Titration curves — the shape of neutralisation

By the end of this lesson

By the end you can sketch and interpret pH titration curves for strong acid–strong base, strong acid–weak base, weak acid–strong base, and weak acid–weak base titrations, identifying equivalence points, half-equivalence points, and buffer regions.

What is a titration curve?

A titration curve plots pH (y-axis) vs volume of titrant added (x-axis).

Key features to recognise:

Initial pH (before any titrant added)
Buffer region (flat-ish part where pH changes slowly)
Half-equivalence point (pH = pKa for weak acid titrations)
Equivalence point (steep vertical jump, stoichiometric point)
Final pH (after excess titrant)

The four combinations

Titration	Initial pH	Equivalence pH	Vertical jump	Buffer region?
Strong acid + Strong base	Low (~1)	7	Large (~3–10 pH units)	No
Weak acid + Strong base	Medium (~3)	>7 (basic)	Medium (~6–10)	Yes (pH = pKa ± 1)
Strong acid + Weak base	Low (~1)	<7 (acidic)	Medium (~3–7)	Yes (pH = pKa ± 1)
Weak acid + Weak base	Medium	Depends on Ka/Kb	Small/none	Yes (two overlapping)

Strong acid + Strong base (e.g. HCl + NaOH)

Initial: pH low (e.g. 0.1 M HCl → pH 1)
Before equivalence: pH rises slowly (dilution effect)
Near equivalence: Very steep jump (pH 3 → 11 over 0.1 mL)
Equivalence point: pH = 7 exactly (neutral salt NaCl)
After equivalence: pH levels off at high value (excess OH⁻)

The jump is huge because [H⁺] changes from 10⁻³ M to 10⁻¹¹ M — 8 orders of magnitude!

Weak acid + Strong base (e.g. CH₃COOH + NaOH)

Initial: pH higher than strong acid (e.g. 0.1 M CH₃COOH → pH ~2.9)
Buffer region: Flat section where CH₃COOH/CH₃COO⁻ buffer resists pH change
Half-equivalence: pH = pKa (here pH = 4.76). [HA] = [A⁻]
Equivalence point: pH > 7 (basic salt CH₃COONa hydrolyses)
Jump: Smaller than strong–strong (pH ~7 → 10)
After equivalence: Same as strong–strong (excess OH⁻)

Half-equivalence = the buffer's sweet spot

At half-equivalence, exactly half the weak acid has been neutralised: $[\text{HA}] = [\text{A}^-]$

Henderson–Hasselbalch: $\text{pH} = \text{p}K_a + \log_{10}\frac{[\text{A}^-]}{[\text{HA}]} = \text{p}K_a + \log_{10}1 = \text{p}K_a$

This is the best buffer capacity — equal amounts of acid and conjugate base.

Strong acid + Weak base (e.g. HCl + NH₃)

Mirror image of weak acid + strong base:

Initial: pH low (strong acid)
Buffer region: NH₄⁺/NH₃ buffer (flat section)
Half-equivalence: pH = pKa(NH₄⁺) = 9.25 (or pOH = pKb(NH₃))
Equivalence point: pH < 7 (acidic salt NH₄Cl hydrolyses)
Jump: Medium (pH ~4 → 7)

Weak acid + Weak base (e.g. CH₃COOH + NH₃)

No sharp equivalence point jump — the curve is shallow throughout
Two buffer regions overlap (CH₃COOH/CH₃COO⁻ and NH₄⁺/NH₃)
Equivalence point pH depends on relative Ka and Kb
Hard to detect equivalence point with an indicator — not used in standard titrations

Titration curve features

Multiple choiceQuestion 1 · Apply

Multiple choiceQuestion 2 · Apply

CalculateQuestion 3 · Apply

Answer the 3 questions above to continue.