Theory and Practice of Precipitation Gravimetry: Composition of Final Precipitate

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 MnO_x, 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.