dilutioncalculator.co.uk§ Verified 20 Jun 2026 · Reviewed by A. Hargreaves, MChem (Hons)
pH shift on dilution
A 10× dilution of a strong acid raises pH by 1 unit, up to a limit set by the autoionisation of water. Weak acids dampen the shift via partial dissociation.
§ 1Strong acid
0.1 M HCl has pH 1. Diluted 10×: 0.01 M, pH 2. Each decade of dilution adds 1 pH unit until concentration approaches 10⁻⁷ M, where water's own hydronium contribution caps the rise. A 10⁻⁹ M HCl is not pH 9; it is pH 6.98.[9]
§ 2Weak acid
0.1 M acetic acid (pKa 4.76) has pH ≈ 2.87. Diluted 10×: pH ≈ 3.37, a shift of only 0.5 units. As the acid dilutes, the fraction dissociated rises, partially compensating for the lower total concentration.
§ 3Buffer behaviour
- A Henderson-Hasselbalch buffer near its pKa is largely dilution-stable for shifts under 5×.
- Outside the buffer region (pH ± 1 of pKa), dilution can shift pH sharply.
- Dilution never reverses an acid into base territory; the limit is pH 7 at 25 °C.