An acidic buffer solution can be prepared by mixing the solutions of
Ammonium acetate and acetic acid
A buffer solution is a solution which resists any change in its pH value on dilution or an addition of solution of an acid or alkali. The process by which added H+ or OH− ions are removed so that pH remains constant is known as buffer action.
Acid buffer solutions: An acid buffer solution consists of solution of a weak acid and its salt with strong base. The best known example is a mixture of solution of acetic acid and sodium acetate.
CH3COOH ⇌ CH3COO− + H+(Weakly ionized)
CH3COONa ⇌ CH3COO− + Na+(Strongly ionized)
Sodium acetate, being salt, ionises completely to form CH3COO− and Na+ ions. On the other hand, acetic acid being a weak acid ionises very less. Moreover, its ionisation is further suppressed by the acetate ions from sodium acetate (common ion effect). When a few drops of an acid (say HCl) are added to it, the H+ ions from the added acid combine with excess of acetate ions to form feebly or nearly unionised CH3COOH. Thus there is no rise in H+ ion concentration and the pH remains unchanged. When a few drops of an acid (say HCl) are added to it, the H+ ions from the added acid combine with excess of acetate ions to form feebly or nearly unionised CH3COOH. Thus there is no rise in H+ ion concentration and the pH remains unchanged. On the other hand, when a few drops of base (say NaOH) are added, OH- of the added base reacts with acetic acid to form water (unionisable) and acetate ions.
Thus there is no increase in the OH− ion concentration, and hence the pH of the solution remains constant. pH values of acid buffers are less than 7 and can be calculated from the following Henderson's equation :
pH = pKa + log[salt][Acid]
Basic buffer solution : A basic buffer solution consists of a mixture of a weak base and its salt with strong acid. The best known example is a mixture of ammonium hydroxide and ammonium chloride.
NH4OH ⇌ NH+4 + OH−
NH4Cl ⇌ NH+4 + Cl−
The NH+4 ions from completely ionised NH4Cl suppress the ionisation of NH4OH which is already a weak ionisable substance. Thus the concentration of OH− ions remains constant. When a few drops of a base (say NaOH) are added, the OH− ions (from NaOH) combine with NH+4 ions to form nearly unionised NH4OH and thus the concentration of OH− ions and hence pH value remains constant.