Buffer Capacity Calculator
Calculate the buffer capacity (β) of a weak acid / conjugate base buffer from the two concentrations, using the Van Slyke equation. Useful for chemists and biochemists designing stable-pH solutions.
Last updated: September 2026
Formula below · 2 sources (pubchem.ncbi.nlm.nih.gov, Wikipedia) · Updated Sep 2026
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About this calculator
Buffer capacity (β) measures how much strong acid or base a buffer can absorb per unit change in pH. For a weak acid / conjugate base pair in the usual buffering range, the Van Slyke equation gives β = 2.303 × [HA] × [A⁻] / ([HA] + [A⁻]), where [HA] is the weak acid concentration and [A⁻] is the conjugate base concentration (mol/L), and β is in mol·L⁻¹ per pH unit. (The full equation adds 2.303([H⁺] + [OH⁻]) for water, which matters only below pH ~3 or above ~11.) The buffer's identity enters only through its pKa, which sets the ratio [A⁻]/[HA] at a given pH; for given concentrations, acetate, phosphate, Tris and bicarbonate have the same β. Maximum buffer capacity occurs when [HA] = [A⁻], i.e., when pH = pKa, where β = 0.576 × C_total. A higher total buffer concentration raises β proportionally, while moving away from the pKa in either direction reduces it.
How to use
Suppose you have an acetic acid/acetate buffer with [HA] = 0.1 M and [A⁻] = 0.1 M. β = 2.303 × 0.1 × 0.1 / (0.1 + 0.1) = 0.02303 / 0.2 = 0.115 mol·L⁻¹·pH⁻¹ (the site displays 0.12). Enter 0.1 in Weak Acid Concentration and 0.1 in Conjugate Base Concentration. The result means adding about 0.0115 mol of strong acid per liter lowers the pH by roughly 0.1 unit. With a 9:1 ratio (0.18 M and 0.02 M, same total) β drops to 2.303 × 0.18 × 0.02 / 0.2 = 0.041 — about a third of the 1:1 value.
Frequently asked questions
What does buffer capacity mean and what are typical values for a biological buffer?
Buffer capacity (β) is defined as the moles of strong acid or base needed to change 1 liter of buffer by 1 pH unit. A higher β means the buffer resists pH changes more strongly. Typical biological buffers like PBS or HEPES at physiological concentrations (10–50 mM) have β values in the range of 0.005–0.05 mol·L⁻¹·pH⁻¹. Intracellular bicarbonate buffers in blood achieve β ≈ 0.06 near pH 7.4. Values below 0.01 are generally considered weak buffers.
How do I choose the right weak acid for a buffer at a specific target pH?
Select a weak acid whose pKa is within ±1 pH unit of your target pH — ideally matching it exactly. When pH = pKa, [HA] = [A⁻] and buffer capacity is maximized. For example, acetic acid (pKa 4.76) works well for buffers near pH 4–6, while phosphate (pKa₂ 7.2) is ideal near physiological pH. Using an acid whose pKa is far from your target pH results in a very low [A⁻]/[HA] ratio, meaning the buffer will be quickly overwhelmed.
Why does buffer capacity decrease when you move away from the pKa value?
Buffer capacity depends on the product [HA][A⁻] in the numerator of the van Slyke equation. This product is maximized when [HA] = [A⁻], which occurs at pH = pKa. As pH moves away from pKa, one species dominates and the other becomes vanishingly small, so their product — and thus β — drops sharply. Practically, a buffer is considered effective only within about one pH unit of its pKa. Beyond that range, it provides little resistance to pH change even at high total concentration.
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