COPRECIPITATION
Co precipitation is used for simultaneous precipitation of more than one component. Catalysts based on more than one component can be prepared easily by co-precipitation.
Precipitates separating from a solution are not as a rule pure but contain larger amount of foreign substances including mother liquor. The precipitates may contain varying amounts of impurities depending upon the nature of the precipitate and the conditions of precipitation. The contamination of the precipitate by substances that are normally soluble under the conditions of precipitation is called coprecipitation. Contamination as a result of coprecipitation should be distinguished from contamination as a result of purely chemical precipitation. A typical example of coprecipitation is encountered when barium chloride is added to a solution of potassium sulphate the precipitated barium sulphate is found, after washing, to contain greater or lesser amounts of potassium sulphate inspite of the fact that this salt is freely soluble under the circumstances. Corecipitation prominently takes place by the two mechanism i.e.
i) adsorption at the surface of the particles
ii) occlusion by incorporation of foreign ions (and solvent)
POSTPRECIPITATION
Coprecipitation is important as far as the contamination of a precipitate with foreign materials is concerned. But there is another process called postprecipitation which makes a precipitate impure. In this case, as the name suggests the primary precipitate separates out in a pure form and the second phase of the foreign substance which is slightly soluble forms afterwards. The second phase is therefore, not coprecipitated but postprecipitated. Postprecipitation can occur only when the supernatant is supresatureated, with respect to some component which crystallizes fairly slowly.
DIFFERENCES IN COPRECIPITATION AND POST PRECIPITATION
We have seen that both coprecipitation and postprecipitation make the precipitates impure and can cause serious problems but the two differ in several aspects.
i) If the precipitate is left in connect with the mother liquor the contamination increases with time in postprecipitation the contamination increases the faster the solution is agitated by either the mechanical or thermal means. The reverse is generally the case with coprecipitation.
ii) In the case of postprecipitation the contamination increases the faster the solution is agitated by either the mechanical or thermal means. The reverse is generally the case with coprecipitation.
iii) The magnitude of contamination by postprecipitation may be much greater than by coprecipitation.
