Table 5.17 Practical gravimetric procedures
Stage Practical manipulations Remarks
sample pre-treatment dissolution of sample, separation or masking of
interfering ionsprevents simultaneous precipitation
and reduces inclusion of impurities
precipitation from hot dilute solution, careful addition or
homogeneous generation of precipitating agent in small
excess, with stirring: digestionpromotes particle growth and reduces
occlusion. N.B. digestion can increase
post-precipitation
filtration cooled solution filtered, precipitate washed with dilute
electrolyte solutiondecreases solubility, reduces adsorbed
impurities and prevents peptization
drying and ignition careful drying at 110– 140 °C access of air during
ignitionprevents sputtering losses, ensures
complete oxidation to a stoichiometric
product
weighing weighings carried out to nearest 0.1 mg reheating and
reweighing until constant weight is obtained; samples
stored in a desiccatorspecial care required for hygroscopic
solidsMagnesium may be precipitated from solution as MgNH 4 PO 4. 6H 2 O, a compound which has been
widely used as a basis for the gravimetric determination of the element over many years. An
examination of its use will serve to illustrate some of the problems associated with inorganic
precipitations. The initial precipitation is made from a solution at pH = 11–12 by the addition of
ammonium phosphate in excess.
Some of the problems encountered in the analysis are:
(a)—
Precipitate Stoichiometry
The precipitate is not stoichiometric and it is necessary to convert it to the pyrophosphate by ignition
(1100°C) to obtain the most precise results.
(b)—
Solubility
For a precipitate used in gravimetric analysis magnesium ammonium phosphate has a rather high
solubility (0.1 g dm–^3 in water at 20°C). Hence solution volumes should be kept as low as possible and
the precipitate must be filtered from cold solution. Furthermore, the solution composition must be
carefully controlled to ensure the maintenance of conditions of