pH is one of the most commonly determined parameters of aqueous (and non-aqueous) solutions because many chemical processes are dependent on pH. Altering pH can often significantly alter the rate of chemical reactions. The solubility and bioavailability of many pharmaceutically active compounds is also determined by solution pH. Furthermore, in chromatography, the retention and selectivity of ionizable analytes can be manipulated by changing the eluent pH. This module will discuss the equipment used to measure pH and how to use it.