What Is a Buffer? How Solutions Resist pH Change
Add one drop of strong acid to a glass of pure water and the pH crashes from 7 to about 3. Add the same drop to your bloodstream and nothing happens — your pH shifts by maybe a hundredth of a unit. The difference is a buffer, and without it you would not survive lunch.
The short answer: a buffer is a solution that resists changes in pH when small amounts of acid or base are added. It works because it contains a weak acid and its conjugate base side by side, one ready to mop up added OH⁻ and the other ready to mop up added H⁺.
What a buffer actually is
A buffer needs two components present in comparable amounts:
- a weak acid (call it HA), and
- its conjugate base (A⁻) — the same species minus its proton.
The classic laboratory buffer is acetic acid (CH₃COOH) with sodium acetate (CH₃COONa). The acetic acid supplies HA; the sodium acetate dissolves to supply A⁻.
The pair works because a weak acid sits in equilibrium rather than committing one way:
CH₃COOH ⇌ H⁺ + CH₃COO⁻
Having a healthy reservoir on both sides is what gives the solution somewhere to put whatever you throw at it.
You can also build a buffer the other way round, from a weak base and its conjugate acid — ammonia with ammonium chloride is the standard example.
How it absorbs an attack
If you add acid (H⁺): the conjugate base is waiting. Acetate ions grab the incoming protons and turn back into acetic acid molecules.
CH₃COO⁻ + H⁺ → CH₃COOH
The added H⁺ is converted into a weak acid that barely ionises, so it never gets to lower the pH much.
If you add base (OH⁻): the weak acid handles it. Acetic acid donates a proton to neutralise the hydroxide.
CH₃COOH + OH⁻ → CH₃COO⁻ + H₂O
Either way, the aggressive ion is swapped for something mild. The ratio of HA to A⁻ shifts a little, but the pH barely moves.
Think of it as a sponge with two sides — one soaks up acid, the other soaks up base.
Why a strong acid can't do this
A common exam question asks why HCl and NaCl don't make a buffer, even though NaCl looks like HCl's conjugate partner.
The answer is that the chloride ion is a useless base. HCl dissociates completely and irreversibly, so Cl⁻ has no interest in taking a proton back. Add acid to that mixture and nothing absorbs it.
A buffer requires a weak acid precisely because a weak acid's reaction is reversible. Reversibility is the mechanism.
The pH a buffer holds
A buffer doesn't hold pH 7 by default — it holds whatever pH suits its acid, given by the Henderson–Hasselbalch equation:
pH = pKa + log([A⁻] / [HA])
Two useful consequences:
- When the acid and its conjugate base are in equal amounts, the log term is zero and pH = pKa. An equimolar acetic acid/acetate buffer sits at pH 4.76.
- A buffer works best within about one pH unit either side of its pKa. Outside that range one component runs low and the buffer loses its grip.
So you choose your buffer by choosing an acid whose pKa is near the pH you need.
Buffer capacity is the other half of the story: a buffer made from concentrated solutions can absorb far more before it breaks than a dilute one at the same pH. Pour in enough acid to consume all the A⁻ and the buffer is exhausted, and the pH falls off a cliff.
Buffers in the real world
- Blood is held between pH 7.35 and 7.45 mainly by the carbonic acid/bicarbonate pair (H₂CO₃ / HCO₃⁻) — which manages to hold at 7.4 despite a pKa of 6.1 only because your lungs continuously blow off CO₂, topping the system up from outside. Drift outside that range and enzymes stop working — a serious medical emergency.
- Shampoos and skin creams are buffered to be mildly acidic, matching skin's natural pH.
- Fermentation and cell culture media are buffered so that the acids microbes produce don't kill them.
- Ocean water is buffered by the same carbonate system, which is why absorbing CO₂ acidifies it slowly rather than suddenly.
Worked examples
1. Which pair makes a buffer: HCl/NaCl or CH₃COOH/CH₃COONa?
The acetic acid pair. HCl is a strong acid, so Cl⁻ won't accept protons back.
2. An equimolar buffer of acetic acid (pKa 4.76) and acetate — what's its pH?
The ratio is 1, log 1 = 0, so pH = 4.76.
3. What happens to that buffer if you add a little NaOH?
The OH⁻ reacts with CH₃COOH, converting some of it to CH₃COO⁻. The ratio rises slightly, so the pH climbs a fraction — but nothing like the jump you'd see in water.
4. You need a buffer at pH 9. Acetic acid (pKa 4.76) or ammonium (pKa 9.25)?
Ammonium — its pKa is close to the target, so the buffer will be effective there.
Common mistakes to avoid
- Thinking a buffer holds pH 7. It holds whatever pH its pKa dictates. A pH 4.76 buffer is completely normal.
- Believing pH doesn't change at all. It changes — just very little, and only until the buffer's capacity is used up.
- Pairing a strong acid with its salt. No reversible equilibrium, no buffering. The weak acid is essential.
FAQ
What is a buffer solution in simple terms?
A solution that keeps its pH nearly constant when small amounts of acid or base are added. It contains a weak acid and its conjugate base, so it can neutralise both.
How does a buffer work?
The conjugate base reacts with any added H⁺, and the weak acid reacts with any added OH⁻. Both convert the added ions into something much weaker, so the pH barely shifts.
Why is blood a buffer?
Blood contains carbonic acid and bicarbonate, which hold its pH between 7.35 and 7.45. Enzymes only function in that narrow band, so the buffer keeps you alive.
Can a buffer be destroyed?
Yes. Add enough acid or base to use up one of the two components and the buffer's capacity is exceeded — after that the pH changes sharply.
The takeaway
A buffer is a weak acid and its conjugate base sharing a solution, each ready to neutralise whatever the other can't. It doesn't lock pH at 7; it locks it near the acid's pKa, and it holds only until its capacity runs out. That small trick is what keeps blood, oceans and half the chemistry lab stable.
Background → What Is pH? The pH Scale Explained Simply and Acids vs Bases: What's the Difference?. Related → [Strong vs Weak Acids] (sibling) explains why only weak acids buffer, [pH vs pOH] (sibling) covers the scale, and [What Is Neutralization?] (sibling) is the reaction a buffer keeps interrupting.
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