It is a special type of introducing contamination, where the impurity precipitates after all the major part of the analytical precipitate has formed, or the precipitation of another substance takes place on the primary precipitate.The second substance is poorly solvable and forms a super-saturated solution.Eg-When a solution of CaCl2(calcium chloride) is treated with H2C2O4(oxalic acid) in the presence of magnesium ion as impurity, the precipitate of calcium oxalate induces the precipitation of magneium oxalate causing an err in the estimation.The larger the time of contact, greater is the post-precipitation.
CaCl2+H2C2O4———————>CaC2H2O4(ppt.)+HCl[The reaction is carried out in he presence of magnesium ion].


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