Interpreting pH Curves (HL) (HL IB Chemistry)

Four Types of Acid-Base Titrations

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 intercept on the y-axis starts at a low pH, roughly 1, due to the relative strength of the hydrochloric acid
• As the NaOH (aq) is added, there is a gradual rise in pH until the titration approaches the equivalence point
• In this case, the pH at equivalence is 7
  • The equivalence point is in the middle of the vertical section of the pH curve
• Once all of the acid has been neutralised, the curve flattens out and continues to rise gradually
• At the end of the titration, the pH will be high due to the relative strength of the 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 on the intercept on the y-axis starts at roughly 3 due to the relative strength of the ethanoic acid
• The initial rise in pH is steep as the neutralisation of the weak acid by the strong base is rapid
• Ethanoate ions (conjugate base to ethanoic acid) are formed which then creates a buffer
  • A buffer consists of a weak acid and its conjugate base or a weak base and its conjugate acid
• At this point, the buffer formed will resist changes in pH so the pH rises gradually as shown in the buffer region
• 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)]
  • At this point, it is important to note that the pK_a of the acid is equal to the pH
    • pK_a = pH at half equivalence 
• The equivalence point in a weak acid - strong base titration is above 7

Diagram to show a strong acid - strong base pH curve

Weak acid - strong base pH curve. The equivalence point is above pH 7

Weak Base + Strong Acid

• In this example, strong hydrochloric acid, HCl (aq), is being added to weak ammonia, NH₃ (aq)

NH₃ (aq) + HCl (aq) → NH₄Cl (aq)

• The pH on the intercept on the y-axis starts at roughly 11 due to the relative strength of the ammonia
• The pH will fall as the ammonia begins to be neutralised and the conjugate acid, NH₄⁺ (aq), is produced
• This again creates a buffer region so the pH will only fall gradually
• The half equivalence point is the stage of the titration at which exactly half the amount of weak base has been neutralised
  • [NH₃ (aq)] = [NH₄⁺ (aq)]
  • At this point, it is important to note that the pK_b of the base is equal to the pOH
  • pK_b = pOH at half equivalence 
• The pH at equivalence for a weak base-strong acid is below 7

Diagram to show a strong acid - weak base pH curve

Weak base - strong acid pH curve. The equivalence point is below pH 7

Weak Acid + Weak Base

• In this example, weak ethanoic acid, CH₃COOH (aq), is being added to weak ammonia, NH₃ (aq)

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

• The starting pH of roughly 11 for the weak base will fall as it begins to neutralise
• The change in pH for this titration is very gradual
  • Note that the vertical section of this pH curve is not steep as with the other three so the equivalence point is difficult to determine
  • Therefore this titration is not performed
• The pH at equivalence for a weak acid-weak base is roughly 7 but it is difficult to determine

Diagram to show a weak acid - weak base pH curve

Weak acid - weak base pH curve. The equivalence point is difficult to determine