PH2

"The pH of a solution is equal to the negative, ten-based logarithm of the activity of the hydrogen ions in the solution. Neutral water dissociates into equal amounts of hydrogen (H+) cations and hydroxyl (OH-) anions. As the product of the concentrations (activities) of the two ions is always a constant 10-14, water has a pH of 7. In acidic solutions the hydrogen ions are in excess, while in basic solutions the hydroxyl ions are in excess. "