Interpreting pH Curves (HL) (DP IB Chemistry): Revision Note

Interpreting pH curves

• There are four types of acid-base titration that you should know about:

  • Strong acid + strong base

  • Weak acid + strong base

  • Weak base + strong acid

  • Weak acid + weak base

Strong acid + strong base

• In this example, sodium hydroxide, NaOH (aq), is being added to hydrochloric acid, HCl (aq)

HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l)

• The pH starts low (around 1) due to the strong acid

• As NaOH is added, pH rises steadily until just before the equivalence point

• The equivalence point occurs at pH 7, in the middle of the vertical section of the curve

• After neutralisation, the pH increases more gradually as excess base is added

• The final pH is high due to the strength of the excess sodium hydroxide

Diagram to show a strong acid - strong base pH curve

Strong acid - strong base pH curve. The equivalence point is at pH 7

Weak acid + strong base

• In this example, strong sodium hydroxide, NaOH (aq), is being added to weak ethanoic acid, CH₃COOH (aq)

NaOH (aq) + CH₃COOH (aq) → CH₃COONa (aq) + H₂O (l)

• The pH starts around 3, due to the weak acid

• pH rises sharply as the weak acid is neutralised by the strong base

• Ethanoate ions (CH₃COO⁻) are formed, creating a buffer

• The buffer region resists changes in pH and causes a gradual rise in the curve

• The half equivalence point is the stage of the titration at which exactly half the amount of weak acid has been neutralised

  • [CH₃COOH (aq)] = [CH₃COO^- (aq)] and pK_a = pH at half equivalence 

• The equivalence point is above pH 7

Diagram to show a weak acid - strong base pH curve

Weak base + strong acid

• In this titration, hydrochloric acid (HCl) is added to ammonia (NH₃)

NH₃ (aq) + HCl (aq) → NH₄Cl (aq)

• The pH starts around 11, due to the weak base

• As acid is added, pH gradually falls as NH₄⁺ forms

• A buffer region forms where pH changes slowly

• At the half-equivalence point:

  • [NH₃ (aq)] = [NH₄⁺ (aq)] and pK_b = pOH at half equivalence 

• The equivalence point is below pH 7

Diagram to show a strong acid - weak base pH curve

Weak acid + weak base

• Ethanoic acid (CH₃COOH) is added to ammonia (NH₃)

NH₃ (aq) + CH₃COOH (aq)→ CH₃COONH₄ (aq)

• pH starts around 11 and gradually falls as neutralisation occurs

• The vertical section is not steep, so the equivalence point is hard to identify

• This titration is not usually performed

• The equivalence point is roughly pH 7, but it is difficult to determine accurately

• There is no suitable indicator:

  • The pH change at the equivalence point is too gradual for any indicator to show a sharp colour change

Diagram to show a weak acid - weak base pH curve