Alkalinity – What Is It?
A trap for baristas is remembering the difference between an alkali (something that is alkaline) and the alkalinity of a substance. The terms are related, but they are not the same thing. The alkalinity of water refers to its buffering capacity, its ability to resist a change in pH.
The buffering capacity of water is mainly due to carbonate and bicarbonate ions. These can neutralise acids added to the water, with the consequence that the addition of acid has a smaller effect on the pH. Here’s how it works:
H2CO3 ⇌ HCO3– + H+
(carbonic acid ⇌ bicarbonate + hydrogen ions)
Bicarbonate ions in water are in an equilibrium with carbonic acid. This means bicarbonate ions are constantly reacting with H+ ions in the water to form carbonic acid. Meanwhile, carbonic acid in the water is constantly breaking down to form H+ and bicarbonate. Because both reactions occur at the same rate, there is no net change.
However, if you add an acid, you are adding more H+ ions into the mix. This makes it more likely that the reaction will proceed from right to left, absorbing some of the H+ ions added. As a result, the change in pH is reduced as some of the H+ ions are used up in the reaction.
Carbonate ions bring about a similar equilibrium:
HCO3– ⇌ . CO32- + H+
(bicarbonate ⇌ carbonate + hydrogen ions)
In this case, the carbonate ions absorb hydrogen ions and become bicarbonates; once again, some of the H+ ions are absorbed, reducing the pH change. Carbonates aren’t very soluble in water, though, so the bicarbonates have the biggest effect on the buffering capacity of the water.