Principles General qualities for precipitation reagents: • precipitate is
nonhygroscopic such that its weight is independent of
humidity. • product has a high
molecular weight, to facilitate measurements of small quantities of the analyte. • precipitation is selective for the ion of interest. In terms of disadvantages, gravimetric analysis usually only provides for the analysis of one of a few elements. Methods can be convoluted. Challenges to precipitations are related to
impurities in the solid, which can be caused by occlusions or surface
adsorption of other ions. Some problems can be averted using
homogeneous precipitation, which is the formation of the precipitate from a single
homogeneous solution, as in the case of
barium sulfate. The
insolubility of precipitates can be affected by other ions in the solution. The solubility of silver chloride (AgCl; Ksp = 1.0 × 10−10 in 0.1 M
NaNO3) can increase by many orders of magnitude in the presence of other anions.
Case studies Potassium Potassium (K) can be quantified using
hexachloroplatinic acid as the precipitating agent. Treatment of a solution containing K+ ions with an excess of this
chloroplatinic acid quantitatively affords of
potassium hexachloroplatinate, which is easily weighed and is non-hygroscopic: : A similar analytical procedure yields a precipitate of
potassium tetraphenylborate from
sodium tetraphenylborate.
Phosphate |thumb|right|150px Addition of aqueous solution of ammonium molybdate to a solution containing hydrogen phosphate gives a precipitate of
ammonium phosphomolybdate.
Complexation: nickel, aluminium Several applications of gravimetric analysis employ organic
ligands that generate precipitates with a specific
metal ion. A solution of
nickel ions is treated with a > 2 equivalents of
dimethylglyoxime to give a bright red precipitate of
nickel bis(dimethylglyoximate). In a similar approach, a solution of
aluminium ions is treated with
8-hydroxyquinoline to give
aluminium tris(8-Hydroxyquinolinate).
Barium Barium sulfate is highly insoluble in water. Using homogeneous precipitation, a sample solution containing
barium ions is treated with an excess of
sulfamic acid. This solution is heated to induce hydrolysis of sulfamic acid to
bisulfate: : The bisulfate readily reacts with barium ions to give the sulfate: : ==Volatilization methods==