Buffer solution

A buffer solution is chemical solution which resists change to its pH or acidity.
It is a solution in water of a mixture of a weak acid or base and its salt. The pH of the solution changes very little when a small amount of strong acid or base is added to it. Buffer solutions are used as a means of keeping pH at a nearly constant value in a wide variety of chemical applications. Many life forms have a relatively small pH range; an example of a buffer solution is blood.
Buffer solutions may be of two types: acidic and basic.
eg. CH3COOH+CH3COONa
eg. NH4OH+NH4Cl
If you add acid to the solution, the concentration of H+ ions will increase; to keep equilibrium a small number of ions will be combined (forming salt and reducing the concentration of H+ ion in the solution). If you add base the concentration of H+ ion will reduce (by consumption or combining) and so a small amount of salt will break into ions and maintain the pH.
Buffer index is the number of moles of acid or base added to 1 litre of buffer solution to change its ph by 1 unit