The ppt should consist of large crystals which are easy to wash filter and free from impurities. Its particle size must be large so that adsorption of foreign ions is minimal. Particle size is inversely proportional to the relative supersaturation of the solution during precipitation. Relative Supersaturation=Q-S/S
Q=conc at any instant
S=equilibrium solubility

A supersaturated solution is one that is unstable because it contains more solute than a saturated solution. It is relieved by precipitation of the excess solute. Therefore we need Q small and S large
1) pption from a hot solution
2) pption from dilute solution
3) ppt at a pH near the acidic end of the pH range this increases S because most pots are more soluble at the acidic end of their range

Conditions for the Mother Liquor