Composition of Final Precipitate
The quantitative application of precipitation gravimetry, which is based on a conservation of mass, requires that the final precipi- tate have a well-defined composition. Precipitates containing volatile ions or sub- stantial amounts of hydrated water are usually dried at a temperature that is suffi- cient to completely remove the volatile species. For example, one standard gravimetric method for the determination of magnesium involves the precipitation of MgNH4PO4 6H2O. Unfortunately, this precipitate is difficult to dry at lower temperatures without losing an inconsistent amount of hydrated water and ammo- nia. Instead, the precipitate is dried at temperatures above 1000 °C, where it decom- poses to magnesium pyrophosphate, Mg2P2O7.
An additional problem is encountered when the isolated solid is non- stoichiometric. For example, precipitating Mn2+ as Mn(OH)2, followed by heating to produce the oxide, frequently produces a solid with a stoichiometry of MnOx, where x varies between 1 and 2. In this case the nonstoichiometric product results from the formation of a mixture of several oxides that differ in the oxidation state of manganese. Other nonstoichiometric compounds form as a result of lattice de- fects in the crystal structure.