Equilibrium

Buffer Solutions

What do you think will happen if the pH of our blood changes drastically from its normal pH of 7.35? Yes, the cells of our body will not function properly and our body systems will fail! Human blood contains a ‘buffer’ that allows it to maintain its pH at 7.35 to ensure normal functioning of cells. Buffer solutions are also important in chemical and biochemical processes where the control of pH is very important. Let’s understand buffer solutions in more detail.

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Buffer Solutions

Buffers are solutions that resist change in pH on dilution or on the addition of small amounts of acids or alkali. 

A lot of biological and chemical reactions need a constant pH for the reaction to proceed. Buffers are extremely useful in these systems to maintain the pH at a constant value. This does not mean that the pH of buffers does not change. It only means that the change in pH is not as much as it would be with a solution that is not a buffer.

Learn more about pH Scale here in more detail.

Types of Buffer Solutions

Buffers are broadly divided into two types – acidic and alkaline buffer solutions. Acidic buffers are solutions that have a pH below 7 and contain a weak acid and one of its salts. For example, a mixture of acetic acid and sodium acetate acts as a buffer solution with a pH of about 4.75.

Alkaline buffers, on the other hand, have a pH above 7 and contain a weak base and one of its salts. For example, a mixture of ammonium chloride and ammonium hydroxide acts as a buffer solution with a pH of about 9.25. Buffer solutions help maintain the pH of many different things as shown in the image below.

buffer solutions

Preparation of a Buffer Solution

If you know the pKa (acid dissociation constant) of the acid and pKb (base dissociation constant) of the base, then you can make a buffer of known pH by controlling the ratio of salt and acid or salt and base. Buffers can either be prepared by mixing a weak acid with its conjugate base or a weak base with its conjugate acid.

For example, phosphate buffer, a commonly used buffer in research labs, consists of a weak base (HPO42-) and its conjugate acid (H2PO4-). Its pH is usually maintained at 7.4.

Understand the Concept of Equilibrium in Chemical Process in detail.

Buffer Action

So, how does a buffer work? Let’s take the example of a mixture of acetic acid (CH3COOH) and sodium acetate (CH3COONa). Here, acetic acid is weakly ionized while sodium acetate is almost completely ionized. The equations are given as follows:

CH3COOH \rightleftharpoons  H+ + CH3COO

CH3COONa \rightleftharpoons  Na+ + CH3COC

To this, if you add a drop of a strong acid like HCl, the H+ ions from HCl combine with CH3COO– to give feebly ionized CH3COOH. Thus, there is a very slight change in the pH value. Now, if you add a drop of NaOH, the OH ions react with the free acid to give undissociated water molecules.

CH3COOH + OH  \rightleftharpoons  CH3COO + H2O

In this way, the OH– ions of NaOH are removed and the pH is almost unaltered.

Learn more about ionization of Acids and Bases

Solved Example for You

Question: Which of the following statement/s is false about buffer solutions?

  1. The pH of a buffer solution does not change on dilution.
  2. Buffer solutions do not have a definite pH.
  3. The pH of a buffer solution changes slightly on the addition of a small amount of acid or base.
  4. The pH of buffer solution does not change on standing for long.

Solution: The option ‘b’ is false. Buffer solutions have a definite pH.

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