result, whereas a predominance of particle growth will form a more tractable precipitate. From the
practical standpoint, the difference (Q–S) should be controlled and kept to a minimum. On mixing
reagent and analyte solutions it is difficult to avoid a momentarily high (Q–S) value (in the region
where the two first make contact), especially if S is small. Furthermore, a low value of S is necessary
for quantitative precipitation, so that a situation often arises in which the rate of homogeneous
nucleation vastly exceeds that of particle growth. Consequently, the analyst frequently has to cope with
colloidal precipitates or suspensions. Where S is larger, crystalline precipitates are more readily
obtained. Colloidal suspensions occur as a result of electrical repulsions between particles which
prevent agglomeration. These repulsive forces develop from ions adsorbed onto the surface of the
precipitate which cause the formation of an electrical double layer. The adsorbed layer will contain an
excess of the precipitate ion which predominates in solution whilst the diffuse counter ion layer will
contain an excess of oppositely charged ions to maintain the overall electrical neutrality of the solution.
In the familiar example where Cl– has been precipitated as AgCl by the addition of an excess of silver
nitrate, the adsorbed layer will contain an excess of Ag+, and the counter ion layer an excess of and
OH–.
Adsorption can be diminished by heating or by the addition of a highly charged strong electrolyte. This
allows coagulation of the precipitate to proceed, but it must be remembered that if a filtered precipitate
of this type is being washed the particles can be dispersed again as a colloid and pass through the filter.
This effect, known as peptization, may be prevented by using hot washing solutions containing a
suitable electrolyte. Precipitates formed by colloid agglomeration are amorphous and porous with very
high surface areas. In almost all cases, precipitates are improved by heating them in contact with the
solution before filtration. This is known as digestion and will promote the formation of larger particles
with a reduced surface area, and more ordered arrangements within crystals. Thus both the surface
adsorption and occlusion of impurities will be reduced.
Purity of Precipitates
Steps are normally taken to prevent the simultaneous precipitation of materials other than the desired
analyte species. Incorporation of impurities into the precipitate may however occur by coprecipitation
or post-precipitation. The former arises during the formation of the precipitate, and the latter after it has
been formed. The various modes of coprecipitation are summarized in Table 5.16.
Post-precipitation involves the deposition of a sparingly soluble impurity of similar properties to the
precipitate on the surface of that precipitate after it has been formed. It is particularly a problem where
similar materials are being separated on the basis of their different rates of precipitation, e.g. calcium
and magnesium oxalates or zinc and mercury sulphides. Copreci-