Buffer Capacity Calculator
Buffer capacity (beta) quantifies how well a buffer resists changes in pH when acid or base is added. The Van Slyke equation relates buffer capacity to the total buffer concentration (C), the acid dissociation constant (Ka), and the hydrogen ion concentration at the current pH. Buffer capacity is highest when pH equals pKa and decreases as the ratio of acid to conjugate base departs from 1:1. Enter the pKa of your weak acid, the total buffer concentration, and the desired pH to calculate the buffer capacity.
Buffer capacity formula (Van Slyke)
beta = 2.303 * C * Ka * [H+] / (Ka + [H+])^2
Where Ka = 10^(-pKa) and [H+] = 10^(-pH). The maximum buffer capacity occurs at pH = pKa and equals beta(max) = 2.303 * C / 4 = 0.576 * C.
Practical buffer design
- For a phosphate buffer at pH 7.4, use KH(2)PO(4) / K(2)HPO(4) (pKa 7.20). Buffer capacity is high near physiological pH.
- Acetate buffers (pKa 4.76) work well in the pH 3.6 to 5.6 range.
- Increasing total buffer concentration C proportionally increases buffer capacity without changing pH.
- The Henderson-Hasselbalch equation (pH = pKa + log([A-]/[HA])) determines which ratio of acid to base to mix.
- At pH = pKa plus or minus 1, buffer capacity drops to about 33% of its maximum value.
Frequently asked questions
What is buffer capacity?
Buffer capacity (beta) is the amount of strong acid or base (in moles) needed to change the pH of one liter of buffer by one pH unit. A higher beta means the buffer resists pH changes more strongly.
What is the Van Slyke equation for buffer capacity?
The Van Slyke equation is: beta = 2.303 * C * Ka * [H+] / (Ka + [H+])^2, where C is the total buffer concentration, Ka is the acid dissociation constant, and [H+] is the hydrogen ion concentration at the pH of interest.
When is buffer capacity maximum?
Buffer capacity is maximum when pH equals pKa (the acid's pKa), i.e., when [H+] = Ka. At this point the Henderson-Hasselbalch ratio [A-]/[HA] is 1:1, giving the maximum buffering power.
What is the useful buffering range?
A buffer is effective within approximately one pH unit on either side of the pKa. Outside this range (pH less than pKa - 1 or pH greater than pKa + 1), the buffer capacity drops significantly.
What does total buffer concentration C mean?
C is the sum of all forms of the buffer species: C = [HA] + [A-]. It equals the initial concentration of the buffering compound before any equilibrium is established.
Official sources
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.