Buffer Capacity
Buffer capacity, β, is a quantitative measure of the resistance of a buffer solution to pH change on addition of hydroxide ions. It can be defined as follows.
where dn is an infinitesimal amount of added base and d(p) is the resulting infinitesimal change in the cologarithm of the hydrogen ion concentration. With this definition the buffer capacity of a weak acid, with a dissociation constant Ka, can be expressed as
where CA is the analytical concentration of the acid. pH is approximately equal to -log10.
There are three regions of high buffer capacity.
- At very low p the first term predominates and β increases in proportion to the hydrogen ion concentration. This is independent of the presence or absence of buffering agents and applies to all solvents.
- In the region p = pKa ± 2 the second term becomes important. Buffer capacity is proportional to the concentration of the buffering agent, CA, so dilute solutions have little buffer capacity.
- At very high p the third term predominates and β increases in proportion to the hydroxide ion concentration. This is due to the self-ionization of water and is independent of the presence or absence of buffering agents.
The buffer capacity of a buffering agent is at a maximum p = pKa. It falls to 33% of the maximum value at p = pKa ± 1 and to 10% at p = pKa ± 1.5. For this reason the useful range is approximately pKa ± 1.
Read more about this topic: Buffer Solution
Famous quotes containing the word capacity:
“People between twenty and forty are not sympathetic. The child has the capacity to do but it can’t know. It only knows when it is no longer able to do—after forty. Between twenty and forty the will of the child to do gets stronger, more dangerous, but it has not begun to learn to know yet. Since his capacity to do is forced into channels of evil through environment and pressures, man is strong before he is moral. The world’s anguish is caused by people between twenty and forty.”
—William Faulkner (1897–1962)