Alkalinity is a measure of the capacity of water to neutralize acids. This is known as the buffering capacity of water, or the ability of water to resist a change in pH when acid is added.
Alkalinity in water (with a pH of 7.0 or higher) is caused primarily by the presence of dissolved acid-neutralizing substances (alkali). It relates to the balance of carbon dioxide in water and is a function of pH.
The three primary alkali ions in water that contribute to alkalinity are:
- Bicarbonate (HCO 3 -)
- Carbonate (CO 3 2-)
- Hydroxide (OH -)
Total Alkalinity
Total alkalinity is a measure in parts per million of all the alkali (all carbonate, bicarbonate and hydroxide ions) present in a sample. Therefore, total alkalinity is a measurement of water’s ability to resist changes in pH.
Total alkalinity is determined by titration to a pH of 4.9, 4.6, 4.5, or 4.3, depending on the amount of carbon dioxide present.
Phenolphthalein Alkalinity
Phenolphthalein alkalinity (p-alkalinity) is determined by titration to a pH of 8.3 (the end point when phenolphthalein indicator changes color) and it registers the total hydroxide and one half the carbonate present. Phenolphthalein alkalinity is a subset of total alkalinity.
Typically, hard water is alkaline, while soft water is more acidic. However, exceptions exist in unusual circumstances.