The solutions which resist change in pH on dilution or with the addition of small amounts of acid or alkali, are called Buffer solutions. e.g.,
Weak acid and its salt with strong base are known as acidic buffer. pH < 7.
e.g., CH3COOH, CH3COONa, etc.
Weak base and its salt with strong acid are known as basic buffer. pH > 7.
e.g., NH4Cl and NH4OH
It is applicable to sparingly soluble salt. There is equilibrium between ions and unionized solid substance.
Consider an equation
Ksp is called the solubility product constant or simply solubility product.
Under equilibrium conditions
The solubility of salts of weak acids like, phosphates, increases at lower pH.