Preparing a Buffer Solution

General Chemistry • Acid Base Equilibrium

Written by STEM Calculators Team Published November 4, 2025 Updated February 24, 2026

Prepare a Buffer at a Desired pH (Henderson–Hasselbalch)

Given a weak acid HA and its salt A⁻ (or a weak base B and its conjugate acid BH⁺), compute the required concentration, moles, and mass of salt to obtain a target pH in volume \(V\). Assumes the added salt is 1:1 (e.g., NaA or BH⁺Cl⁻) and the final volume is \(V\).

Rate this calculator

0.0 /5 (0 ratings)

Be the first to rate.

Your rating

Name (optional) Review (optional)

You can update your rating any time.

Preparing a Buffer for a Target pH

A buffer made from a weak acid HA and its conjugate base A⁻ (or a weak base B and its conjugate acid BH⁺) satisfies the Henderson–Hasselbalch relation. For acid buffers:

\[ \begin{aligned} \mathrm{pH} &= \mathrm{p}K_a + \log\!\frac{[A^-]}{[HA]} \\ \Rightarrow\quad [A^-] &= [HA]\;10^{\mathrm{pH}-\mathrm{p}K_a}. \end{aligned} \]

For base buffers, use \(\mathrm{p}K_a = 14 - \mathrm{p}K_b\) of the conjugate acid and \(\displaystyle \mathrm{pH} = \mathrm{p}K_a + \log\!\frac{[B]}{[BH^+]}\), so \(\displaystyle [BH^+] = [B]\;10^{-(\mathrm{pH}-\mathrm{p}K_a)}\).

From concentration to mass

Once the required conjugate concentration is known, multiply by the final volume \(V\) to get the moles of salt to dissolve, then multiply by its molar mass \(M\):

\[ \begin{aligned} n_{\text{salt}} &= [\text{conjugate}]\,V, \\ m_{\text{salt}} &= n_{\text{salt}}\,M. \end{aligned} \]

Why Henderson–Hasselbalch works here

In typical buffer preparations, the salt provides most of the conjugate species. The weak acid/base ionization (\(x\)) is small compared with the stoichiometric concentrations, so the equilibrium concentrations are well approximated by the initial ones. A quick check with the exact equilibrium expression confirms the obtained pH:

\[ \begin{aligned} K_a &= \frac{[A^-][H_3O^+]}{[HA]} \;=\; \frac{x\,(C_{\text{conj}}+x)}{C_{\text{weak}}-x} \quad\Rightarrow\quad x = \frac{-\big(C_{\text{conj}}+K_a\big)+\sqrt{\big(C_{\text{conj}}+K_a\big)^2+4K_aC_{\text{weak}}}}{2}. \end{aligned} \]

Here \(x=[H_3O^+]\) for acid buffers (or \(x=[OH^-]\) for base buffers). The resulting \(\mathrm{pH}=-\log_{10}x\) should closely match the target value when buffer assumptions hold.

Notes: (1) This tool assumes 1:1 salts (e.g., NaA, BH⁺Cl⁻) and that volume change upon dissolution is negligible. (2) For very dilute buffers or extreme pH targets, verify assumptions with the equilibrium check.

Frequently Asked Questions

How does this buffer preparation calculator use the Henderson-Hasselbalch equation?

It uses pH = pKa + log10([A-]/[HA]) for acid buffers (or the equivalent form for base buffers) to determine the needed conjugate-to-weak ratio. That ratio is combined with your chosen C_weak and final volume to compute the salt amount.

Should I enter Ka or pKa when preparing a buffer?

Either works as long as you select the matching option in the calculator. Use pKa if you have it directly; otherwise enter Ka and the tool will convert it internally to the logarithmic form.

What does the salt molar mass input affect in the results?

After the tool finds the required moles of conjugate salt, it multiplies moles by the molar mass (g/mol) to compute the grams of salt to weigh out.

Why might the calculated buffer pH be slightly different from the target pH?

Henderson-Hasselbalch assumes the weak equilibrium shift is small compared with the stoichiometric concentrations. For very dilute buffers or extreme pH targets, that assumption can be weaker, so the equilibrium check can show a small deviation.

Prepare a Buffer at a Desired pH (Henderson–Hasselbalch)

Explanation & steps

Rate this calculator

Frequently Asked Questions

Related calculators