What ion is being measured when testing for alkalinity?
Alkalinity measures a samples buffering capacity (resistance to pH change)
Alkalinity does not measure specific ions in a sample; it is a measure of a sample's capacity to resist a pH change. If acid is added to water having low alkalinity, the pH of the water will decrease significantly. If the same amount of acid is added to the same amount of water having high alkalinity, the pH of the water will not change significantly. This resistance to pH change is primarily due to bicarbonate, carbonate and hydroxide ions, however the test does not measure these ions specifically. All the ions that contribute to alkalinity are represented together as CaCO3 by convention.