# Henderson-Hasselbalch Calculator

Calculate buffer pH with the Henderson-Hasselbalch equation. Enter pKa and conjugate acid/base concentrations to find pH, ratio, and buffer effectiveness.

## What this calculates

Calculate the pH of a buffer solution using the Henderson-Hasselbalch equation: pH = pKa + log₁₀([A⁻]/[HA]). Enter the pKa of your weak acid and the concentrations of the conjugate base and acid to determine pH, the base-to-acid ratio, and buffer effectiveness.

## Inputs

- **pKa** — min -2, max 16 — The acid dissociation constant (-log Ka) of the weak acid.
- **Conjugate Base [A⁻]** (mol/L) — min 0 — Molar concentration of the conjugate base (e.g., acetate ion).
- **Weak Acid [HA]** (mol/L) — min 0 — Molar concentration of the weak acid (e.g., acetic acid).

## Outputs

- **pH** — The calculated pH of the buffer solution.
- **[A⁻]/[HA] Ratio** — The ratio of conjugate base to weak acid concentration.
- **Buffer Type** — formatted as text — Whether the buffer is acidic or basic relative to pKa.
- **Buffer Capacity Assessment** — formatted as text — Assessment of the buffer effectiveness based on the ratio.

## Details

The Henderson-Hasselbalch equation is the cornerstone of buffer chemistry. It provides a direct relationship between pH, the pKa of a weak acid, and the ratio of conjugate base to acid concentrations in a buffer solution.

The Equation: pH = pKa + log₁₀([A⁻]/[HA])

Where [A⁻] is the molar concentration of the conjugate base and [HA] is the molar concentration of the weak acid. When [A⁻] = [HA], the ratio is 1, log(1) = 0, and pH = pKa. This is why buffers are most effective at pH values near the pKa.

Buffer Effectiveness

A buffer is effective when the [A⁻]/[HA] ratio is between 0.1 and 10, corresponding to a pH range of pKa minus 1 to pKa plus 1. Outside this range, the buffer has insufficient capacity to resist pH changes. The total concentration of buffer components also matters: higher concentrations provide greater buffer capacity.

Common Buffer Systems

- Acetic acid / Acetate (pKa = 4.76): useful for pH 3.8–5.8

- Phosphate (pKa2 = 7.20): useful for pH 6.2–8.2

- Tris (pKa = 8.07): useful for pH 7.1–9.1

- Bicarbonate (pKa = 6.35): physiological buffer system

## Frequently Asked Questions

**Q: What is the Henderson-Hasselbalch equation used for?**

A: The Henderson-Hasselbalch equation calculates the pH of a buffer solution from the pKa of the weak acid and the ratio of conjugate base to acid concentrations. It is widely used in biochemistry, pharmaceutical chemistry, and laboratory work to design buffers at specific pH values.

**Q: How is this different from the Buffer Solution calculator?**

A: This calculator focuses specifically on the forward calculation: given pKa and concentrations, find pH. The Buffer Solution calculator allows solving for any variable (pH, pKa, or concentrations). Use this calculator for quick pH determinations from known buffer compositions.

**Q: What are the limitations of the Henderson-Hasselbalch equation?**

A: The equation assumes ideal behavior and works best for dilute solutions. It becomes inaccurate for very concentrated solutions, polyprotic acids without well-separated pKa values, or when the acid or base concentration is very close to zero. It also does not account for ionic strength effects.

**Q: How do I choose a buffer for a specific pH?**

A: Select a weak acid with a pKa close to your desired pH (within 1 pH unit). The closer the pKa is to your target pH, the more effective the buffer. For example, for pH 7.4 (physiological), use a phosphate buffer (pKa2 = 7.20) or HEPES (pKa = 7.5).

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Source: https://vastcalc.com/calculators/chemistry/henderson-hasselbalch
Category: Chemistry
Last updated: 2026-04-21