Explain the principle of TGA.

Thermogravimetry is the technique in which change in the weight is recorded as the function of temperature Or time. The phenomenon is used for qualitative as well as quantitative analysis.

Only solid substances can be analyzed by this method.

In TGA following changes are possible: $$1)Reactants(S)\stackrel{\mathrm{\Delta }}{⟶}product(S)+gas$$ $$2)Reactants(S)+gas\stackrel{\mathrm{\Delta }}{⟶}product(S)$$

The second type of changes usually we do not perform in the laboratory. The first type of changes involves loss in the mass or weight. This loss may be due to gas which is obtained by decomposition of reactants on heating or this loss may be due to H₂O present with the reactants. This water is essential water. The essential water is water of crystallization or it is water of constituent’s.

e.g. i) The loss due to gas is $$CaC{O}_3\stackrel{\mathrm{\Delta }}{⟶}CaO+C{O}_2$$

ii) The loss due to water of crystallization is $$CuS{O}_4.5H_{2}O\stackrel{\mathrm{\Delta }}{⟶}CuS{O}_4.H_{2}O+4H_{2}O$$

iii) The loss of water due to constituent is $$2Fe{\left(OH\right)}_3\stackrel{\mathrm{\Delta }}{⟶}{Fe}_2{O}_3\left(S\right)+3H_{2}O$$

The graph is plotted mass Vs temperature or time. The graph thus obtained is known as Thermogram. Thermogram is characteristic curve for a given compound. In this graph the horizontal portion i.e. plateau indicate regions where there is no change in the weight while curved portion indicates the loss in the mass. The heating temperature varies from Ambient to 1500°C.