How to use it
- Choose whether to find the pH or the ratio for a target pH.
- Pick the buffer pair, or enter a custom pKa.
- For the pH, enter the concentrations of base and acid. For the ratio, enter the target pH and the total buffer concentration.
- Read the pH or the ratio and the amounts of each form.
Formulas
- pH = pKa + log₁₀([A⁻] ÷ [HA])
- Ratio for a target pH: [A⁻] ÷ [HA] = 10^(pH − pKa)
- Split of a total concentration C: [A⁻] = C × r ÷ (1 + r), [HA] = C ÷ (1 + r)
The pKa values in the list are rounded values at 25 °C. pKa changes with temperature and ionic strength (Tris especially), so for precise work measure the pH after mixing.
Worked example
An acetate buffer with 0.15 mol/L sodium acetate and 0.10 mol/L acetic acid: pH = 4.76 + log₁₀(1.5) = 4.76 + 0.18 = 4.94.
To make 0.1 mol/L acetate buffer at pH 5.00: ratio = 10^(5.00 − 4.76) = 1.7378, so 0.0635 mol/L acetate and 0.0365 mol/L acetic acid.
Tips
- The equation is an approximation. It works best for concentrations well above about 0.001 mol/L and for pH values not far from the pKa.
- Polyprotic acids have several pKa values; use the one between the two forms in your buffer, such as 7.21 for phosphate near neutral pH.
- Blood: with pKa 6.1 for the bicarbonate system, pH 7.4 needs a ratio of about 20 : 1.