pH, Buffer & Titration Curve Calculator

pH ↔ pOH ↔ [H⁺], buffer recipes with Henderson–Hasselbalch checked exactly, and titration curves.

Calculation

Known value (convert)

pH 3.2, pOH 4, H+ 1e-3 M or OH- 0.01 M.

Buffer acid

More options

Ka, pKa, Kb or pKb

For your own buffer acid or a weak analyte in a titration.

Weak acid [HA]

Buffer: give [HA] and [A⁻] for the pH, or leave both empty and give a target pH.

Conjugate base [A⁻]

Target pH (buffer recipe)

Total buffer concentration

Recipe: [HA] + [A⁻]. Empty: 0.1 M.

Buffer volume

Gives the amounts and grams to weigh, and is needed for additions.

Add strong acid

Amount of HCl (or another strong acid) added to the buffer volume.

Add strong base

Sample (titration)

Sample concentration

Sample volume

Titrant concentration

Plot up to (titrant volume)

Empty: twice the equivalence volume.

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More about this tool

How it works

Three pH calculators in one. Convert between pH, pOH, [H₃O⁺] and [OH⁻]. Design or check a buffer: the pH of a weak acid and its conjugate base (Henderson–Hasselbalch next to the exact charge balance), the concentrations and grams for a target pH, the buffer capacity and the pH after adding a strong acid or base. Draw the titration curve of a strong or weak acid or base with the equivalence and half-equivalence points, the steepest point and matching indicators. Nine common buffer acids are built in. Part of the Proofline math notebook.

pH = −log₁₀[H₃O⁺], pOH = 14.00 − pH, [OH⁻] = Kw/[H₃O⁺] with Kw = 1.0 × 10⁻¹⁴ at 25 °C.
Buffers: the exact pH solves the charge balance [H⁺] + [Na⁺] = [A⁻] + [OH⁻] + [Cl⁻] with [A⁻] = C·Ka/(Ka + [H⁺]) by bisection (Proofline’s chemistry engine); Henderson–Hasselbalch pKa + log([A⁻]/[HA]) is shown next to it. A recipe for a target pH is found by bisection on the share of A⁻, so it is exact, not Henderson–Hasselbalch. Buffer capacity β = 2.303·(Kw/[H⁺] + [H⁺] + C·Ka·[H⁺]/(Ka + [H⁺])²).
Titrations: every point solves the same charge balance with the dilution by the titrant, from the start through the buffer region and the equivalence point to the excess. The steepest point is the maximum of |dpH/dV|; indicators are suggested when their transition range contains the equivalence pH (± 0.3).
Verified: 0.1 M acetic acid titrated with 0.1 M NaOH starts at pH 2.88, passes pKa at half equivalence and reaches pH 8.72 at equivalence; strong acid against strong base gives pH 7.00 at equivalence; an equimolar acetate buffer has pH = pKa.

Dilute-solution model at 25 °C: activities are taken as concentrations, so real pH values can differ by a few tenths at higher ionic strength; always check a buffer with a calibrated pH meter. Polyprotic acids are treated one step at a time. Nothing here is advice on handling chemicals or radioactive material safely.

Sources

D. C. Harris, C. A. Lucy: Quantitative Chemical Analysis, 9th ed. (W. H. Freeman, 2016) — chapters 8–11 (systematic treatment of equilibrium, buffers, titrations) and Appendix G (pKa values) (W. H. Freeman)
P. Atkins, J. de Paula, J. Keeler: Atkins’ Physical Chemistry, 11th ed. (Oxford University Press, 2018) — the building-up principle, Hund’s rule; real gases and the van der Waals equation (Topic 1C) (Oxford University Press)

Part of Proofline

Proofline Math notebook

Solve, graph, test and visualise in one notebook — with data, findings and a report.

Open Proofline

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Calculate pH, design buffers and plot titration curves

Convert between pH, pOH and concentrations, calculate or design a buffer with its capacity, and plot a titration curve with the equivalence point and suitable indicators.

What it is for

Three common lab questions in one place. Convert turns a pH, pOH, [H₃O⁺] or [OH⁻] into the other three. Buffer gives the pH of a weak acid and its conjugate base, or the recipe for a target pH, along with the buffer capacity and the effect of adding strong acid or base. Titration draws the full pH curve of a strong or weak monoprotic acid or base. It marks the equivalence and half-equivalence points and suggests indicators.

The calculation uses the exact charge balance, including water, not just the Henderson–Hasselbalch shortcut. The shortcut is shown next to it so you can see when it is good enough. Nine common buffer acids are preset, from acetic acid to Tris and phosphate. Any Ka, pKa, Kb or pKb can be typed in.

Activities are taken as concentrations, so real solutions with high ionic strength can differ by a few tenths of a pH unit. Always check the pH with a calibrated meter.

Worked example

Acetate buffer, 0.1 M acetic acid and 0.1 M sodium acetate, 100 mL. The pH is 4.76, equal to the pKa. The exact and Henderson–Hasselbalch values agree. The buffer capacity is 0.115 mol/(L·pH). Add 1 mmol HCl and the pH drops only to 4.67. The recipe calls for 0.601 g of acetic acid and 0.820 g of sodium acetate.

Phosphate buffer for pH 7.4 at 0.1 M total, 1 L. The tool calculates 0.0386 M NaH₂PO₄ and 0.0614 M Na₂HPO₄, a ratio of 1.592. That means 4.63 g and 8.72 g of the anhydrous salts.

Titration of 25 mL of 0.1 M acetic acid with 0.1 M NaOH. Equivalence is at 25 mL with pH 8.72, because the acetate formed is a weak base. The curve starts at pH 2.88 and passes pH 4.75 at the half-equivalence point. Suitable indicators are phenolphthalein and thymolphthalein. The same titration with HCl instead gives an equivalence pH of exactly 7.00, and bromothymol blue fits.

How to read the result

pH (exact) solves the full equilibrium. Henderson–Hasselbalch is the textbook approximation. They differ for very dilute buffers or when the ratio is far from 1.
Buffer capacity β is the strong base, in mol per litre, needed to raise the pH by one unit. It is largest at pH = pKa.
On the titration curve, the steepest point is the equivalence point. At half equivalence the pH of a weak acid equals its pKa, which is how pKa is measured.
Indicators are chosen so that their colour change lies within the steep part of the curve.

Tips and common mistakes

Write constants with their type: Ka 1.8e-5, pKa 4.76 or pKb 4.75.
A buffer works best within about one pH unit of the pKa. The tool warns when the target is outside that range.
For weak bases such as ammonia, half equivalence gives the pKa of the conjugate acid, 14 − pKb.
Polyprotic acids are treated one step at a time. Phosphate uses the second pKa for pH 7.

FAQ

Why is the equivalence pH of acetic acid not 7?

At equivalence the flask contains sodium acetate, a weak base, which makes the solution slightly alkaline.

Does temperature matter?

Yes. Kw and Ka change with temperature. The tool uses values at 25 °C.

Can it titrate polyprotic acids?

Not yet. The titration mode covers monoprotic acids and bases. For a polyprotic acid you can treat each step separately when the pKa values are more than about three units apart.

How accurate is the recipe?

Within the ideal model it is exact. In practice, adjust with a pH meter, because activity effects shift the pH by up to a few tenths.

Guide last updated: November 8, 2026

For scripts and AI agents

JSON API: POST https://prooflinephbuffer.usefulweb.app/api/v1/calculate (same calculation as this page)
Interface description: OpenAPI 3.1 · AI guide: llms.txt
MCP server (Streamable HTTP): https://prooflinephbuffer.usefulweb.app/mcp · tool name proofline_ph_buffer

Example request

curl -s -X POST https://prooflinephbuffer.usefulweb.app/api/v1/calculate \
  -H 'content-type: application/json' \
  -d '{"mode":"convert","value":"pH 3.2"}'

Feedback · ideas & bug reports welcome