Alkalinity is the ability of a substance to resist a change in pH and defined as the buffering capacity of a solution. This resistance affects how much acid is needed to adjust the pH of a solution. Alkalinity is typically measured as mg/L of Calcium Carbonate (CaCO3). The higher the alkalinity of the water, the more resistant it will be to a change in pH. The lower the alkalinity, the easier that the pH can change. This concept is reflected in the differences with groundwater versus surface water. In certain areas the alkalinity is very high because rainwater dissolves calcium carbonate as it percolates through the soil. Because alkalinity is a general parameter that influences a wide variety of processes it is an important parameter to measure for growers (including hydroponics and aquaponics), saltwater aquarists, food and beverage industries, and pools and spas to name a few.

Alkalinity can be measured as Phenolphthalein Alkalinity or Total Alkalinity. The Phenolphthalein Alkalinity is determined by neutralizing the sample to a pH of 8.3 using a dilute hydrochloric acid solution and a phenolphthalein indicator. This process converts hydroxide ions to water, and carbonate ions to bicarbonate ions:

OH- + HCl → H2O + Cl-→ CO3-2 + HCl → HCO3- + Cl-

Since bicarbonate ions can be converted to carbonic acid with the addition of hydrochloric acid, the Phenolphthalein Alkalinity measures total hydroxide ions, but only half of the bicarbonate contribution. To completely convert the carbonate ions, hydrochloric acid is added until the sample pH is 4.5, which is known as Total Alkalinity: 

HCO3 - + HCl → H2CO3 + Cl-