pH's a measure of the acidity or basicity of a solution. It's defined as the cologarithm of the activity of dissolved hydrogen ions (H+). Hydrogen ion activity coefficients can't be measured experimentally, so they're based on theoretical calculations. The pH scale isn't an absolute scale; it's relative to a set of standard solutions whose pH's established by international agreement. The concept of pH was first introduced by Danish chemist Søren Peder Lauritz Sørensen at the Carlsberg… (More on Ph)