Pharma gyan Pandit

Precipitation from Homogeneous Solution

Precipitation from homogeneous solution is a technique in which a precipitating agent is generated in a solution of the analyte by a slow chemical reaction. Local reagent excesses do not occur because the precipitating agent appears gradually and homogeneously throughout the solution and reacts immediately with the analyte. As a result, the relative supersaturation is kept low during the entire precipitation. In general, homogeneously formed precipitates, both colloidal and crystalline, are better suited for analysis than a solid formed by direct addition of a precipitating reagent.

e.g.Two precipitates of PbCrO₄ are shown in this photo. The precipitate in the beaker on the left was formed by pouring quickly a solution 0.1 M Pb(NO₃)₂ into a solution of 0.1 M K₂CrO₄. The precipitate forms rapidly and consists of very small particles. The precipitate in the beaker on the right was formed by heating a solution of 0.1 M Pb(NO₃)₂, 0.1 M Cr(NO₃)₃, and 0.1 M KBrO₃. Under these conditions, the slow oxidation of Cr³⁺ to CrO4^2– results in a precipitate that forms slowly and consists of much larger particles. This latter process is called homogeneous precipitation, which generally results in a precipitate that is easier to isolate by filtration and that has fewer impurities.