Neutralization and titration
Add 0.100 M sodium hydroxide to 25.00 mL of 0.100 M hydrochloric acid and the pH jumps from 3.70 to 10.30 between 24.9 mL and 25.1 mL.
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When an acid meets a base, the hydronium ions from the acid and the hydroxide ions from the base combine to form water: H₃O⁺ + OH⁻ → 2H₂O. The other ions are left behind as a salt. Hydrochloric acid and sodium hydroxide give sodium chloride and water, HCl + NaOH → NaCl + H₂O. Sulfuric acid, with two protons to give, needs two sodium hydroxide units: H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O. This kind of reaction is called neutralization.
Antacids in the stomach
The stomach holds a solution of roughly 0.03 M hydrochloric acid, which helps digest food; that alone would give a pH of about 1.5. Heartburn happens when some of this acid leaks up into the oesophagus, whose lining is not protected against it. Antacids are bases that neutralize the excess. Milk of magnesia is a suspension of magnesium hydroxide: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O. Each unit of Mg(OH)₂ neutralizes two units of HCl. OpenStax notes that magnesium antacids can have a laxative effect and aluminium hydroxide tends to cause constipation, so some products combine the two. This is general information, not medical advice.
Titration: measuring an unknown acid
Neutralization gives chemists a way to measure concentration. In a titration a solution of known concentration, the titrant, is added little by little from a burette to a measured volume of the unknown solution until the two have reacted in exactly the ratio of the equation. That moment is the equivalence point. If 25.00 mL of 0.100 M HCl contains 0.100 × 0.02500 = 0.00250 mol of acid, it takes exactly 0.00250 mol of NaOH to neutralize it, which is 25.00 mL of 0.100 M NaOH.
Watch the pH in the lab as you add base. At first it rises slowly: after 20.0 mL it is only 1.95. Then, close to the equivalence point, it shoots up. Between 24.9 mL and 25.1 mL, a difference of a few drops, the pH jumps from 3.70 to 10.30. For a strong acid and a strong base the equivalence point is at pH 7.00, because sodium chloride does not react with water. After that the pH is set by the extra hydroxide and levels off, reaching 12.52 at 50.0 mL.
Weak acids and choosing an indicator
Switch the lab to acetic acid. The curve starts higher, at pH 2.87, and rises more gently. Halfway to the equivalence point, at 12.50 mL, half the acid has become acetate and the pH equals the pKa, about 4.74. At the equivalence point the flask contains sodium acetate, and acetate is a weak base, so the pH is 8.72, not 7. That matters for the indicator. Phenolphthalein, changing between 8.3 and 10, brackets the steep part of both curves. Methyl orange would change colour far too early for acetic acid, over the first few millilitres, so it would give a wrong answer.
| NaOH added (mL) | pH, 0.100 M HCl | pH, 0.100 M acetic acid |
|---|---|---|
| 0.0 | 1.00 | 2.87 |
| 12.5 | 1.48 | 4.74 |
| 24.9 | 3.70 | 7.14 |
| 25.0 | 7.00 | 8.72 |
| 25.1 | 10.30 | 10.30 |
| 50.0 | 12.52 | 12.52 |
- HCl + NaOH → ? + H₂O. What is the salt?
- NaCl
- NaH
- ClOH
- Na₂O
- How many moles of HCl are in 25.00 mL of 0.100 M HCl?
- What volume of 0.200 M NaOH neutralizes 25.00 mL of 0.100 M HCl, in mL?
- 20.00 mL of HCl needs 16.00 mL of 0.100 M NaOH. What is the HCl concentration in M?
- What is the pH at the equivalence point of HCl titrated with NaOH?
- Why is the equivalence point of acetic acid with NaOH above 7?
- Acetate ions are a weak base
- Acetic acid is strong
- NaOH is weak
- Water boils
- Which indicator suits the acetic acid titration?
- Phenolphthalein
- Methyl orange
- None can work
- Any indicator
- How many moles of HCl does 0.010 mol of Mg(OH)₂ neutralize?
- At the half-equivalence point of a weak acid titration, pH equals…
- pKa of the acid
- 7
- 14
- pKw
- Stomach acid is roughly 0.03 M HCl. What is its pH, to one decimal place?
- Neutralization: acid + base → salt + water; the net reaction is H₃O⁺ + OH⁻ → 2H₂O.
- Titration finds an unknown concentration from the volume of titrant at the equivalence point.
- Strong acid with strong base: equivalence at pH 7.00. Weak acid with strong base: above 7 (8.72 for acetic acid), so choose phenolphthalein.
Sources for this course (10)
- OpenStax Chemistry 2e (openstax.org/books/chemistry-2e): 4.2 neutralization and antacids (stomach roughly 0.03 M HCl, milk of magnesia Mg(OH)2, Al(OH)3)
- 14.1 Brønsted-Lowry acids and bases (Boyle 1680, Davy 1815, Arrhenius 1884, Brønsted and Lowry 1923, autoionization about two in a billion molecules, Kw = 1.0 × 10⁻¹⁴ at 25 °C, about 5.6 × 10⁻¹³ at 100 °C)
- 14.2 pH and pOH (pH = −log[H3O+], stomach acid 1.2 × 10⁻³ M gives 2.92, vinegar about 4 × 10⁻³ M gives 2.4, blood 7.3, KOH 0.0125 M gives 12.10, neutral pH 6.31 at 80 °C, rainwater 5 to 6, acid rain below 5, pH meter resolution 0.001, Figure 14.2: 1 M HCl 0, gastric juice about 1.5, lime juice about 2, wine about 3.5, coffee about 5, milk of magnesia about 10.5)
- 14.3 relative strengths (six strong acids and six strong bases, Ka, percent ionization, 0.100 M acetic acid 1.3%)
- 14.6 buffers (normal blood pH about 7.4)
- 14.7 titrations (25.00 mL of 0.100 M HCl or acetic acid with 0.100 M NaOH, Table 14.2, equivalence pH 7.00 and 8.72, half-equivalence pH = pKa, Ka acetic acid 1.8 × 10⁻⁵, methyl orange Ka 4.0 × 10⁻⁴ red HIn and yellow In⁻, interval about pKa ± 1, phenolphthalein colourless below pH 8.3)
- LibreTexts Acid-Base Indicators (litmus about 5 to 8, methyl orange 3.1 to 4.4, phenolphthalein 8.3 to 10.0; chem.libretexts.org) and bromothymol blue 6.0 to 7.6 yellow to blue (Prairie South indicator table via LibreTexts Chemistry 30)
- PubChem acetic acid pKa 4.76 at 25 °C, sodium bicarbonate 0.1 M pH 8.3, ammonia 0.1 N pH 11.1, sodium hydroxide (pubchem.ncbi.nlm.nih.gov)
- NOAA Ocean acidification (noaa.gov: surface ocean pH has fallen by 0.1 since the industrial revolution, about a 30 percent increase in acidity)
- IUPAC CIAAW abridged standard atomic weights.
Checked: 2026-10-17