time consuming. Newer instrumental methods may determine several compo-
nents simultaneously, rapidly and are generally applicable down to trace levels.Precipitation As noted in Topic C1, many elements will form compounds insoluble in water
or other solvents. Provided the compound is stable, or may be converted into a
stable compound easily, these insoluble precipitates may be used for analysis.
The technique for obtaining a precipitate may be summarized as follows:
(i) The sample should be dissolved as completely as possible in a suitable
solvent. Any residue that does not dissolve (for example, silica present in
the metal sample of the above example), may be filtered off at this stage.
(ii) Unless there are undesirable changes when the sample is heated, the solu-
tions should be warmed. This speeds up reactions and helps to form a more
granular precipitate.
(iii) The precipitating reagent must be chosen to give as insoluble a precipitate
as possible. Preferably, a reagent that will produce the largest mass of
precipitate should be used. For example, aluminum may be precipitated
and heated to give the oxide, Al 2 O 3 when 10.0 mg of aluminum will
produce 18.9 mg precipitate. If ‘aluminon’ (8-hydroxyquinoline, C 9 H 6 ON) is
used, 170.0 mg of precipitate results.
(iv) The precipitating reagent should be added slowly, with stirring to the
warm solution. To check whether precipitation is complete, the precipitate
is allowed to settle, and more reagent added. If further precipitate does not
form, the reaction is complete. If the solution appears cloudy, it is possible
that a colloidal form of the solid is present. This may be coagulated by
further warming or adding more reagent.
(v) The reaction mixture is filtered. Various means may be used for this. The
simplest is a quantitative filter paper (ashless), which has been dried and
weighed previously. These may be dried, or burnt (‘ashed’).Another useful filter is a sintered glass orporcelain crucible, dried and
weighed as before. Glass will withstand heating to about 300∞C and porcelain to
over 800∞C.Purification The precipitate should be washed to remove traces of solution. This may cause
difficulties, as the washing may redissolve the solid. Using a wash liquid with a
common ionreduces the solubility.
For example, if 0.18 g of a precipitate of lead sulfate is washed with 1 dm^3 of
distilled water, it will dissolve 0.046 g of precipitate or 25%, as the solubility
product is Ksp= 2.3¥ 10 -^8 (mol dm-^3 )^2. If the precipitate is washed with 1 dm^3 of
0.01 M sulfuric acid, then the amount dissolved is much less, 0.7 mg or 0.4%.
Drying can be done in stages. To remove water, the filtered solid in its container
is placed in a desiccatorand left for a few hours. A vacuum desiccatoris even
more efficient for removing solvents at low temperature. Heating in ovens,
furnaces or directly with burners will raise the temperature to remove materials
or to decompose the precipitate to a more stable form. For example, ‘basic
aluminum succinate’ is a good precipitate for aluminum, but must be ignited to
constant weight at about 1200∞C to convert to aluminum oxide.
The stages involved in drying and decomposition can be studied using
thermogravimetry (see Topic G1).Drying and
heating
96 Section C – Analytical reactions in solution