Heating to constant mass is a gravimetric technique in analytical chemistry used to determine the mass of an evolved product relative to the initial mass of unreacted material. The technique requires the production of a volatile gas, such as carbon dioxide evolved from the thermal decomposition of calcium carbonate.
The weights of the reactants and products must be proportional and reproducible for the technique to give meaningful information. The formation of products must be irreversible, and the products that remain after heating to constant mass must be thermally stable. Knowing the heat input to the reactants and the time needed to constant mass enables the calorimetric determination of the reaction enthalpy.
Heating to constant mass can also be a drying technique used to dehydrate a chemical compound, such as iron oxide. The thermal decomposition of a metal carbonate to a metal oxide and carbon dioxide can also be used to source this metal oxide.
The material yield after constant mass heating can also give information on the purity of the original reactants. Too much or too little mass can indicate impurities that remained or were evaporated during the heating process, respectively.
Heating to constant mass requires a specialized, sensitive balance that samples the mass of the reactants and products continuously. This analytical balance may also feature built-in heating and stirring elements to facilitate the heating process.