No discussion of gravimetric analysis would be complete without some discussion of the impurities which may be present in the precipitates. This is anything unwanted which precipitates with the thing you do want. You cannot avoid it - all you can do is minimise it by careful precipitation and thorough washing. Here unwanted material is adsorbed onto the surface of the precipitate. Digestion of a precipitate reduces the amount of surface area and hence the area available for surface adsorption.
Washing can also remove surface material.
This is a type of coprecipitation in which impurities are trapped within the growing crystal. Sometimes a precipitate standing in contact with the mother liquor becomes contaminated by the precipitation of an impurity on top of the desired precipitate. Washing and Filtering Problems with coprecipitation and surface adsorption may be reduced by careful washing of the precipitate. With many precipitates, peptization occurs during washing. Drying the solid Generally the solids are dried at about 120oC but conditions for drying can vary considerably.