Co - precipitation- this is due to anything unwanted that precipitates with our desired ppt

Surface adsorption- here unwanted material is adsorbed unto the surface of the ppt. It is reduced by digestion as during digestion larger crystals are produced.Washing can also help reduce surface material

Occlusion- this is a type of co- pption in which impurities are trapped within the growing crystal

Post-pption-a ppt standing in contact with the mother liquor can become contaminated by the pption of an impurity on top of the desired ppt

Mixed crystal formation- this occurs if two ions have the same charade and their ionic diameters are similar therefore they can fit into the same crystal lattice creating a mixed crystal

Peptisation- this occurs during washing due to part of the crystal converting to a colloid. This colloidal form is able to pass through the pores of the filter paper. This can be minimized if during washing if minimal water is used.

Impurities in Precipitation Gravimetry