Buffer capacity control in pharmaceutical solutions.

Pharmaceutical solutions usually contain buffers to provide control over pH. This is important to prevent pH change during storage which can lead to undesirable results. Moreover, buffer capacity of these buffer systems should also be evaluated. The value of buffer capacity is related to pH of solution and is directly proportional to total buffer concentration. After choosing optimum pH, buffer system, and buffer capacity then the total buffer concentration and the concentration of each buffer specie can be calculated.

The question that rises now is what is the desired buffer capacity for pharmaceutical solutions. The answer to this question is related to two issues. Firstly, the degree of expected factors that the solution could be subjected to that cause change in pH. These factors include dissolution of atmospheric CO₂,  alkali leaching from a glass container, and/or degradation in solution. Generally a buffer capacity of 0.01 - 0.2 is suitable.
Secondly, the effect of the solution pH on the tissue that it is applied on. For example, if the solution is an ophthalmic preparation (eye drops)  that requires to have pH relatively far from that of lacerimal fluid (7.4) then the buffer capacity should be minimal. Because at this pH the solution could irritate the eye. Therefore, a small buffer capacity would allow rapid change of pH after the solution mixes with the lacrimal fluid. Thus, limiting the extent of irritation. Similarly, intravenous solutions should be non-buffered or have minimal buffer capacity. In addition, some cases require attention to buffer capacity values to enhance oral absorption of the drug with minimal gastric upset.

Note: when choosing a buffer system, choose one that has a pKa value for its acid close to the desired pH. Thus, the pH will be close to the maximum buffer capacity.