Many body fluids e.g., blood or urine have 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 change 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 buffer solution around pH 4.75 and a mixture of ammonium chloride and ammonium hydroxide acts as a buffer around pH 9.25.
You will learn more about buffer solutions in higher classes.