pH Calculator
Calculate ideal equilibrium pH for strong or weak acids and bases or conjugate buffers. Explore dilution with a curve that includes the contribution of water.
Find the pH of an acid, base or conjugate buffer, then explore what happens when you dilute it. This ideal model includes water's contribution, even near neutral pH. Calculations stay in your browser.
Original solution · dilution 1×
Acidic · ideal model at 25 °C
Before dilution: pH 1 → now 1
Strong acid · initial 0.1 mol/L equivalents
Fixed 25 °C, Kw = 10⁻¹⁴. Concentrations approximate activities. At appreciable ionic strength, measured pH can differ; this is an ideal equilibrium estimate, not an exact laboratory recipe.
What does dilution change?
Final acid equivalent concentration: 0.1 mol/L. All initial components are divided by the same dilution factor.
Horizontal axis: log₁₀ of dilution factor, from 1× to 100,000,000×. Diluent is ideal pure water without dissolved CO₂; no extra acid, base or salts are added. This is a dilution curve, not a titration curve.
Equilibrium composition after dilution
Conjugate base A⁻
Undissociated HA
Mass balance includes both forms.
Start with an example
Illustrative solutions at 25 °C. Each example restores every setting, including dilution.
Model, formulas and limitations
pH = −log₁₀[H₃O⁺], pOH = −log₁₀[OH⁻], with [H₃O⁺][OH⁻] = 10⁻¹⁴ at 25 °C. For fully dissociated strong acid, [H₃O⁺] = (C + √(C² + 4Kw))/2, including water. Strong base uses the corresponding hydroxide expression. Enter equivalents directly: 0.02 mol/L Ba(OH)₂ supplies 0.04 mol/L OH⁻; sulfuric acid's second dissociation requires a different model.
For a weak acid HA and its conjugate salt MA, C = initial HA + initial A⁻, s = initial A⁻, and K = Ka. The positive root H + s = Kw/H + C·Ka/(Ka + H) satisfies electroneutrality and mass balance. A weak base B/BH⁺ uses the same balance in OH⁻ with Kb. Initial components are divided by D for dilution, and equilibrium is solved again.
The Henderson–Hasselbalch initial-ratio approximation pH ≈ pKa + log₁₀(initial A⁻/initial HA) can work for a buffer when dissociation changes those concentrations little. It may fail in dilute or extreme-ratio mixtures. This tool solves the equilibrium rather than forcing that approximation or assuming pH remains exactly unchanged after dilution.
One monoprotic acid/base pair only. No polyprotic phosphate/carbonate, mixed acid systems, activity correction, temperature calculation, pool dosing or biological interpretation. Use your own appropriate pKa/pKb at 25 °C. The numeric answer is a model estimate, not measurement precision.
References: OpenStax pH and pOH, weak acid/base equilibrium and buffers. For practical transfer volumes, use the serial dilution calculator; to determine concentration from measured reaction volumes, use the titration calculator.
