Buffer Capacity Calculator
Approximate how resistant a buffer is to pH change using the Van Slyke buffer capacity equation.
Buffer capacity, β
0.106114 mol·L⁻¹·pH⁻¹
Base to acid ratio
1
Fraction acid (α)
0.6401
Estimated buffer capacity β = 0.106114 mol·L⁻¹·pH⁻¹. This means approximately that many moles of strong acid or base per liter are required to shift the pH by one unit near the specified conditions.
How to Use This Calculator
Gather buffer data
Measure or plan the concentrations of the weak acid and its conjugate base. Input values in moles per liter.
Enter pKa
Use the dissociation constant of the acid. Consult reference tables for the precise pKa.
Specify working pH
Provide the pH at which the buffer operates. Buffer capacity depends on pH relative to pKa.
Review β and ratios
The calculator reports buffer capacity, the base-to-acid ratio, and the acidic fraction of the buffer species.
Formula
β = 2.303 · (Cₐ + C_b) · (Kₐ · [H⁺]) / (Kₐ + [H⁺])²
This Van Slyke approximation estimates buffer capacity for a weak acid/conjugate base pair. Cₐ and C_b are the molar concentrations, Kₐ is 10^-pKa, and [H⁺] is 10^-pH.
Example
For an acetate buffer with 0.1 M acid, 0.1 M base, pKa 4.75, and pH 4.75, β ≈ 0.058 mol·L⁻¹·pH⁻¹.
Full Description
Buffer capacity quantifies a buffer’s resistance to pH change. High values indicate that large amounts of strong acid or base are required to alter the pH. The Van Slyke approximation provides a convenient estimate for simple buffers near their pKa.
The calculation relies on knowing the acid and base concentrations and the working pH. Because buffer behavior is most effective within ±1 pH unit of the pKa, capacity drops sharply when operating far from that range.
Use this tool to compare buffer formulations, evaluate robustness, or plan titrations that rely on sustained pH control.
Frequently Asked Questions
How accurate is this approximation?
It is most accurate near the buffer’s pKa and for moderate ionic strength. Complex buffers may require full equilibrium calculations.
Can I use total buffer concentration?
Yes. Provide the molar concentrations of the acid and conjugate base present. If only total buffer concentration is known, split it according to the base-to-acid ratio.
What if pH equals pKa?
The buffer is at maximum capacity when pH = pKa and concentrations of acid and base are equal.
Does temperature matter?
Temperature affects pKa and the dissociation equilibria. Adjust pKa for temperature when high precision is required.
Can this handle polyprotic acids?
This version assumes a monoprotic acid/base pair. For polyprotic systems, calculate capacity for each dissociation step or use specialized models.