*3.3.1 pH measurement during the preformulation stage*

The negative logarithm of hydrogen ion concentration of drug substances is known as pH. The formula for calculation of pH is pH = − Log [H+] and based on the pH scale it is categorised as an acidic drug with 1–7 pH, ±7 is a neutral drug, and 7–14 pH is alkaline/basic drugs.

Most of the drugs are available in salt form or they are either weak bases or acids. Therefore, it is very important to have a complete understanding of molecule ionisation behaviour at specific pH values. Hence, at the preformulation stage, the effect of drug ionisation, ionic strength, pH, and temperature are simultaneously studied to understand dosage form stability, solubility, bioavailability, and efficacy of drug molecules [54].
