Many body fluids e.g., blood or urine have a definite pH and any deviation in their pH indicates malfunctioning of the body.

The control of pH is also very important in many chemical and biochemical processes.

Many medical and cosmetic formulations require that these be kept and administered at a particular pH.

The solutions which resist changes in pH on dilution or with the addition of small amounts of acid or alkali are called Buffer Solutions.

Buffer solutions of known pH can be prepared from the knowledge of pKa of the acid or pKb  of base and by controlling the ratio of the salt and acid or salt and base.

A mixture of acetic acid and sodium acetate acts as a buffer solution around pH 4.75

A mixture of ammonium chloride and ammonium hydroxide acts as a buffer around pH 9.25.

To prepare a buffer of acidic pH we use weak acid and its salt is formed with a strong base.

The quantity is the ratio of concentration of conjugate base (anion) of the acid and the acid present in the mixture.

Since acid is a weak acid, it ionises to a very little extent and concentration of [HA] is negligibly different from concentration of acid taken to form a buffer.

Also, most of the conjugate base, [A—], comes from the ionisation of salt of the acid.

Therefore, the concentration of conjugate base will be negligibly different from the concentration of salt

pH= pKa+ log [Salt]/[Acid] is called Henderson–Hasselbalch equation.