pH calculator for acids, bases, buffers and titrations
This pH calculator covers the first-year acid–base syllabus in one place. Convert between pH, pOH, hydrogen-ion and hydroxide concentrations by typing any one of them; find the pH of a strong acid or base from its molarity, including very dilute solutions where water's own ionisation matters; solve weak acids and bases exactly from Ka or Kb with the quadratic equation and see whether the square-root shortcut is acceptable; work out buffer pH with the Henderson–Hasselbalch equation; and plot a strong acid–strong base titration curve.
A built-in table of about fifty acids and bases lists pKa values at 25 °C; click a row and its Ka or Kb is loaded into the weak-acid and buffer modes. Each answer shows the equation and the substituted numbers, so the page doubles as a worked example.
All calculations run in your browser and assume 25 °C, Kw = 1.0 × 10⁻¹⁴ and ideal behaviour (concentration in place of activity), the convention in textbooks and most lab work.
How to use the pH Calculator
- Choose a mode: conversion, strong acid/base, weak acid/base, buffer or titration curve.
- For a strong acid or base, type the molarity and pick the substance type (one or two protons or hydroxides).
- For a weak acid or base, enter the concentration and either Ka/Kb (1.8e-5 style notation is fine) or pKa/pKb — the fields stay in sync — or click a row of the pKa table.
- For a buffer, enter pKa and the amounts of conjugate base and acid; add a target pH to get the ratio you need.
- For a titration, enter the analyte concentration and volume and the titrant concentration; change the titrant volume to read the pH anywhere on the curve.
pH Calculator features
Exact weak-acid solution
The quadratic is solved exactly and the √(Ka·C) approximation is shown beside it with a 5 % rule check and the percent ionisation.
Water included for dilute strong acids
10⁻⁸ M HCl comes out at pH 6.98, not 8, because the hydrogen ions from water are counted.
Buffer design
Henderson–Hasselbalch pH, the usable pKa ± 1 range and, for a target pH, the base-to-acid ratio and amounts.
Titration curve
An SVG plot of pH against titrant volume with the equivalence point marked, for a strong acid titrated by a strong base or the reverse.
pKa reference table
About fifty acids, bases and biological buffers with pKa, Ka and type; one click loads the value.
The equations used
pH is defined as pH = −log₁₀[H⁺] and pOH as −log₁₀[OH⁻]; at 25 °C they add to 14 because [H⁺][OH⁻] = Kw = 10⁻¹⁴. A strong acid is fully dissociated, so [H⁺] equals its molarity times the number of acidic protons: 0.01 M HCl has pH 2.00 and 0.01 M NaOH has pOH 2.00, so pH 12.00. For very dilute solutions the calculator solves [H⁺] = (C + √(C² + 4Kw)) ÷ 2, keeping 10⁻⁸ M acid on the acidic side of neutral.
A weak acid HA only partly ionises: Ka = x² ÷ (C − x), where x is [H⁺]. Rearranged, x² + Ka·x − Ka·C = 0, whose positive root is x = (−Ka + √(Ka² + 4Ka·C)) ÷ 2. If x is below 5 % of C the shortcut x ≈ √(Ka·C) is fine; for 0.1 M acetic acid x = 1.33 × 10⁻³ M, 1.3 % ionised, pH 2.88. Weak bases use Kb the same way to give [OH⁻], then pH = 14 − pOH.
| Solution | Method | pH |
|---|---|---|
| 0.01 M HCl | strong, −log(0.01) | 2.00 |
| 0.1 M acetic acid, Ka 1.8 × 10⁻⁵ | weak acid quadratic | 2.88 |
| 0.1 M ammonia, Kb 1.8 × 10⁻⁵ | weak base → pOH 2.88 | 11.12 |
| 0.1 M acetate + 0.1 M acetic acid | Henderson–Hasselbalch, pKa 4.76 | 4.76 |
| 25 mL 0.1 M HCl + 50 mL 0.1 M NaOH | excess OH⁻ 0.033 M | 12.52 |
Buffers and the Henderson–Hasselbalch equation
A buffer is a weak acid together with its conjugate base. Its pH follows pH = pKa + log₁₀([A⁻] ÷ [HA]), so equal amounts give pH = pKa and a ten-fold excess of base adds one pH unit. Buffers work well only within about one unit of the pKa, which is why acetate (pKa 4.76) buffers around pH 4–5.5, phosphate (pKa₂ 7.20) around 6–8 and Tris (conjugate-acid pKa 8.07) around 7–9. Enter a target pH and the calculator returns the ratio and splits your total concentration into acid and base. The dilution calculator prepares the stocks and the molar mass calculator weighs out the salts.
Reading a strong acid–strong base titration curve
When a strong base is added to a strong acid, the pH rises slowly while acid is in excess, jumps several units within a drop or two of the equivalence point, then flattens as base accumulates. The equivalence volume is Veq = CaVa ÷ Cb, and because the salt of a strong acid and strong base does not hydrolyse, the equivalence pH is exactly 7.00 at 25 °C. The curve is calculated from the excess acid or base divided by the total volume, with water's ionisation included. Weak-acid titrations have a buffer region and an equivalence pH above 7; they are not plotted, but the weak-acid mode gives the starting pH and the buffer mode the half-equivalence point (pH = pKa).
pH Calculator FAQ
How do I calculate pH from molarity?
For a strong acid, pH = −log₁₀(molarity × H⁺ per molecule); for a strong base find pOH the same way and subtract from 14. For a weak acid solve x² + Ka·x − Ka·C = 0 and take pH = −log₁₀(x). The pH calculator does both and shows the steps.
What is the pH of 0.1 M acetic acid?
About 2.88. With Ka = 1.8 × 10⁻⁵ the exact quadratic gives [H⁺] = 1.33 × 10⁻³ M; the approximation √(Ka·C) gives 2.87.
How do I convert pKa to Ka?
Ka = 10^(−pKa). Acetic acid's pKa of 4.76 corresponds to Ka = 1.7 × 10⁻⁵. Type either value and the other is filled in.
Why is the pH of 10⁻⁸ M HCl not 8?
An acid cannot make water basic. At such low concentrations the 10⁻⁷ M of H⁺ from water itself dominates, and the exact calculation gives pH 6.98.
How do I make a buffer of a specific pH?
Choose a weak acid whose pKa is within one unit of the target, enter the target pH in the buffer mode, and the calculator gives the base-to-acid ratio and the amounts.
Does temperature affect the results?
Yes. Kw rises with temperature (neutral pH is about 6.6 at 50 °C) and pKa values shift, Tris by about −0.03 per °C. All values here are for 25 °C.