Buffer Calculator
Name the pH you want and the pair you own: the split between acid form and base form falls out of the logarithm, and the capacity tells you how much abuse the recipe survives.
Buffer Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- pKa table values — 25 °C, dilute; TRIS drifts hard with temperature
- Capacity is the van Slyke idealisation for one weak-acid pair
In short: A 100 mmol acetate buffer at pH 4.756000 (pKa 4.756): the ratio is 10^(0) = 1.000000, so the recipe splits 50.000000 mmol acid form and 50.000000 mmol base form — the pKa point is the perfect 50/50. Made to 500 mL that is 0.200000 mol/L total, and the capacity ceiling is ln10 × 0.200000/4 = 0.115129 mol/L per pH unit, right at the pKa. Push the target one unit up and the split leans to 9.090909 against 90.909091 mmol.
Formula
ratio = 10^(pH − pKa) · acid = T/(1+ratio) · base = T·ratio/(1+ratio) · βmax = ln10·C/4
Preparation runs Henderson–Hasselbalch backwards: the ratio of base form to acid form that holds your target pH is ten to the (pH minus pKa), and the total splits between the two forms in that ratio. The van Slyke capacity is largest exactly at the pKa — ln10 times the molar concentration divided by four — and falls off on both sides as the mix leans one way.
Worked Example
- Enter the pH you want to hold.
- Enter the weak acid’s pKa (25 °C table value).
- Enter the total buffer and the final volume.
- Read the acid/base split and the capacity ceiling.
Defaults: pH 4.756, pKa 4.756, 100 mmol to 500 mL → 50.000000/50.000000 mmol, βmax 0.115129. TRIS at pH 7.5: ratio 0.275423 → 78.405368 acid / 21.594632 base.
Strengths & Limits Of This Model
Where this engine is strong
- Recipe split and capacity from one entry
- The temperature trap named on the card
Where it stops
- No ionic-strength correction
- No polyprotic accounting
Practical Use Cases
Cell culture baths
pH that survives the incubator
Assay buffers
recipe sheets in mmol
Teaching
HH backwards, one recipe at a time
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Buffer Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why does the split land at exactly 50/50 when the target pH equals the pKa?
Because the ratio is ten to the difference: pH minus pKa is zero, ten to the zero is one, and a 1:1 ratio of a fixed total is half and half. The pKa is not just a table entry — it is the pH where the pair is perfectly balanced, and the capacity is largest exactly there.
How far from the pKa can I push a buffer before it stops buffering?
The capacity falls to about a third of its ceiling one unit away and keeps sliding; past two units the mix is mostly one form and barely resists anything. The working rule is to pick a weak acid whose pKa sits within about one unit of your target — the ratio card shows how lopsided the recipe gets beyond that.
Why does the recipe quote both forms in mmol?
Because you can stock the conjugate base as a weighed salt: sodium acetate IS the base form. Weigh both solids to the millimole the recipe names, dissolve, adjust, make to the mark. The molar amounts — not the grams — are what the arithmetic couples, which is why the card prints mmol and leaves the gram conversions to the balance.
What is the capacity ceiling actually measuring?
Moles of strong acid or base a litre of the buffer absorbs per unit of pH movement — the van Slyke capacity. At the pKa it simplifies to ln10 × C / 4; for the default 0.200000 mol/L that is 0.115129 mol per litre per pH unit. It is a ceiling because the capacity slides off as soon as the target leaves the pKa.
Why does TRIS get a temperature warning all to itself?
Because its pKa slides about −0.028 pH per degree — a TRIS bath adjusted to 7.5 at 25 °C reads near 8.1 in the 4 °C cold room and near 7.2 in a 37 °C incubator. Acetate barely drifts. Prepare and adjust at the temperature you will use; the card names the trap so the cold room cannot spring it.
Can I enter a target pH outside 0–14?
The arithmetic still runs — the logarithm does not care — but the recipe leaves aqueous chemistry behind. The page prints the honest split and lets the ladder convention go; a target beyond the scale is usually a sign the wrong buffer system is chosen, not that the exponent machine broke.
How is this different from the Henderson–Hasselbalch page?
Direction of travel. That page ANALYSES an existing pair — you have the moles, it returns the pH. This page PREPARES — you have the pH you want, it returns the recipe. Same logarithm, opposite doors, and the two pages link to each other because bench work runs both directions in the same afternoon.
Where do the table pKa values come from?
From dilute aqueous measurement at 25 °C — conventional single values. Real pKa drifts with temperature and ionic strength (TRIS famously so), so the printed recipe is a starting point that the pH meter finishes. The convention is good to a hundredth or two; the meter settles the rest.