A water’s alkalinity mostly comes from the presence in solution of hydroxide ions (HO^-), carbonate ions CO₃^2-_(aq) and hydrogen carbonate ions HCO₃^-_(aq) (phosphate ions PO₄^3- or silicate ions SiO₃^2- may also contribute in a minor way).

The alkalimetric title (AT) is used to determine the total concentration of carbonate ions CO₃^2-_(aq) and strong base (HO^-) in a water. AT is the volume (in mL) of hydrochloric acid (at a concentration of 2.0x10^-2 mol/L) required to dose 100 mL of water in the presence of phenolphthalein as a color indicator. If the addition of phenolphthalein to the test solution does not result in a violet color, then AT is zero.

The complete alkalimetric title (CAT) is used to determine the total concentration of hydrogen carbonate ions HCO₃^-_(aq), carbonate ions CO₃^2-_(aq) and strong base (HO^-) of a water. CAT is the volume (in mL) of hydrochloric acid (at a concentration of 2.0x10^-2 mol/L) required to dose 100 mL of water in the presence of bromocresol green (or methyl orange) as a color indicator.

The unit of AT and CAT is the French degree (°fH): 1 °fH = 3.4 mg/L (HO^-) = 6.0 mg/L (CO₃^2-) = 12.2 mg/L (HCO₃^-)