A buffer is an aqueous solution consisting of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. A buffer resists changes in pH when a small amount of strong acid or base is added and hence it maintains the pH of a solution. Many organisms thrive only in a relatively small pH range so they utilize buffer systems to maintain a constant pH. Buffers are more effective within 1.0 pH unit range of its pKa value.
a. Acetic acid and sodium acetate mixture
b. Ammonium hydroxide and ammonium chloride mixture
c. Potassium dihydrogen phosphate and dipotassium hydrogen phosphate mixture.
d. Sodium carbonate and sodium bicarbonate mixture
Buffer solutions achieve their resistance to pH change because of the presence of an equilibrium between the acid HA and its conjugate base A–.
HA ↔H+ + A–
When some strong acid is added to an equilibrium mixture of the weak acid and its conjugate base, the equilibrium is shifted to the left, in accordance with Le Chatelier’s principle. Because of this, the hydrogen ion concentration increases by less than the amount expected for the quantity of strong acid added. Similarly, if strong alkali is added to the mixture the hydrogen ion concentration decreases by less than the amount expected for the quantity of alkali added.
The quantity of strong acid or base that is added to change the pH of one liter of buffer solution by one pH unit is known as buffer capacity.