ABSTRACT

In this case the terminology Ka has been used and implies that K (c)-values have been determined at different concentrations and extrapolated to zero concentration (where activity = con­ centration). Rather than work with small numbers, the term pKa is used, and it is:

(3.9)

so the pKa for acetic acid would be 4.75. If an acid is dissolved in water in a concentration of C, and

if it dissociates to an extent of a , then the remaining concentration of HA will be C (l-a) and the concentration of both H+ and A-will be Ca. When this is inserted in Eq. 3.7, Eq. 3.10 results:

(3 .10)

When a and C are small, then this becomes:

(3 .11)

One often talks about the "natural” pH of an acid, and this refers to the pH of an aqueous solution of the drug substance. But this pH (as seen in Eq. 2.18) is a function of concentration. If the concentration is equal to the Ka, then the hydrogen ion concentration is equal to the Ka.