Carbonate alkalinity (CO3-2) is one of the three principle forms of alkalinity present in many surface waters and may be classified stoichiometrically by comparing the values for total (T) and phenolphthalein (P) alkalinity. Carbonate alkalinity is equal to twice the phenolphthalein alkalinity (2P) when the phenolphthalein alkalinity is less than or equal to half the total alkalinity and is equal to twice the difference between the total and phenolphthalein alkalinity [2(T-P)] when the phenolphthalein alkalinity is greater than half the total alkalinity. The results are typically expressed in milligrams per liter of equivalent calcium carbonate.
