AP | A-Level | IB | NCERT 11 + 12 – FREE NOTES, RESOURCES AND VIDEOS!
OCR (A) A-Level
YouTube Channel
YouTube Channel
@chemistry.student
*Revision Materials and Past Papers* 2.1.1 Atomic structure and isotopes 2.1.2 Compounds, formulae and equations 2.1.3 Amount of substance 2.1.4 Acids 2.1.5 Redox 2.2.1 Electron structure 2.2.2 Bonding and structure 3.1.1 Periodicity 3.1.2 Group 2 3.1.3 The halogens 3.1.4 Qualitative analysis 3.2.1 Enthalpy 3.2.2 Reaction Rates 3.2.3 Chemical equilibrium 4.1 Basic concepts and hydrocarbons 4.1.2 Alkanes 4.1.3 Alkenes 4.2.1 Alcohols 4.2.2 Haloalkanes 4.2.3 Organic synthesis 4.2.4 Analytical techniques 5.1.1 How fast? 5.1.2 How far? 5.1.3 Acids, bases and buffers 5.2.1 Lattice enthalpy 5.2.2 Enthalpy and entropy 5.2.3 Redox and electrode potentials 5.3.1 Transition elements 5.3.2 Qualitative analysis 6.1.1 Aromatic compounds 6.1.2 Carbonyl compounds 6.1.3 Carboxylic acids and esters 6.2.1 Amines 6.2.2 Amino acids, amides and chirality 6.2.3 Polyesters and polyamides 6.2.4 Carbon–carbon bond formation 6.2.5 Organic synthesis 6.3.1 Chromatography and qualitative analysis 6.3.2 Spectroscopy Required Practicals

5.1.3 Acids, bases and buffers

Brønsted–Lowry acids and basesBuffers, action, uses and calculationsNeutralisationpH and [H+(aq)]

Buffers: action, uses and calculations

Specification Reference 5.1.3 (i)–(m)

Quick Notes

  • A Buffer solution resists pH change when small amounts of acid or base are added to it.
  • Buffers are typically made by:
    • Mixing a weak acid and its conjugate base (e.g. ethanoic acid + sodium ethanoate)
    • Mixing Excess weak acid with strong base, forming some salt in situ.
  • Acidic buffers maintains equilibrium:
    HA ⇌ H+ + A−
    • If acid (H+) is added: A− removes it by forming more HA.
    • If base (OH−) is added: HA donates H+ to neutralise OH−.
  • Buffers are important in biological, industrial, and laboratory settings.
  • In blood: carbonic acid–hydrogencarbonate buffer maintains pH ~7.4.

Full Notes

Buffers and calculations have been outlined in more detail here.
This page is just what you need to know for OCR A-level :)

A buffer solution maintains a relatively constant pH despite the addition of small amounts of acid or base. They ‘minimise’ change to pH.

Buffers are essential in biological systems and many industrial processes where a near constant pH is important. For example, in living organisms buffers maintain an optimum pH to prevent enzymes from being denatured.

Acidic Buffers:

Acidic buffers are made from a weak acid and its salt (that contains the acids conjugate base).

For example The weak acid ethanoic acid (CH3COOH) and its salt sodium ethanoate (CH3COONa).

AQA A-Level Chemistry diagram showing acidic buffer prepared from CH3COOH(aq) and CH3COONa(aq) producing a mixture containing CH3COOH and CH3COO−

When added to a solution of the ethanoic acid, the CH3COONa would dissociate and release CH3COO− ions, which is the conjugate base (A−) of the ethanoic acid.

How Acidic Buffers Work

An equilibrium is established in the buffer system between HA, A− and H+.

AQA A-Level Chemistry equilibrium HA(aq) ⇌ H+(aq) + A−(aq) for acidic buffer

The concentration of HA and A− in the mixture must be much greater than the concentration of H+. This ensures the position of equilibrium is sensitive to changes in H+ concentration change more than changes to HA and A− concentration. Equilibrium position can shift to keep H+ ion concentration nearly constant.

Example: Ethanoic Acid/Sodium Ethanoate Buffer
CH3COOH ⇌ H+ + CH3COO−

When an acid (H+) is added:

AQA A-Level Chemistry diagram showing A− reacting with added H+ to form HA in an acidic buffer

Buffers in Blood

Blood pH is controlled by a hydrogencarbonate buffer system:

H2CO3 ⇌ H+ + HCO3−

This maintains blood pH around 7.4.

Summary