Answer to Question #240508 in Inorganic Chemistry for Ather

Question

In analytical process, the efficiency at each step is 90%. A chemist is analyzing a sample 

for Nickel using DMG. He follows these steps: dissolution, precipitation, filtration, drying 

and weighing. If he starts with 1000 ppm of Nickel sample. Write down the steps and yield 

at end of reaction. (M.W. (Ni = 58.7g; DMG = 116g, d= 1.37g/cc)

Answer 1

Precipitative gravimetric analysis requires that the substance to be weighed be readily removed by filtration. In order for a non-filterable precipitate to form, it must be supersaturated with respect to its solubility product constant. However, if it is too far above the saturation limit, crystal nucleation may occur at a rate faster than crystal growth (the addition of molecules to a crystal nucleus, eventually forming a non-filterable crystal). When this occurs, numerous tiny micro-crystals are formed rather than a few large ones. In the extreme case, micro-crystals may behave as colloids and pass through a fibrous filter. To avoid this, precipitating solutions may be heated. Because the solubility of most salts increases with increasing temperature, this treatment will lower the relative degree of super saturation and slow the rate of nucleation. Also, one might add the precipitant slowly with rapid mixing to avoid the occurrence of locally high concentrations.

Precipitative gravimetry is often practiced at high ionic strengths. This is to reduce the electric double layer thickness (salting-out effect) of the slowly forming crystals. When this occurs, electrostatic repulsion between the crystal and its precipitating molecules is reduced. Crystal growth can then occur more rapidly. It is very important that the precipitate be pure and has the correct stoichiometry.