The condition of precipitation:
The precipitate should first be sufficiently insulouble that the amount lost due to solubility will be negligible. It should consist of large crystals so that they can be easily filtered and also minimize the contamination (because of a large surface area).
The precipitation process involves heterogenous equilibra and the reaction is not instantaneous.
First, supersaturation occurs, i.e. in the solution phase contains more of the dissolved salt than at equilibrium. This is a unstable condition and the driving force will be made the system to approach equilibrium (saturation). This is started by nucleation. In other words, the higher the degree of supersaturation, the greater the rate of nucleation. Also the formation of a greater number of nuclei per unit time will give smaller crystal size.
Von Weimarn find out the relation of this:
Relative supersaturation = (Q-S)/S
where Q is the concentration of the mixed reagents before precipation occurs and is the degree of supersaturation, and S is the solubility of the precipitate at equilibrium.
Obviously, we want to keep Q low and S high during precipitation. Several steps are commonly taken to maintain favorable conditions for precipitation:
1. Precipitate from dilute solution. This keeps Q low.
2. Add dilute precipitating agents slowly, with effective stirring. This also keeps Q low. Local excesses of the reagent are prevented by stirring.
3. Precipitate from hot solution.This increase S. The solubility should not be too great.
4.Precipitate at low pH. It is because many precipitates are more soluble in acid medium and this slows the rate of precipitation. They are more soluble because the anion of the precipitate combines with protons in the solution.
Some other consideration:
When the precipitation is performed, a slight excess of precipitating reagents is added to decrease the solubility by mass action (common ion effect) and to assure complete precipitation. A large excess of agent should be avoided because this increase chances of adsorption on the surface of the precipitate, and also wasteful. Completeness of precipitation is checked by waiting until the precipitate has settled and then adding a few drops of precipitating reagent tto the clear solution above it. If no new precipitate forms, precipitation is complete.