What is the Buffer Calculator?
Buffer solutions resist pH change when small amounts of acid or base are added. The Henderson–Hasselbalch equation relates pH to the ratio of conjugate base and weak acid (or conjugate acid and weak base). This calculator solves for pH, pKa/pKb, component concentrations, or the conjugate-to-weak ratio for acid buffers (HA/A⁻) and base buffers (B/BH⁺). Includes presets for acetate, phosphate, TRIS, and ammonia buffers. Runs instantly in your browser with no signup.
How to use the Buffer Calculator
- Choose acid buffer (HA/A⁻) or base buffer (B/BH⁺).
- Select what to solve: pH, pKa/pKb, conjugate concentration, weak species, or ratio only.
- Enter the known values and pick M or mM for concentrations.
- Use a common-buffer preset or example to get started quickly.
- Copy the results for lab prep or homework.
Common use cases
- Calculating pH of 0.1 M acetic acid / 0.1 M acetate (pH ≈ 4.76)
- Finding the acetate-to-acid ratio needed for pH 5.0
- Designing a TRIS or ammonia buffer for biochemical assays
Frequently asked questions
- What is the Henderson–Hasselbalch equation?
- For an acid buffer: pH = pKa + log₁₀([A⁻]/[HA]). For a base buffer: pOH = pKb + log₁₀([BH⁺]/[B]), then pH = 14 − pOH at 25 °C.
- What pH do I get when [A⁻] equals [HA]?
- When the conjugate and weak species concentrations are equal, log₁₀(1) = 0, so pH = pKa (or pOH = pKb for base buffers).
- Which pKa should I use for phosphate buffer?
- Phosphate has three pKa values. Near pH 7, use pKa₂ = 7.21 for the H₂PO₄⁻ / HPO₄²⁻ pair.
- Does ionic strength affect buffer pH?
- Yes — activity coefficients can shift measured pH slightly from ideal Henderson–Hasselbalch predictions, especially at high ionic strength.
