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Short answer
In a titration with a weak acid and a strong base, the pH starts higher than for a strong acid, climbs quickly and then slowly through a buffer region, jumps steeply at the equivalence point, and levels off in excess base. The equivalence point sits above pH 7 because the conjugate base of the weak acid reacts with water. For 0.100 M acetic acid titrated with 0.100 M NaOH it is 8.72, so phenolphthalein is a suitable indicator and litmus or methyl orange are not.
Definition and scope
This page covers the titration of a monoprotic weak acid (HA) with a strong base such as NaOH. The reaction is HA + OH– to A– + H2O. The titration curve plots pH against the volume of base added. Reading the acid dissociation constant from the half-equivalence point is treated separately in half-titration point; this page covers the whole curve, which calculation applies at each stage, and how to choose an indicator. For the wider field, see analytical chemistry and the related method of gravimetric titration.
What happens at each stage of the curve
The weak-acid curve starts at a higher pH than a strong-acid curve of the same concentration, because the acid is only partially ionised, and it stays higher until the equivalence point (LibreTexts, College of the Canyons CHEM 202, section 6.05). The curve then passes through four distinct stages, and each uses a different calculation (LibreTexts, Titration of a Weak Acid with a Strong Base).
| Stage | What is in the flask | Calculation | 0.3 M HF (Ka 6.6 x 10-4), 25 mL, 0.3 M NaOH |
|---|---|---|---|
| Before any base | Weak acid only | ICE table for the partial ionisation of HA | pH 1.86 |
| Before equivalence | HA and A– (a buffer) | Henderson-Hasselbalch, within its limits | pH 3.00 at 10 mL; 3.18 at 12.50 mL |
| At equivalence | A– only | Kb = Kw/Ka, ICE table, pOH, then pH = 14 – pOH | pH 8.18 at 25 mL |
| After equivalence | A– plus excess OH– | Concentration of the excess strong base | pH 11.77 at 26 mL |
There is a quick initial rise in pH as the first base is added, caused by the common-ion effect of the conjugate anion, then a gradual rise through the buffer region. At half-neutralisation [HA] equals [A–] and pH equals pKa; in the HF example 12.50 mL is half of 25 mL and the pH of 3.18 matches -log(6.6 x 10-4).
When Henderson-Hasselbalch can be used
The same source states that the Henderson-Hasselbalch equation applies only if the ratio of base to acid is between 0.10 and 10 and both [A–]/Ka and [HA]/Ka exceed 100. Close to the start and close to equivalence one of those conditions fails, and the ICE-table route is needed instead. After equivalence the curve follows the strong-acid curve, because the pH is set by the excess hydroxide.
Worked example: weak acid strong base titration
LibreTexts (section 6.05) works through 25.0 mL of 0.100 M acetic acid titrated with 0.100 M NaOH, with equivalence after 25.0 mL of base. The equivalence pH is 8.72, against 7.00 for a strong acid and strong base. The curve starts at about pH 3. Approximate points on the published curve:
| 0.100 M NaOH added | Approximate pH |
|---|---|
| 5 mL | about 4.1 |
| 10 mL | about 4.7 |
| 15 mL | about 5.0 |
| 20 mL | about 5.5 |
| 22.5 mL | about 6.0 |
| 25.0 mL (equivalence) | 8.72 |
| 35 mL | about 12.4 |
| 50 mL | about 12.6 |
The HF table above gives exact values at every stage, which makes it a useful check when you set up the arithmetic yourself. A further example in the same source uses 15 mL of 0.15 M acetic acid with 0.1 M NaOH, with equivalence at 22.5 mL, which follows from 15 mL x 0.15 M = 2.25 mmol divided by 0.1 M.
Why the equivalence point is above pH 7
At equivalence all the acid has been converted to its conjugate base. That base hydrolyses water, producing hydroxide, so the solution is basic. LibreTexts puts it as “The pH at the equivalence point is also higher (8.72 rather than 7.00)”. To calculate it, convert Ka to Kb with Kb = Kw/Ka, solve for [OH–] with an ICE table, take the pOH, and subtract from 14.
Choosing an indicator
The indicator must change colour across a pH range that includes the steep part of the curve. Checking three common indicators against the acetic acid curve (LibreTexts, section 6.05):
| Indicator | Approximate range | Behaviour on this curve | Suitable? |
|---|---|---|---|
| Phenolphthalein | about pH 8.4 to 10 | Brackets the equivalence pH of 8.72 | Yes |
| Litmus | about pH 4.6 to 8 | Changes gradually over roughly 12 to 25 mL of base, not sharply at equivalence | No |
| Methyl orange | about pH 3 to 4.2 | Changes within the first 8 mL or so, well before equivalence | No |
Phenolphthalein is colourless when [H3O+] is above 5.0 x 10-9 M and pink in more basic solution. Choose the indicator from the expected equivalence pH, not from habit.
Where it matters for labs
Before running a titration, calculate the expected equivalence pH from the acid’s Ka and the concentrations in use, then confirm the indicator range brackets it. Titrations also involve a strong base and an acid, so review your handling controls first; see PPE selection for chemical handling and types of laboratory glassware for the burette and flask side of the work.
Common misunderstandings
- “Equivalence means pH 7.” That holds for a strong acid with a strong base. With a weak acid it is above 7 (8.72 for 0.100 M acetic acid).
- “Any indicator that changes near neutral will do.” Litmus changes gradually over a wide volume and methyl orange changes early, so neither marks the equivalence point of the acetic acid curve.
- “Use Henderson-Hasselbalch everywhere.” It is valid only within the ratio and 100-fold limits above.
- “The curve after equivalence depends on the weak acid.” It matches the strong-acid curve, since the pH is set by excess hydroxide.
FAQ
What is an example of a weak acid strong base titration?
Acetic acid titrated with sodium hydroxide: 25.0 mL of 0.100 M acetic acid needs 25.0 mL of 0.100 M NaOH, and the equivalence pH is 8.72. Approximate points along the curve are in the worked example above.
Why does the pH rise quickly at first?
The conjugate anion formed by the first additions suppresses further ionisation of the acid (the common-ion effect), so the pH jumps before settling into the buffer region.
Which indicator suits a weak acid strong base titration?
One whose range brackets the equivalence pH. For acetic acid and NaOH that is phenolphthalein.
Sources
- LibreTexts, College of the Canyons CHEM 202, 6.05 Acid-Base Titrations: https://chem.libretexts.org/Courses/College_of_the_Canyons/CHEM_202%3A_General_Chemistry_II_OER/06%3A_Acid-Base_Equilibria_in_Mixtures/6.05%3A_Acid-Base_Titrations, checked 2026-10-29.
- LibreTexts, Titration of a Weak Acid with a Strong Base: https://chem.libretexts.org/Bookshelves/Ancillary_Materials/Demos_Techniques_and_Experiments/General_Lab_Techniques/Titration/Titration_of_a_Weak_Acid_with_a_Strong_Base, checked 2026-10-29.
