The buffer solution calculator uses the Henderson-Hasselbalch equation (pH = pKa + log([A⁻]/[HA])) to compute buffer pH from weak acid pKa and conjugate base/acid concentrations. Also calculates the ratio needed to achieve a target pH.
Henderson-Hasselbalch Calculator
Mode
Best buffering when target is within ±1 of pKa
Results
Step-by-Step
Henderson-Hasselbalch Equation
Maximum buffer capacity occurs at pH = pKa (ratio = 1:1). Effective buffering range is roughly pKa ± 1.
How to Use the Buffer Solution Calculator
Buffer solutions resist pH changes when acids or bases are added. This calculator applies the Henderson-Hasselbalch equation to compute pH from known concentrations, or to find the ratio needed to achieve a target pH.
Step 1: Select an Acid
Choose a common buffer acid from the dropdown (acetic acid, phosphate, HEPES, Tris, etc.) or enter a custom pKa. The pKa determines the optimal pH range for the buffer.
Step 2: Mode — Compute pH
Enter the concentrations of conjugate base [A⁻] and weak acid [HA]. Example: 0.1 M sodium acetate and 0.1 M acetic acid (pKa 4.75): pH = 4.75 + log(0.1/0.1) = 4.75 + 0 = 4.75.
Step 3: Mode — Target pH
Enter a target pH and the calculator finds the required [A⁻]/[HA] ratio. Example: To prepare a pH 5.0 acetic acid buffer (pKa 4.75): log(ratio) = 5.0 - 4.75 = 0.25, so ratio = 10^0.25 = 1.78. Use 1.78 parts sodium acetate per 1 part acetic acid.
Buffer Capacity Warning
Buffers work best when the target pH is within ±1 unit of the pKa. If pH = pKa ± 2, the ratio is 100:1 or 1:100 — essentially no longer a buffer. For biological applications at pH 7.4, choose a buffer with pKa between 6.4 and 8.4 (phosphate, HEPES, or Tris are common choices).
Frequently Asked Questions
What is the Henderson-Hasselbalch equation?
The Henderson-Hasselbalch equation is: pH = pKa + log([A⁻]/[HA]), where [A⁻] is the concentration of the conjugate base and [HA] is the concentration of the weak acid. It's used to calculate the pH of buffer solutions and to prepare buffers at a desired pH.
What is a buffer solution?
A buffer solution resists changes in pH when small amounts of acid or base are added. It consists of a weak acid and its conjugate base (or weak base and conjugate acid). Buffers are essential in biological systems, laboratory work, and industrial processes.
What are common buffer systems?
Acetic acid/acetate buffer (pKa 4.75, pH range 3.75–5.75), phosphate buffer (pKa 7.2, pH range 6.2–8.2), HEPES buffer (pKa 7.5, biological research), carbonate/bicarbonate (pKa 10.3), and Tris buffer (pKa 8.1, widely used in biochemistry).
Is this calculator free?
Yes, completely free with no signup required. All calculations run in your browser.
What is buffer capacity?
Buffer capacity is the amount of acid or base a buffer can absorb before the pH changes significantly. Maximum buffer capacity occurs when [A⁻] = [HA] (ratio = 1, pH = pKa). Capacity decreases as the ratio moves away from 1:1.