# Blood plasma is maintained at a pH of approximately 7.4 by a carbonic acid / hydrogencarbonate buffer system. The relevant equilibrium is: H₂CO₃(aq) ⇌ H⁺(aq) + HCO₃⁻(aq). Explain how this buffer system resists a change in pH when a small amount of hydrochloric acid is added to blood plasma.

> Edexcel A-Level Chemistry (9CH0) — 11.1 pH, Ka and buffer solutions · Explain · 5 marks

> Blood plasma is maintained at a pH of approximately 7.4 by a carbonic acid / hydrogencarbonate buffer system. The relevant equilibrium is: H₂CO₃(aq) ⇌ H⁺(aq) + HCO₃⁻(aq).

## Mark scheme (5 marks)

1. Hydrochloric acid fully dissociates / the added acid provides extra H⁺ ions
2. The hydrogencarbonate ions (HCO₃⁻) react with / remove the added H⁺ ions
3. The equilibrium shifts to the left
4. The concentration of H⁺ ions remains approximately constant / the pH remains approximately 7.4
5. The buffer only works for small / limited amounts of added acid, not large quantities

## Key terms

- [equilibrium](https://www.gradenine.co.uk/glossary/equilibrium)
- [pH](https://www.gradenine.co.uk/glossary/ph)

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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