Impurities in Precipitates

Precipitates tend to carry down from the solution other constituents that are normally soluble, causing the precipitate to become contaminated. This process is called coprecipitation. In other wards, coprecipitation is a phenomenon in which otherwise soluble compounds are removed from solution during precipitate formation.

There are four types of coprecipitation: 1.surface adsorption,

2.mixed-crystal formation, 3.occlusion,

4. mechanical entrapment.

Surface adsorption

Adsorption is a common source of coprecipitation and is likely to cause significant contamination of precipitates with large specific surface areas, that is, coagulated colloids.

Although adsorption does occur in crystalline solids, its effects on purity are usually imdetectable because of the relatively small specific surface area of these solids.

The net effect of surface adsorption is therefore the carrying down of an otherwise soluble compound as a surface contaminant. In order to minimizing adsorbed impurities on colloids:

q Using digestion process to improve the purity.

q Washing a coagulated colloid with a solution containing a volatile electrolyte. The adsorbed layers can often be removed by washing.

q Reprecipitation is effective way to minimize the effects of adsorption.

Mixed-crystal formation

Mixed-crystal formation, one of the ions in the crystal lattice of a solid is replaced by an ion of another element. For this exchange to occur, it is necessary that the two ions have the same charge and that their sizes differ by no more than about 5%. This problem occur with both colloidal suspensions and crystalline precipitates. Ex (Pb ion replace some of the barium ion). In order to minimizing this type of coprecipitation:

oThe interfering ion may have to be separated before the final precipitation step. oA different precipitating reagent that does not give mixed crystals with the ions

interested may be used.

Occlusion

Occlusion is a type of coprecipitation in which a compound is trapped within a pocket formed during rapid crystal growth material that is not part of the crystal structure is trapped within a crystal. For example, water may be trapped in pockets when AgNO₃ crystals are formed.

Occluded impurities are difficult to remove. Digestion may help some but is not completely effective. The impurities cannot be removed by washing. Reprecipitation that go on at the elevated temperature of digestion open up the pockets and allow the impurities to escape into the solution.

Mechanical entrapment

Mechanical entrapment occurs when crystals lie close together during growth. Here, several crystals grow together and in so doing trap a portion of the solution in a tiny pocket.

Mechanical entrapment can be minimumize when the rate of precipitate formation is low—that is under conditions of low supersaturation. In addition, digestion is often remarkably helpful in reducing this types of coprecipitation.