Buffer Preparation Calculator (Henderson-Hasselbalch, Grams to Weigh Out)
The ratio of conjugate base to acid for a target pH, and the moles and grams of each to weigh out for a given volume and total concentration.
Results
What this tool does
Henderson–Hasselbalch says pH = pKa + log₁₀([A⁻] ÷ [HA]), so to reach a target pH you mix the weak acid and its conjugate base in the ratio 10^(pH − pKa) — equal amounts give a pH equal to the pKa. Enter the molar mass of the form you will actually weigh: sodium acetate trihydrate weighs about two thirds more than the anhydrous salt. A buffer works best within about one pH unit of its pKa, and its capacity grows with concentration. The equation ignores ionic strength and temperature, so the pH you measure will differ a little: dissolve, check with a calibrated pH meter and trim with a little acid or base.
Formula
[A⁻] ÷ [HA] = 10^(pH − pKa) · n_HA = c V ÷ (1 + ratio) · n_A⁻ = c V × ratio ÷ (1 + ratio)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
ph | pH you want the buffer to have | — |
pk | pKa | — |
c | Total buffer concentration (acid + conjugate base) | mol/L |
v | Volume | L |
ma | Molar mass of the acid form, HA | g/mol |
mb | Molar mass of the conjugate base as the salt you weigh out | g/mol |
GA | Acid form (HA) to weigh out | g |
GB | Conjugate base salt to weigh out | g |
R | Ratio [A⁻] ÷ [HA] | — |
NA | Amount of acid form | mol |
NB | Amount of conjugate base | mol |
Q | Within one pH unit of the pKa? | — |
Worked example
- pH you want the buffer to have5
- pKa4.76
- Total buffer concentration (acid + conjugate base)0.1 mol/L
- Volume1 L
- Molar mass of the acid form, HA60.05 g/mol
- Molar mass of the conjugate base as the salt you weigh out82.03 g/mol
- Acid form (HA) to weigh out2.193 g
- Conjugate base salt to weigh out5.207 g
- Ratio [A⁻] ÷ [HA]1.7378
- Amount of acid form0.0365 mol
- Amount of conjugate base0.0635 mol
- Within one pH unit of the pKa?Yes — it buffers well
Limitations
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.