Philosophy

Four Stages of the Data Processing Cycle Hierarchy of Data Storage Elements

The acid dissociation constant for a weak acid, HA, is defined as

[pic]

Simple manipulation with logarithms gives the Henderson-Hasselbalch equation, which describes pH in terms of pKa

[pic]

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 hydronium 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+.

B + H2O [pic] BH+ + OH-.

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.

buffer capacity = [pic]

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[3]

[pic]

where Kw is the self-ionization constant of...