Those solution which resist the change in pH, when a small amount of an acid or a base is added to it, are called buffer solution. The PH of a buffer solution does not change on dissolution of the walls of the container and on keeping for a long time.
How to prepare a Buffer Solution?
There are two ways:
- By mixing a weak acid and salt of it with a strong base.
Example:
- CH3COOH + CH3COONa
- H3PO4 + NaH2 PO4
- H2CO3 + NaHCO3
They give acidic buffers having pH value less than seven.
2. By Mixing a weak base and salt of it with a strong acid.
Example:
- NH4OH + NH4Cl
They give basic buffers having pH values more than seven.
Application of buffer solution.
- Many industrial processes such as electroplating, manufacture of leather, manufacture of photographical materials and the preparation of dyes require the use of buffers
- The pH of human blood is buffered at 7.4. This is maintained by a mixture of bicarbonates, phosphates and complex protein systems. For the normal range, the pH of blood is from 7.35 to 7.45. In case it decreases up to 7 or goes up to 8 death may occur.
- Buffer solutions are extensively used by an analytical chemist.
- Buffer tablets are available in the market which can be used to calibrate the pH meter.
- In bacteriological research, one uses the buffer solutions in culture media, because the growth of bacteria needs a definite pH.
How do such solutions act as buffers?
Le Chatelier’s principle and common ion effect can help us to understand the buffer action of the solutions. Let us consider the buffer solution consisted of CH3COOH and CH3COONa. They are dissociated in water. Sodium acetate being a very strong electrolyte as compared to acetic acid furnishes sufficient CH3COO– ion as compared to CH3COOH
\[
\mathrm{CH_3COOH \; \rightleftharpoons \; CH_3COO^-_{(aq)} + H^+_{(aq)}}
\]
\[
\mathrm{CH_3COONa \; \rightleftharpoons \; CH_3COO^-_{(aq)} + Na^+_{(aq)}}
\]
When a few drops of an acid, say HCl are added in this solution, the H+ ions provided by HCI are taken up by CH3COO– (mostly obtained from CH3COONa), so incoming protons are consumed and pH is retained.
When a few drops of a base say NaOH is added from outside, then the protons already present in the solution are consumed, To compensate to those protons, there happens a further dissociation of CH3COOH and pH is retained.
Buffer Capacity
The number of moles of an acid or a base required by one dm-3of a buffer solution for changing its pH by one unit, is called buffer capacity of a
solution.
Explanation:
The buffer capacity of a solution is the capability of a buffer to resist the change of pH. It is measured quantitatively that how much extra acid or a base solution can absorb before the buffer is essentially destroyed.
The molarities of the two components of buffer solution determine the buffer capacity. Let us learn how to calculate to change of pH of a buffer after addition of an acid or a base.
Quantitative aspect of buffer capacity.
in order to have quantitative understanding of buffer capacity of a solution, let us take following example.
Consider that we have prepared a buffer solution of pH = 4.82. This buffer can be prepared by mixing 0.11 molar CH3COONa and 0.09 molar CH3COOH Now add 0.01 moles of NaOH in 1dm3 of buffer solution. This NaOH is a strong base and is 100% dissociated, If we dissolve 0.01 moles of NaOH in 1dm3 of solution in distilled water, then the pH of the system goes from 7.0 to 12.0. There happens a big change of pH.
But when we have added 0.01 moles of NaOH in the above-mentioned buffer, it changes the pH of the buffer from 4.82 to 4.92 that is only 0.1. This is a very small change and this is due to the buffer capacity of solution under consideration.
Reason:
When 0.01 moles of NaOH is added, it releases 0.01 moles of OH–. It will decrease the concentration of CH3COOH from 0.09 to 0.08 moles. This
neutralization will bring about a change in the concentration of CH3COO–. The concentration of CH3COO– will increase from 0.11 to 0.12 moles.
According to these new concentrations of CH3COOH and CH3COO–, we can do the following calculations:
\[
\text{pH} = pK_a + \log \frac{[\text{salt}]}{[\text{acid}]}
\]
\[
\text{pH} = 4.74 + \log \frac{0.12}{0.08}
\]
\[
\text{pH} = 4.74 + \log 1.5
\]
\[
\text{pH} = 4.74 + 0.176
\]
\[
\text{pH} = 4.92
\]
Before adding 0.01 NaOH, the pH of buffer is 4.82 Hence, very very small change in pH has happened.