If and only if the released material is known to be entirely the target volatile species, the loss equals its mass. A sample that loses water and carbon dioxide together cannot have its water content inferred from the total loss alone. Some analytes are converted to a gas and trapped in a separate absorber; the absorber's mass gain can then quantify the gas if capture is complete and selective.

TGA plots sample mass or percent of starting mass against temperature or time. A horizontal plateau suggests a stable composition over that interval. A downward step may reflect evaporation, dehydration, decomposition, sublimation or oxidation products escaping. In an oxygen atmosphere, oxidation can also cause mass gain if oxygen becomes part of the remaining solid. Therefore a downward step is not automatically “water loss,” and a flat region does not prove a pure compound. A derivative curve, mass-loss rate versus temperature, can help separate overlapping events but does not identify gases by itself.

For a hydrate, CuSO₄·5H₂O → CuSO₄ + 5H₂O is an idealised overall dehydration equation. Theoretical water mass fraction is 5M(H₂O)/M(CuSO₄·5H₂O). A measured loss near that fraction supports but does not alone prove the formula. Some hydrates lose water in stages; an intermediate plateau can correspond to a lower hydrate. The final anhydrous material may decompose if heating continues too far.

For carbonate, CaCO₃ → CaO + CO₂ on heating under suitable conditions. A sample mass loss of 44.01 g per 100.09 g pure CaCO₃ corresponds to CO₂ leaving. The actual decomposition temperature and degree of completion depend on atmosphere, CO₂ partial pressure, heating rate and particle size. If residue absorbs moisture or CO₂ during cooling, the before–after result can be biased. A desiccator and specified handling help.

TGA extends the method by following the mass path rather than just two endpoints, helping choose a stable weighing temperature and identify when multiple changes occur.

Step-by-step reasoning

1. State which volatile species or residue is the measurand. 2. Write expected decomposition or volatilisation equations and theoretical mass changes. 3. Choose atmosphere, heating rate and final temperature suited to selective conversion. 4. Record initial and final mass, or inspect TGA steps and plateaus. 5. Compare observed fraction with stoichiometry while checking for overlapping events or reabsorption.

Visual explanation

Draw a TGA trace beginning at 100% mass, descending to an intermediate plateau, then descending again to a final plateau. Label the first step “possible dehydration” and the second “possible decomposition,” with a question mark until products are identified. Beside it show a simple before–after balance calculation and note that it would combine both losses.

Real-world analogy

A luggage scale before and after a journey tells how much mass disappeared but not what object was removed. Watching the luggage mass continuously as items are taken out reveals separate steps, but you still need to know which item left at each step. TGA supplies the timing and magnitude of mass changes; chemistry supplies their identities.

Real-world example

A laboratory compares a mineral powder's TGA curve in nitrogen and in air. A loss in both atmospheres might be dehydration or carbonate decomposition, while an additional mass gain in air could suggest oxidation. The difference helps develop a model, but confirming evolved gases may require a coupled detector or independent test.

Why?

Why specify the heating atmosphere? Oxygen can react with the sample, and CO₂ partial pressure can shift carbonate decomposition. The same material may show different onset temperatures and net mass changes in air, nitrogen or a CO₂-rich stream. A formula inferred from one atmosphere may not transfer unchanged to another.

Common misconception

“All mass lost on heating is water” is false. CO₂, solvents or decomposition fragments may escape. Another mistake is treating the first flat portion of a TGA trace as proof that no chemical reaction occurred; reactions with no net mass change or very slow processes can be invisible to the balance.

Worked example

Pure CaCO₃, 1.000 g, fully decomposes to CaO and CO₂. Expected CO₂ loss is 1.000 × 44.01/100.09 ≈ 0.4397 g, leaving about 0.5603 g CaO. If measured residue is 0.6000 g, incomplete decomposition, impurity or reabsorption could explain the difference; one should not declare the starting sample 100% CaCO₃ from this mass alone.

Quick check

1. A TGA trace shows a 10% mass loss. Can you immediately call the sample 10% water? Answer: No. The lost species may be water, CO₂, solvent or another volatile product. Identification requires chemical context, controlled conditions and possibly gas analysis.

Exam focus

Calculate theoretical mass fractions from balanced equations and identify what the final weighing form is. Explain how TGA steps differ from one before–after mass difference. Mention atmosphere, heating rate and overlap when interpreting a curve, and use a desiccator to limit post-heating reabsorption.

Advanced insight

Coupling TGA to mass spectrometry or infrared gas analysis can identify evolved molecules and strengthen assignment of overlapping losses. A temperature plateau and matching mass fraction remain useful evidence, but multiple independent observations give a more defensible chemical interpretation of a complex sample.

Summary

Volatilisation gravimetry infers a component from controlled mass loss or captured gas mass. TGA follows mass continuously as temperature changes, revealing steps and plateaus. Neither a loss nor a plateau identifies a compound alone; balanced chemistry, atmosphere and product verification make the mass analytically meaningful.

Practice questions

1. What fraction of pure CaCO₃ mass is lost as CO₂ on complete decomposition? Answer: 44.01/100.09 ≈ 0.4397, or about 43.97% of initial pure CaCO₃ mass.

2. Why might a TGA sample gain mass in air? Answer: Oxygen may react with and become incorporated into the solid, so oxidation can increase residue mass even during heating.

3. What does a mass plateau indicate, and what does it not prove? Answer: It indicates little measurable mass change over that interval. It does not by itself prove chemical purity or rule out reactions that have little net mass effect.