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Buffer Solutions And Ocean Acidification

Learn and apply Buffer Solutions And Ocean Acidification in the published Chemistry course sequence.

  • GCE A-Level H1 Chemistry 8873-2027
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Buffer Solutions and Ocean Acidification: Orientation

H1 requires qualitative buffer reasoning: identify the pair, then show which member removes added H⁺ or OH⁻.

H1 8873 scope
  • Explain buffer action and carbonate/hydrogencarbonate ocean buffering; buffer-pH calculations are excluded.

Definitions (Must Know)

  • A buffer solution resists large pH changes when small amounts of acid or base are added.
  • An acidic buffer contains a weak acid and a substantial concentration of its conjugate base.

Detailed Explanations

For HA/A−, added acid is consumed by A− + H+ → HA. Added base is consumed by HA + OH− → A− + H2O.

Additional atmospheric CO2 dissolves and contributes to carbonic-acid formation. The resulting equilibria increase [H+] and consume carbonate ions, lowering ocean pH.

A buffer limits pH change; it does not prevent change. Its capacity is finite because one component can be consumed.

In seawater, dissolved carbon dioxide shifts carbonate equilibria, increasing [H⁺] and reducing carbonate-ion availability.

Worked Examples

Modelled example 1

Explain how a buffer removes added acid

Core

Problem

Explain how an ethanoic acid/ethanoate buffer responds to a small addition of hydrochloric acid.
Study the worked solution

  1. Identify the added species

    Method

    Treat hydrochloric acid as a source of added H⁺.

    Reason

    It is the increase in hydrogen-ion concentration that the buffer must limit.

    Working

    The disturbance is added H⁺.

  2. Choose the removing component

    Method

    Use ethanoate, the conjugate-base member of the pair.

    Reason

    A conjugate base accepts a proton.

    Working

    CH₃CO₂- + H⁺ → CH₃CO₂H.

  3. State the pH effect

    Method

    Conclude that the pH falls only slightly for a small addition.

    Reason

    Most of the added H⁺ is converted to weak ethanoic acid rather than remaining free in solution.

    Working

    The increase in [H⁺] is much smaller than in unbuffered water.

Guided practice 2

Explain both buffer responses

About 6 min

Problem

A buffer contains HA and A⁻. Explain its separate responses to small additions of hydrochloric acid and sodium hydroxide.

Try this before viewing the solution

Hints

Hint 1: assign one component to each addition
The conjugate base accepts added H⁺; the weak acid supplies a proton to added OH⁻.
Hint 2: write two equations
Write one equation that forms HA and another that forms A⁻ + H₂O.
View solution step by step

  1. Respond to added acid

    Method

    Use A⁻ to remove H⁺.

    Reason

    A⁻ is the proton-accepting member of the conjugate pair.

    Working

    A⁻ + H⁺ → HA.

  2. Respond to added base

    Method

    Use HA to remove OH⁻.

    Reason

    The weak acid donates a proton, converting hydroxide to water.

    Working

    HA + OH⁻ → A⁻ + H₂O.

  3. Connect both reactions to pH

    Method

    State that each small disturbance is largely consumed.

    Reason

    Both members are present in appreciable amounts and can respond in opposite directions.

    Working

    The pH changes, but by much less than it would without the buffer pair.

Common misconception 3

Separate resistance from perfect constancy

Find and correct the mistake

Learner claim

A learner says an HA/A⁻ buffer keeps exactly the same pH however much acid is added because A⁻ removes all added H⁺. Correct the claim.

Choose the correct boundary

A buffer can

View solution step by step

  1. Correct “exactly the same”

    Method

    Replace prevention of change with resistance to large change.

    Reason

    Consuming added acid changes the relative amounts of HA and A⁻.

    Working

    The pH normally changes slightly even while buffering is effective.

  2. Apply the capacity limit

    Method

    Restrict the claim to small additions while both components remain appreciable.

    Reason

    The supply of A⁻ is finite and can be consumed by added H⁺.

    Working

    After enough acid is added, too little A⁻ remains and the pH can fall sharply.

Examiner practice 4

Explain ocean acidification as an equilibrium chain

4 marks

Problem

Explain how increasing atmospheric carbon dioxide can lower ocean pH and reduce the concentration of carbonate ions, CO₃²⁻. [4 marks]

Try this before viewing the solution

View solution step by step

  1. Dissolve more carbon dioxide

    1 mark

    Method

    Link the atmospheric increase to dissolved CO₂.

    Reason

    Gas exchange allows a higher atmospheric amount to drive more carbon dioxide into seawater.

    Working

    Atmospheric CO₂ increases, so dissolved CO₂ increases.

  2. Form acidic species

    1 mark

    Reason

    Dissolved carbon dioxide contributes to carbonic-acid formation.

    Working

    CO₂(aq) + H₂O(l) ⇌ H₂CO₃(aq).

  3. Increase hydrogen ions

    1 mark

    Reason

    Carbonic acid dissociation shifts the carbonate system toward more H⁺.

    Working

    H₂CO₃(aq) ⇌ H + (aq) + HCO₃-(aq), so ocean pH decreases.

  4. Consume carbonate

    1 mark

    Method

    Show added hydrogen ions converting carbonate to hydrogencarbonate.

    Reason

    This removes CO₃²⁻ from the carbonate pool.

    Working

    H + (aq) + CO₃²⁻(aq) → HCO₃-(aq), so carbonate-ion concentration decreases.

Challenge 5

Interpret acidification while seawater remains alkaline

Minimal support

Problem

Over several decades, a seawater sample’s mean pH falls from 8.20 to 8.05 while its carbonate-ion concentration also decreases. A learner says this is not acidification because both pH values exceed 7, and that the change proves the carbonate buffer did nothing. Assess both claims.

Try this before viewing the solution

Hints

Hint 1: interpret the direction
Ocean acidification describes a decrease in pH; it does not require the final pH to be below 7.
Hint 2: interpret buffering
A buffer limits a change rather than preventing it, and its components are altered while they absorb a disturbance.
View solution step by step

  1. Interpret the pH data

    Method

    Identify the downward pH change as acidification.

    Reason

    The term describes movement toward lower pH, not necessarily crossing the neutral value.

    Working

    The sample remains alkaline at pH 8.05 but has become less alkaline than at pH 8.20.

  2. Interpret the buffer

    Method

    Reject the claim that any pH change proves zero buffering.

    Reason

    Buffering resists large changes and has finite capacity; it does not hold pH exactly constant.

    Working

    The carbonate system can moderate the fall while the pH still changes.

  3. Use the carbonate evidence

    Method

    Connect the lower carbonate concentration to hydrogen-ion uptake.

    Reason

    H⁺ converts CO₃²⁻ into HCO₃- as the system responds.

    Working

    The observed carbonate decrease is consistent with the buffering chemistry, not evidence that nothing happened.

Mind Stretchers

Attempt the independent prompts before opening a hint or solution.

  • Write equations showing how an NH3/NH4+ buffer responds to added H+ and OH−.
  • Explain why rising atmospheric CO2 can reduce the concentration of CO3²− in seawater.

Mind stretcher 1: Buffer capacityExtension

Question. Two buffers contain the same HA:A⁻ ratio, but one is ten times more concentrated. Predict which better resists the same added amount of acid.

Show Hint

The ratio affects initial pH; total amounts affect capacity.

Show Answer

The more concentrated buffer has larger amounts of A⁻ available to remove added H⁺, so it has greater buffer capacity even though the starting pH may be similar.

Mind stretcher 2: Ocean transferExtension

Question. Explain why increasing atmospheric CO₂ can make calcium-carbonate shell formation more difficult.

Show Hint

Link dissolved CO₂ to H⁺ and then carbonate ions.

Show Answer

More CO₂ dissolves and forms acidic species, increasing [H⁺]. H⁺ reacts with CO₃²⁻ to form HCO₃⁻, lowering [CO₃²⁻] available for CaCO₃ formation.