Bicarbonate
Bicarbonate is an ion made up of hydrogen, carbon, and oxygen that, together with carbonate, forms the basis of a reef aquarium’s alkalinity. At the pH range most reef tanks are kept in, bicarbonate is actually the more abundant of the two forms, making it the dominant contributor to a tank’s buffering capacity at any given moment.
Understanding bicarbonate’s role helps explain why alkalinity behaves the way it does, buffering pH changes while also supplying corals with material for skeletal growth.
Why It’s Important
- Serves as the primary buffering ion in seawater, helping resist sudden shifts in pH.
- Represents the most abundant form of dissolved inorganic carbon at typical reef tank pH levels.
- Supplies raw material that corals convert into carbonate for use in calcification.
- Plays a central role in the broader carbonate system that governs a tank’s overall water chemistry stability.
How It Works
Bicarbonate exists in a dynamic equilibrium with carbon dioxide and carbonate, shifting between these forms depending on the water’s pH. At the pH of a typical reef tank, somewhere around 8.1 to 8.4, bicarbonate is the dominant form present, with a smaller amount existing as carbonate. As pH rises, more of the available carbon shifts toward carbonate; as it falls, more shifts back toward bicarbonate and dissolved CO2.
Corals draw on this pool, using carbonate ions directly in calcification while the surrounding bicarbonate continuously replenishes the carbonate supply as it’s consumed, keeping the overall system in balance.
This equilibrium helps explain a pattern many reef keepers notice: pH tends to dip overnight. Without light, corals and algae stop photosynthesizing and consuming CO2, while respiration continues, raising dissolved CO2 levels. That extra CO2 shifts the bicarbonate-carbonate balance and pulls pH downward until daylight returns and photosynthesis resumes, illustrating just how directly this ion balance connects to the tank’s daily rhythm.
Bicarbonate and Related Parameters
Bicarbonate is inseparable from alkalinity and carbonate, since these three concepts describe the same underlying chemistry from different angles. It’s also connected to pH and the broader carbonate system, since the balance between bicarbonate and carbonate shifts directly with pH changes, and to CO2 accumulation, which can push that equilibrium in ways that affect both pH and alkalinity readings.
Common Misconception
A common misconception is treating bicarbonate and carbonate as interchangeable terms describing the same thing. While closely related and constantly interconverting, they are distinct ions, and the ratio between them shifts with pH. This is part of why alkalinity is measured as a combined total rather than reported as separate bicarbonate and carbonate concentrations in most hobbyist test kits.
The Bottom Line
Bicarbonate is the dominant form of dissolved inorganic carbon in reef aquarium water, working alongside carbonate to make up total alkalinity. Its role in buffering pH and supplying material for coral calcification makes it a quiet but essential part of the water chemistry every reef keeper is ultimately managing when they test and dose for alkalinity.
