Buffer solutions

Aug 26, 2022, 16:45 IST

About Buffer Solutions

A buffer solution is a solution which resists a sudden/high change in its pH when such a small amount of acid or base is added to it. This does not mean that the pH of the buffer solution does not change but that the change in pH would be less than the pH change that would have been for a solution that is not a buffer.

Types of Buffer solutions

There are three types of buffer solutions:

i) Acidic buffer ii) Basic buffer iii) Salt Buffer

Acidic Buffer solutions

Such a solution consists of a mixture of Weak Acid and it’s salt with a strong base such that the acid & salt concentration are related as

For eg., Mixture of CH3COOH + CH3COONa

Mixture of HCOOH + CH3COOK

Mixture of HCN + NaCN

Mixture of H3PO4 + NaH2PO4

Explanation of Buffer Action

Let us consider a buffer solution made up of CH3COOH and CH3COONa. The weak acid dissociates to a very small extent more so due to the common ion effect of its salt.Now let us assume that this solution contains 10 moles of CH3COOH and 10 moles of CH3COONa. The salt being a strong electrolyte would completely dissociate while the acid would be hardly dissociated. We assume that the amount of CH3COO– is 10 moles as the contribution from CH3COOH would negligible. We also assume that the amount of CH3COOH to be 10 moles as it would be very weakly dissociated. Therefore the solution contains 10 moles of CH3COOH and 10 moles of CH3COO– ions.

Let us now add 1 moles of H+ ions to this solution. These 1 moles of H+ ions would react with the 1 moles of CH3COO– ions to produce 1 moles of CH3COOH. (This is because the reaction of CH3COOH to give CH3COO– ions and H+ ions has an equilibrium constant value of approximately 10–5. Therefore, the reverse reaction, that is the reaction of CH3COO– ions to combine with H+ ions to give CH3COOH would have an equilibrium constant of approximately 105. So this reaction can be assumed to be complete). This 1 mole of CH3COOH formed would ionize weakly because it is a weak acid & also due to the common ion effect of CH3COO– ions. So the amount of H+ ion produced back is much less than the 1 mole that was added. So effectively, 1 mole of H+ ions was consumed and an amount much less than that is produced back which causes an insignificant change in pH of the buffer solution.Similarly let us add 1 mole of NaOH ions to the same buffer solution containing 10 moles of CH3COOH and 10 moles of CH3COO– ions, These 1 mole of OH– ions added would be consumed by the 1 moles of CH3COOH to produce 1 mole of CH3COO– ions. The CH3COO– ions produced would be hydrolyzed weakly and thus the OH– ion produced is much less than the amount that was added. This causes a minor change in pH.