Buffer capacity is defined as the moles of acid or base necessary to change the pH of one liter of solution by one unit.
Buffer Capacity = (number of moles of OH- or H3O+ added) divided by ( pH change) x (volume of buffer in L)
-Buffering capacity increases as the molar concentration (molarity)of the buffer salt/acid solution increases.
That's only partially true.The pH of a buffer solution is independent of buffer concentration only if there is a sufficient quantity present. A buffer consists of a solution of a weak acid (base) and the salt of weak acid (base). The addition of either acid or base converts acid(base) to the acid (base) salt, or the salt back to acid (base) when another acid or base is added to the solution.If there is not enough buffer present compared to the added acid or base, the buffer can be swamped, and the pH will change significantly. So it is only partially true that the buffer is independent of the buffer concentration.If the amount of acid or base added, is small compared to the buffer concentration then the pH is not sensitive to the amount of buffer present
-the closer the bufferd pH is to the pKa,the greater the buffering capacity
The solution is at a maximum buffer capacity (the minimum dpH/dn) when pH = pKa, as can be seen algebraically:
Ka = [H3O+][A-]/[HA]; [H3O+] = Ka[HA]/[A-]
log [H3O+] = log Ka + log ([HA]/[A-])
pH = pKa - log([HA]/[A-])
This rearranged form of the ionization constant of an acid is sometimes called the Henderson-Hasselbalch equation.
Since pH = pKa if and only if [HA] = [A-], maximum buffer capacity of a conjugate acid-base pair is always found at pH = pKa. Reasonable buffer capacity is still obtained if the ratio of [HA] to [A-] is somewhere between 0.1 and 10, but outside this range it decreases rapidly. A chemical buffer can be effective about pH = pKa +/- 1. To make up a buffer solution one would first choose a reasonable acid-base conjugate pair on the basis of its pKa value and any other chemical constraints (one would not use the HCN/CN- pair, for example, in biological systems because cyanide is toxic). One would then adjust the concentrations such that both are reasonable (say 10-3 molar or above) while the desired pH is still obtained. The conjugate acid-base pairs glycinium ion/glycine, hydrogen citrate/citrate, dihydrogen phosphate/monohydrogen phosphate, and boric acid/hydrogen borate are often used for the preparation of aqueous buffers.