[acetate

Figure 2. Neutralization curve obtained during the titration of 1.0 M acetic acid, plotted as a function of the acetate ion concentration.

The selection of a buffer system for use in a pharmaceutical dosage form is relatively straight-forward. It is evident from the preceding discussion that the most important prerequisite for a buffer is the approximate equality of the pKA value of the buffer with the intended optimal pH value for the formulation. Knowledge of the pH stability profile of a drug substance enables one to deduce the pH range for which formulation is desirable, and the basis for the most appropriate buffer system would be the weak acid or base whose pK\ or pKB value was numerically equal to the midpoint of the pH range of stability.

There are, of course, other considerations that need to be monitored, such as compatibility with the drug substance. Boylan (1986) has provided a summary of the selection criteria for buffering agents:

1. The buffer must have adequate capacity in the desired pH range.

2. The buffer must be biologically safe for the intended use.

3. The buffer should have little or no deleterious effect on the stability of the final product.

4. The buffer should permit acceptable flavoring and coloring of the product.

Basis for buffering system

pK1

pK>

pK>

Martell and Smith reference*

Acetic acid

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