The pH of water is a measure of the acid–base equilibrium and, in most natural waters, is
controlled by the carbon dioxide–bicarbonate–carbonate equilibrium system. An increased
carbon dioxide concentration will therefore lower pH, whereas a decrease will cause it to rise.
Temperature will also affect the equilibria and the pH. In pure water, a decrease in pH of
about 0.45 occurs as the temperature is raised by 25 °C. In water with a buffering capacity
imparted by bicarbonate, carbonate, and hydroxyl ions, this temperature effect is modified
(1). The pH of most raw water lies within the range 6.5–8.5 (1).