Ph vs. pH

Upper and lower case is often critical. In chemistry, the symbol Ph is frequently used to represent a phenyl group, whereas pH stands for potentium Hydrogenium (Latin for potential of hydrogen). The pH value of a solution measures its acidity or basicity. The quantity pH is dimensionless and defined as the negative value of the decimal logarithm of the hydrogen ion activity, a_H, in an aqueous solution:

pH = -log₁₀ a_H

For pH details see: IUPAC Gold Book/pH quantity