Criteria of precipitation of an electrolyte

A very useful conclusion is derived from the solubility product concept. No precipitation of the electrolyte occurs if the ionic product is less than the solubility product, i.e., the solution has not reached the saturation stage.

Case I:    When K_ip<K_sp, then solution is unsaturated in which more solute can be dissolved.

Case II:   When K_ip = K_sp, then solution is saturated in which no more solute can be dissolved.

Case III: When K_ip > K_sp, then solution is supersaturated and precipitation takes place.

When the ionic product exceeds the solubility product, the equilibrium shifts towards left hand side, i.e., increasing the concentration of undissociated molecules of the electrolyte. As the solvent can hold a fixed amount of electrolyte at a definite temperature, the excess of the electrolyte is thrown out from the solution as precipitate.

Thus, for the precipitation of an electrolyte, it is neces­sary that the ionic product must exceed its solubility product For example, if equal volumes of 0.02 M AgN0₃ solution and 0.02 M K₂Cr0₄ solution are mixed, the precipita­tion of Ag₂Cr0₄ occurs as the ionic product exceeds the solubility product of Ag₂Cr0₄ which is 2 × 10^–12.