pH Calculator
pH, pOH, [H⁺] and [OH⁻]; strong and weak acids and bases; buffers.
About this tool
Choose a mode. Convert works between pH, pOH, [H⁺] and [OH⁻]. Strong acid or base takes the concentration and the number of H⁺ or OH⁻ ions per formula unit. Weak acid or base takes the concentration and pKa or pKb, with presets such as acetic acid and ammonia. Buffer takes the pKa and the concentrations of the acid and its conjugate base. Results show pH, pOH, both ion concentrations and, for weak species, the fraction ionised, with the result marked on a colour pH scale.
How pH is calculated
pH is the negative base-10 logarithm of the hydrogen-ion concentration in moles per litre: pH = −log₁₀[H⁺]. Because the scale is logarithmic, each unit is a tenfold change, so pH 3 is ten times more acidic than pH 4. At 25 °C, water's ion product makes pH + pOH = 14. Strong acids and bases dissociate completely, so 0.01 M hydrochloric acid has pH 2 and 0.01 M sodium hydroxide has pH 12. The calculator includes water's own ions, so a very dilute acid at 10⁻⁸ M correctly gives pH 6.98 rather than an impossible 8. Weak acids only partly ionise. The exact quadratic from Ka gives 0.1 M acetic acid (pKa 4.76) a pH of about 2.88, with only about 1.3% of the acid ionised. Buffers resist pH change, and the Henderson–Hasselbalch equation, pH = pKa + log([A⁻] ÷ [HA]), gives 4.94 for 0.15 M acetate with 0.10 M acetic acid. Honest limits: these are ideal results at 25 °C. Concentrated solutions deviate because of activity effects, temperature shifts the neutral point, and polyprotic acids are simplified here. The dilution calculator prepares solutions, the molar mass calculator converts grams to moles and the logarithm calculator explains the maths. How to calculate percentages is a gentle refresher.
Frequently asked questions
How do I calculate pH from concentration?
For a strong acid, pH = −log₁₀ of the H⁺ concentration. 0.01 M HCl gives pH 2. Weak acids need the Ka, using the quadratic or this calculator.
What is the pH of 0.1 M acetic acid?
About 2.88, using pKa 4.76. Only about 1.3% of the acid molecules are ionised.
What is the Henderson–Hasselbalch equation?
pH = pKa + log([A⁻] ÷ [HA]). It gives the pH of a buffer from the acid's pKa and the ratio of base to acid.
Is my data saved?
No. Everything runs in your browser and nothing is stored or sent.