buffer
Buffers are solutions that maintain a relatively constant pH when an acid or a base is added. They therefore protect, or buffer, other molecules in solution from the effects of the added acid or base. Buffers contain either a weak acid HA and its conjugate base A or a weak base B and its conjugate acid BH+ , and they are critically important for the proper functioning of biological systems. In fact, every biological fluid is buffered to maintain its physiological pH.