pH calculator
pH = −log₁₀[H⁺] and, at 25 °C, pH + pOH = 14. This calculator converts between pH, pOH, [H⁺] and [OH⁻], gives the pH of strong and weak acids and bases (solving the full equilibrium, including water), and calculates buffer pH or the base-to-acid ratio for a target pH with the Henderson–Hasselbalch equation.
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What it does
How to use the pH calculator
- Choose a calculatorConvert a value, or calculate for a strong acid/base, a weak acid/base or a buffer.
- Enter the valuesType the concentration and, for weak acids, bases and buffers, the pKa (or pick a common one).
- Read the resultGet the pH with pOH, [H⁺] and [OH⁻], plus the fraction ionised or the buffer ratio.
How it works
pH = −log₁₀[H⁺] pOH = −log₁₀[OH⁻] [H⁺][OH⁻] = Kw = 1.0 × 10⁻¹⁴ (25 °C)
Weak acid: [H⁺]³ + Ka[H⁺]² − (KaC + Kw)[H⁺] − KaKw = 0
Buffer: pH = pKa + log₁₀([A⁻]/[HA])
Strong acids are treated as fully dissociated with water’s own ions included, so a 10⁻⁸ M HCl solution correctly gives a pH just below 7. Weak acids and bases are solved exactly from the charge balance rather than with the √(KaC) shortcut. Concentrations stand in for activities, which is accurate for dilute solutions.
Worked example
0.1 M acetic acid (pKa 4.76): [H⁺] = 1.31 × 10⁻³ M, so pH = 2.88, with 1.3 % of the acid ionised. A phosphate buffer at pH 7.4 (pKa 7.20) needs [HPO₄²⁻]/[H₂PO₄⁻] = 100.2 = 1.58.
Frequently asked questions
How do you calculate pH from concentration?
For a strong acid, pH = −log₁₀ of the acid concentration (times the number of protons released). For a weak acid you also need Ka: [H⁺] ≈ √(Ka × C) when the acid is only slightly ionised, or solve the equilibrium exactly, as this calculator does.
What is the Henderson–Hasselbalch equation?
pH = pKa + log₁₀([A⁻]/[HA]). It gives the pH of a buffer from the ratio of base to acid forms, and buffers work best within one pH unit of the pKa.
Why is pH + pOH = 14?
Because water’s ion product Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C. At other temperatures Kw changes, so the sum is not exactly 14.
Can pH be negative?
Yes, for very concentrated strong acids (above 1 M), although activities then differ from concentrations and measured values deviate from the simple formula.
Does Tris buffer pH change with temperature?
Yes, strongly: Tris’s pKa drops by about 0.03 per °C, so set the pH at the temperature you will use it.
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