How to use
- Choose a mode. Find the pH works out the pH from the concentrations of the acid and base forms; Find the recipe takes a target pH and works out how much of each form to use.
- Pick a buffer or enter a pKa of your own. Choosing from the list fills the pKa in.
- Find the pH — enter the concentrations of the base form and the acid form.
- Find the recipe — enter a target pH and a total concentration and you get the concentration of each form. Add a total volume and the molecular weights and you also get the mass (g) to weigh out. Leave them empty and only the concentrations are shown.
Exponents can be entered with e — for example, 1.5×10⁻⁵ is entered as 1.5e-5.
Formula and practical notes
A buffer is a solution holding a weak acid and its conjugate base together. Acid coming in is taken up by the conjugate base and base coming in by the acid, so the pH does not move much.
Starting from the acid dissociation equilibrium and taking −log of both sides gives this:
- Henderson-Hasselbalch
- pH = pKa + log₁₀( [base form] ÷ [acid form] )
Two things can be read straight off it. When the two forms are at the same concentration the log term is zero, so pH = pKa. And what matters is the ratio, not the absolute concentration — dilute the whole thing two-fold and the pH stays (approximately) where it was.
Working out the recipe
The target pH fixes the ratio between the two forms.
- Ratio
- r = [base form] ÷ [acid form] = 10^(pH − pKa)
Splitting the total concentration by that ratio gives each one.
- Base form
- [base form] = total × r ÷ (1 + r)
- Acid form
- [acid form] = total × 1 ÷ (1 + r)
Multiply by volume and molecular weight and you have the mass to put on the balance.
- Mass
- mass(g) = concentration(mol/L) × volume(L) × MW(g/mol)
Practical notes
- The pKa of Tris depends strongly on temperature. At roughly −0.028 / °C, a buffer titrated to pH 8.0 at 25℃ sits near 8.6 at 4℃. Set the pH at the temperature you will actually use it. The pKa values in this calculator are for 25℃.
- This calculation is an approximation. The Henderson-Hasselbalch equation ignores ionic strength and activity coefficients. At high concentrations, or with a lot of salt present, the real pH can differ from the calculated one. Always confirm with a pH meter and adjust.
- Buffering capacity only holds near the pKa. Once the target pH is more than one unit from the pKa, one form makes up almost everything and there is little buffering left. Pick a different buffer in that case.
- Where a system has several pKa values, as phosphate and citrate do, pick the one that matches your target pH. Phosphate has three — 2.15 / 7.20 / 12.35 — and 7.20 is the one used near neutral.
- For a hydrate, use the molecular weight that includes the water of crystallisation. It changes the mass considerably.
- Think about what the buffer does to the experiment too. Phosphate precipitates with calcium and magnesium and inhibits some enzymes. Tris reacts with aldehydes.