Buffer Solutions
Definition: Solution which resist changes in pH on addition of small quantities of acid or alkali or on dilution is known as buffer solution. They consist of weak acids and their conjugate bases or weak bases and their conjugate acids.
Buffer action: The ability of buffer solution to resist changes in pH on addition of small quantities of acid or alkali is called buffer action.
There are two types of buffers:
- Acidic buffers
- Basic buffers
The mixture of weak acid and its salt with strong base is known as acidic buffer.
The mixture of weak base and its salt with strong acid is known as basic buffer.
Example of a buffer solutions:
- Citric acid + sodium citrate
- Acetic acid + sodium acetate
- Glycerine + HCl
General equation important in buffer systems:
- HA(aq) H+(aq) + A–(aq)
How a buffer resists changes in pH:
Buffers are a mixture of a weak acid and its conjugate base or weak base and its conjugate acid. If acid (H+) is added, then the equilibirum will shift to the left to use up some of the added H+ and make more undissociated HA. This keeps the H+ concentration very nearly the same. Therefore pH change is minimal. If base (OH-) is added, then H+ reacts with OH- to make water, and the equilibrium shifts to the right to make more H+, thereby keeping the concentration of H+ very nearly the same. Therefore pH change is minimal.
Calculation of pH of buffers
pH of acidic buffer/Hendersen's equation for acidic buffer
pH of basic buffer/Hendersen's equation for basic buffer
Buffer Capacity (ß):
Buffer capacity is the ratio of number of moles of acid or base added to one litre of buffer to change its pH value by one unit.
Buffer systems formed by weak acids have maximum buffering capacity at the pH = pKa of the acid. Buffer systems formed by weak bases have maximum buffering capacity at the pH = 14 - pKb of the base.