Topic 3 of 3

Buffering and oceans

Explain the response and its limits.

A-Level 8873, revised syllabus (2026-2027)

Keep an acid and its conjugate base available

A buffer removes small additions of acid or base, rather than preventing all pH change.

An acidic buffer contains appreciable amounts of a weak acid and its conjugate base, for example CH₃CO₂H and dissolved sodium ethanoate. Added H⁺ is consumed by ethanoate; added OH^- is consumed by the weak acid. Because most added strong acid/base is removed, the pH changes much less than it would in water.

Two complementary responses

Add a little acid
CH₃CO₂^-(aq) + H⁺(aq) → CH₃CO₂H(aq). The conjugate base removes the added proton.
Add a little alkali
CH₃CO₂H(aq) + OH^-(aq) → CH₃CO₂^-(aq) + H₂O(l). The weak acid removes hydroxide.

A basic buffer uses a weak base and its conjugate acid, such as NH₃/NH₄⁺. NH₃ consumes H⁺; NH₄⁺ consumes OH^-. Buffers help keep enzyme conditions suitable and prevent unwanted pH shifts in formulations. Their capacity is finite: a large acid/base addition can use up one component.

Check your understandingWhy is sodium ethanoate solution alone not the intended two-component acidic buffer?Think it through, then reveal the answer

It supplies ethanoate that can consume acid, but does not initially contain an appreciable reservoir of weak acid to consume added alkali. A useful buffer needs both members in appreciable amounts.

Apply the same proton bookkeeping to the ocean

Acidification is a fall in pH, even while seawater remains alkaline.

The required ocean pair is CO₃^2-/HCO₃^-. Carbonate can accept a proton to form hydrogencarbonate; hydrogencarbonate can donate a proton to react with added hydroxide. This is one part of the wider seawater carbonate system.

The ocean pair responds in both directions
Addition	Main buffer reaction	Role
Acid	CO₃^2-(aq) + H⁺(aq) → HCO₃^-(aq)	Carbonate removes H⁺.
Alkali	HCO₃^-(aq) + OH^-(aq) → CO₃^2-(aq) + H₂O(l)	Hydrogencarbonate removes OH^-.

More atmospheric CO₂ promotes more dissolved CO₂. Its reaction with water can be represented overall as CO₂(aq) + H₂O(l) ⇌ H⁺(aq) + HCO₃^-(aq). The extra H⁺ is partly taken up by carbonate, increasing hydrogencarbonate and reducing carbonate availability. Buffering limits the change; it cannot cancel a sustained large input, so ocean pH falls.

Worked example

Read a pH change as a concentration change

A water sample changes from pH 8.2 to 8.0. Has [H⁺] increased by only 0.2?

pH is logarithmic, so compare 10^-8.0 with 10^-8.2.
Their ratio is 10^0.2 = 1.5849.
Both pH values exceed 7 at 25 degrees C, yet the second sample has more H⁺.

Answer

[H⁺] is about 1.58 times as large, a roughly 58% increase. The sample has acidified without becoming acidic in the pH-below-7 sense.

Check your understandingWhy does the existence of an ocean buffer not imply that atmospheric CO₂ has no effect on pH?Think it through, then reveal the answer

The components are consumed and redistributed as they respond. A sustained CO₂ input changes the equilibria and the carbonate/hydrogencarbonate balance. A buffer resists change within its capacity; it does not lock pH at a constant value.