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A buffer is a solution which can maintain a constant pH. If we add an acid or a base to a buffer solution, it can resist the change in pH to a certain extent. You have to remember that we are talking about small additions of acid or base, and such changes will not alter the buffer-pH much. But if we add a large quantity of acid or base to a buffer, obviously there will be significant change in its pH.
Buffers are conjugate acid-base mixtures. Buffer systems are very important for all types of organisms. The bicarbonate buffer system present in our body plays an important role in maintaining a reasonably constant pH. They are usually made of a weak acid with its conjugate base, or it can also be a mixture of a weak base with its conjugate acid. Consider the theory behind the working of a buffer system. Regardless of the type of buffer, both component species in a buffer system are in a state of equilibrium. Consider a weak acid-conjugate base buffer. A perfect example is a mixture of acetic acid and its salt. We can write the equation as follows:

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Let's say we are adding some acid into this buffer system. What happens to it? Well, as the acid is added, an increase in H3O+ is imperative. So the backward reaction is favored or in other words, the reaction proceeds to the left. This consumes the increased H3O+ and the system will try to attain equilibrium again. This process restores or maintains the pH, and that is exactly the function of a buffer system.
Consider what happens when we add a strong base (for example, KOH) to the same buffer system. The extra added OH ions will consume the H3O+. As this happens, the reaction shifts to the right. So more and more acetic acid will ionize restoring the hydrogen ion concentration, and thereby the pH of the solution is maintained. Isn't that amazing?

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