Enrichment · Passion Projects · Chemistry

The ratio that
holds the pH where
you set it.

Choose a buffer pair and type the pH, concentration and volume you need. The calculator gives the ratio of the two forms and the grams of each to weigh, and warns when the pair is a poor fit for that pH.

Make a bufferWhat you get

The buffer calculator

The worked example is 500 mL of 0.1 M phosphate buffer at pH 7.4 for an enzyme lab. Nothing you type leaves this browser.

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What you get

Enrichment · Passion ProjectsFree

What you finish with
The base-to-acid ratio, the moles and grams of each form, how to make the buffer up, and whether the pair suits the pH. It prints as a recipe and downloads as a spreadsheet.
Who it is for
Biology and chemistry students, teachers preparing lab solutions, and students doing science fair or research projects.
Format
Browser calculator for a phone, tablet or computer, by touch or keyboard. Prints a recipe; downloads a spreadsheet (.csv); export and import a file.
Access
Free. No account, no sign-in and nothing to install. Everything you type stays in this browser; nothing is sent to Caveat.
Where it lives
The Passion Projects space in Caveat Enrichment.
Support
Questions, a correction or a request: team@caveatacademic.com.

How to use it

  1. Choose the buffer pair, or Other and type its pKa and molar masses.
  2. Type the target pH, the total concentration and the volume.
  3. Read the grams of each form and the warnings.
  4. Make it up and finish the pH with a calibrated meter.
  5. Print the recipe for the bench.

Make a buffer

How the recipe is worked out

Henderson-Hasselbalch, then moles to grams.

The worked example: 0.1 M phosphate, pH 7.4pH − pKa7.40 − 7.21= 0.19Base ÷ acid1.54910^0.19Moles0.05 mol0.0304 base + 0.0196 acidGrams4.313 + 2.354× 141.96 and × 119.98The worked example: 0.1 M phosphate, pH 7.4pH − pKa7.40 − 7.21= 0.19Base ÷ acid1.54910^0.19Moles0.05 mol0.0304 base + 0.0196 acidGrams4.313 + 2.354× 141.96 and × 119.98
The pH sets the ratio of the two forms; the concentration and volume set how much of both there is.

The arithmetic

pH = pKa + log₁₀([base] ÷ [acid]), so the ratio of base form to acid form is 10^(pH − pKa). The total moles are concentration × litres; the base form gets ratio ÷ (1 + ratio) of them and the acid form the rest. Grams are moles × molar mass of the form you actually have, so a hydrate needs its own molar mass in Other.

Why the meter wins

The equation uses concentrations where real solutions behave by activity, so ionic strength nudges the pH, usually by a tenth or two. pKa also moves with temperature, strongly for Tris. A buffer works best within about one pH unit of its pKa. Calibrate the meter, adjust with a little acid or base, then make up to volume.

Sources

  1. Good, N. E., et al. (1966). Hydrogen ion buffers for biological research. Biochemistry 5(2), 467 to 477: MES, HEPES and their pKa values.
  2. Rumble, J. R., ed. CRC Handbook of Chemistry and Physics: dissociation constants of organic acids and bases (acetic acid 4.76, Tris 8.07) and of inorganic acids (phosphoric acid pKa2 7.21).
  3. IUPAC. Compendium of Chemical Terminology (the Gold Book): buffer and pKa (goldbook.iupac.org).
  4. OpenStax. Chemistry 2e, section 14.6: Buffers, and the Henderson-Hasselbalch equation (openstax.org).

The worked example is made up. Follow your school's lab safety rules; acids and bases need goggles. Your figures stay in this browser. Support: team@caveatacademic.com.