Is HCO₃⁻ an Acid or a Base?
Short answer: it can be either. The hydrogencarbonate ion, HCO₃⁻ (you may know it as bicarbonate), acts as an acid with some partners and as a base with others. A species like this is called amphiprotic (you will also see the word amphoteric, and at A-level the two are treated as the same idea, so do not lose sleep over the distinction). What actually earns marks is showing, with equations, when it donates a proton and when it accepts one. So let me give you clear examples of both.
What HCO₃⁻ is
HCO₃⁻ is what carbonic acid, H₂CO₃, becomes after it loses one proton. It still has one H it can give away, and it still has a lone pair that can grab a proton. Those two features are what let it face both ways. You meet it as sodium hydrogencarbonate (baking soda, NaHCO₃), as the buffer that holds your blood near pH 7.4, and as the calcium hydrogencarbonate that makes water “hard”.
When HCO₃⁻ acts as an ACID (it donates H⁺)
Put HCO₃⁻ with something more willing to take a proton (a base) and it gives its H⁺ away, leaving the carbonate ion, CO₃²⁻.
- With hydroxide, for example warming it with aqueous sodium hydroxide:
HCO₃⁻ + OH⁻ → CO₃²⁻ + H₂O
- Its own slight ionisation in water:
HCO₃⁻ ⇌ H⁺ + CO₃²⁻
In both cases HCO₃⁻ is the acid and CO₃²⁻ is its conjugate base. The pair differs by exactly one H⁺.
When HCO₃⁻ acts as a BASE (it accepts H⁺)
Put HCO₃⁻ with something more willing to give a proton (an acid) and it takes the H⁺, forming carbonic acid, H₂CO₃, which immediately breaks up into water and carbon dioxide (the fizz).
- With a strong acid such as hydrochloric acid:
HCO₃⁻ + H⁺ → H₂CO₃ → H₂O + CO₂
fully: NaHCO₃ + HCl → NaCl + H₂O + CO₂
- This is why any hydrogencarbonate fizzes when acid is added, and it is the standard test that a solution contains HCO₃⁻ (or CO₃²⁻).
- With a carboxylic acid, a favourite in organic chemistry: a compound that liberates CO₂ from sodium hydrogencarbonate must contain a -COOH group, because -COOH is acidic enough to protonate HCO₃⁻. Phenols and alcohols are not, so they give no fizz. That single observation separates a carboxylic acid from a phenol.
In each case HCO₃⁻ is the base and H₂CO₃ is its conjugate acid. Again the pair differs by one H⁺.
The quick rule
Look at what HCO₃⁻ is up against:
- meets a stronger acid → HCO₃⁻ behaves as a base (accepts H⁺, giving H₂CO₃, then CO₂ + H₂O)
- meets a stronger base → HCO₃⁻ behaves as an acid (donates H⁺, giving CO₃²⁻)
Same ion, opposite behaviour, decided entirely by its partner.
Two conjugate pairs, one buffer
Because it has a partner on each side, HCO₃⁻ sits in two conjugate acid-base pairs at once:
| Conjugate pair | HCO₃⁻’s role |
|---|---|
| H₂CO₃ / HCO₃⁻ | HCO₃⁻ is the base |
| HCO₃⁻ / CO₃²⁻ | HCO₃⁻ is the acid |
The first pair is the buffer that keeps your blood near pH 7.4:
CO₂(aq) + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
Add acid and it shifts left to mop up H⁺; add base and it shifts right. HCO₃⁻ can steady both because it acts either way.
(For the “so which is it really” instinct: dissolve pure NaHCO₃ and the solution is very slightly alkaline, around pH 8, because its pull as a base just edges out its pull as an acid. But inside a reaction, its partner decides.)
How the A-level asks about HCO₃⁻
The exam rarely asks “is it an acid or a base” outright. It drops HCO₃⁻ into the blood buffer and makes you use it. Here is the question from the 2022 H2 A-level paper (Ten-Year Series):
A hydrogencarbonate buffer solution is used to maintain an almost constant pH in blood. The equilibrium that applies to this buffer solution is shown.
H₂CO₃(aq) ⇌ HCO₃⁻(aq) + H⁺(aq) Ka = 2.5 × 10⁻⁴ mol dm⁻³ at 37 °C
The pH of blood should be maintained at a value of 7.4. Which value of the ratio [H₂CO₃] / [HCO₃⁻] gives a pH of 7.4 at 37 °C?
A 3.4 × 10⁻⁵ B 1.6 × 10⁻⁴ C 6.3 × 10³ D 3.0 × 10⁴
Work it with pH = pKa + lg([HCO₃⁻] / [H₂CO₃]):
- pKa = -lg(2.5 × 10⁻⁴) = 3.60
- 7.4 = 3.60 + lg([HCO₃⁻] / [H₂CO₃]), so lg([HCO₃⁻] / [H₂CO₃]) = 3.80
- [HCO₃⁻] / [H₂CO₃] = 6.3 × 10³, so the ratio asked, [H₂CO₃] / [HCO₃⁻], is its inverse: 1.6 × 10⁻⁴, answer B
Option C is the trap, it is the right number for the upside-down ratio. The exam is really checking whether you can place HCO₃⁻ correctly in its acid-base pair, then read the ratio in the direction asked. Notice too that hydrogencarbonate outnumbers carbonic acid about 6300 to 1 at blood pH, which is why blood copes so well with added acid.
Frequently asked questions
Is HCO₃⁻ an acid or a base? Both. It is amphiprotic: it donates a proton to a stronger base (acting as an acid, forming CO₃²⁻) and accepts a proton from a stronger acid (acting as a base, forming H₂CO₃). Its partner decides which.
What is the conjugate base of HCO₃⁻? CO₃²⁻, the carbonate ion, formed when HCO₃⁻ loses one H⁺. Its conjugate acid is H₂CO₃, formed when it gains one H⁺.
Is a solution of sodium hydrogencarbonate acidic or alkaline? Slightly alkaline, about pH 8. Its tendency to accept a proton just outweighs its tendency to donate one, so a solution of NaHCO₃ is weakly basic.
Practise this
Drill conjugate pairs and buffers in the Ionic Equilibria topic on ChemBank, and for more on where HCO₃⁻ does real work, read our guide to buffers in the body. Written an explanation and not sure it would score? Paste it into Chem-Check and it will check your Brønsted-Lowry reasoning.
Book a trial lesson and see how we turn “both, I think?” into a full-mark answer.