A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. It has the property that the pH of the solution changes very little when a small amount of strong acid or base is added to it. Buffer solutions are used as a means of keeping pH at a nearly constant value in a wide variety of chemical applications. Many life forms thrive only in a relatively small pH range; an example of a buffer solution is blood.
Acid dissociation constant
The acid dissociation constant for a weak acid, HA, is defined as
Simple manipulation with logarithms gives the Henderson-Hasselbalch equation, which describes pH in terms of pKa
In this equation [A−] is the concentration of the conjugate base and [HA] is the concentration of the acid. It follows that when the concentrations of acid and conjugate base are equal, often described as half-neutralization, pH=pKa. In general, the pH of a buffer solution may be easily calculated, knowing the composition of the mixture, by means of an ICE table.
The calculated pH may be different from measured pH. Glass electrodes found in common pH meters respond not to the concentration of hydrogen ions ([H+]), but to their activity, which depends on several factors, primarily on the ionic strength of the media. For example, calculation of pH of phosphate-buffered saline would give the value of 7.96, whereas the actual pH is 7.4.
The same considerations apply to a mixture of a weak base, B and its conjugate acid BH+.
The pKa value to be used is that of the acid conjugate to the base.
In general a buffer solution may be made up of more than one weak acid and its conjugate base; if the individual buffer regions overlap a wider buffer region is created by mixing the two buffering agents.
Buffer solutions are necessary to keep the correct pH for enzymes in many organisms to work. Many enzymes work only under very precise conditions; if the pH strays too far out of the margin, the enzymes slow or stop working and can denature, thus permanently disabling their catalytic activity. A buffer of carbonic acid (H2CO3) and bicarbonate (HCO3−) is present in blood plasma, to maintain a pH between 7.35 and 7.45.
Full article ▸