|Acids and bases|
|Acid dissociation constant
Self-ionization of water
|Lewis · Mineral
Organic · Strong
Superacids · Weak
|Lewis · Organic
Strong · Superbases
Non-nucleophilic · Weak
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.
Buffer solutions are used to maintain a certain level on the pH scale.
The acid dissociation constant for a weak acid, HA, is defined as
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.
One should remember that 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 capacity 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(pH) is the resulting infinitesimal change in pH. With this definition the buffer capacity can be expressed as
where Kw is the self-ionization constant of water and CA is the analytical concentration of the acid, equal to [HA]+[A-]. The term Kw/[H+] becomes significant at pH greater than about 11.5 and the second term becomes significant at pH less than about 2. Both these terms are properities of water and are independent of the weak acid. Considering the third term, it follows that
Their resistance to changes in pH makes buffer solutions very useful for chemical manufacturing and essential for many biochemical processes. The ideal buffer for a particular pH has a pKa equal to that pH, since such a solution has maximum buffer capacity.
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 its 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.
Industrially, buffer solutions are used in fermentation processes and in setting the correct conditions for dyes used in colouring fabrics. They are also used in chemical analysis and calibration of pH meters.
The majority of biological samples that are used in research are made in buffers, especially phosphate buffered saline (PBS) at pH 7.4.
|HCl, Sodium citrate||1 - 5|
|Citric acid, Sodium citrate||2.5 - 5.6|
|Acetic acid, Sodium acetate||3.7 - 5.6|
|K2HPO4, KH2PO4||5.8 - 8 |
|Na2HPO4, NaH2PO4||6 - 7.5 |
|Borax, Sodium hydroxide||9.2 - 11|
By combining substances with pKa values differing by only two or less and adjusting the pH a wide-range of buffers can be obtained. Citric acid is a useful component of a buffer mixture because it has three pKa values, separated by less than two. The buffer range can be extended by adding other buffering agents. The following two-component mixtures (McIlvaine's buffer solutions) have a buffer range of pH 3 to 8.
|0.2M Na2HPO4 /mL||0.1M Citric Acid /mL||pH...|
Other interesting universal buffers are Carmody buffer and Britton-Robinson buffer, developed in 1931.
|Buffer Range||Temp Effect
dpH/dT in (1/K) **
|Full Compound Name|
|SSC||7.0||6.5-7.5||189.1||saline sodium citrate|
** Values are approximate.