Table 5.16 The major types of coprecipitation and their relation to the precipitate type
Type of coprecipitation Mode of contamination Precipitate type most affected
isomorphic inclusion substitution of the precipitate lattice with impurity ions
of similar crystallinitycrystallinenon-isomorphic inclusion solid solution of the impurity within the precipitate crystalline
occlusion physical trapping of impurities within precipitate
particlescrystalline and colloidalsurface adsorption chemisorption of impurities from the solution onto the
precipitate surfacecolloidalpitation can be reduced by the normal practical procedures of precipitation from hot dilute analyte
solutions, followed by digestion of the precipitate. Inclusion phenomena when they occur are very
difficult to overcome and it may be necessary to remove the impurity ion before precipitation. At the
same time the degree of post-precipitation will be increased by digestion and where this form of
contamination is paramount rapid filtration is essential. The extent to which coprecipitation may affect
an analysis is well illustrated by the familiar precipitation of barium sulphate. Substances that may
appear as impurities in this precipitate include sulphuric acid, alkali metal sulphates, ammonium
sulphate, iron(III) sulphate, barium phosphate, barium carbonate, barium chloride, barium nitrate and
barium chlorate. The excellent precision and accuracy often claimed for analyses based on this
precipitate undoubtedly result from a compensation of errors.
Practical Gravimetric Procedures
A typical gravimetric analysis procedure may be divided into five stages: sample pretreatment;
precipitation; filtration; drying and ignition; weighing. The operations involved in the various stages are
summarized by Table 5.17. Two aspects, however, need slight amplification. Firstly, the possible
generation of the precipitating agent within the solution in a homogeneous precipitation procedure
should be considered. This method, in which the precipitation is generated within the solution, has the
advantage of preventing local high concentrations of reagent and thus promoting particle growth as well
as minimizing the occlusion of impurities. Some homogeneous precipitation reactions are shown in
Table 5.18. Secondly, the filtration method must be selected to fit the treatment of the precipitate. Where
the material is merely to be dried and weighed, a sintered glass crucible is generally the most
satisfactory. If an ignition step is to be used, however, the precipitate may be collected on a filter paper
and transferred to a silica or platinum crucible for ignition or filtered on an asbestos pad in a Gooch
crucible. Sintered silica crucibles are also used for these precipitates.