Acid–Base Titration

How the model works

The base neutralises the acid: HA + OH⁻ → A⁻ + H₂O. The equivalence point is reached when moles of base added equal the moles of acid present at the start:

V_eq = C_acid · V_acid / C_base

The simulator calculates pH exactly at every volume. It solves the full charge balance [H⁺] + [Na⁺] = [OH⁻] + [A⁻] with Ka and Kw = 10⁻¹⁴, rather than piecing together approximations.

• Strong acid (HCl): pH stays low, then jumps steeply around 7 at equivalence.
• Weak acid (acetic, pKa 4.76): the curve starts higher and has a flat buffer region. At half-equivalence, pH = pKa. The equivalence point lies above 7, because acetate is a weak base.

A good indicator changes colour within the steep part of the curve.

Assumes 25 °C, ideal dilute solutions (activities equal concentrations), a monoprotic acid and perfect mixing.

Guided experiments

1. Strong acid. 25 mL of 0.10 M HCl with 0.10 M NaOH. Prediction: equivalence at 25.0 mL and pH 7.00, with a jump from about pH 4 to 10 within ±0.1 mL. Titrate HCl
2. Weak acid and half-equivalence. Acetic acid, same amounts. Prediction: at 12.5 mL, pH = 4.76 = pKa. Equivalence is still at 25.0 mL but at pH ≈ 8.72. Titrate acetic acid
3. Wrong indicator. Weak acid with methyl orange (3.1–4.4). Prediction: it changes colour far too early, inside the buffer region. Phenolphthalein (8.2–10) matches. Use methyl orange

Challenge

20.0 mL of an unknown HCl solution needs 16.4 mL of 0.125 M NaOH to reach equivalence. What is the acid's concentration?

References

• OpenStax, Chemistry 2e, §14.7 “Acid-Base Titrations”.