Estimate weak-acid buffer capacity
Buffer capacity describes how much strong acid or base is required per litre for a small pH change. Enter total concentration of one monoprotic weak-acid/conjugate-base pair, its pKa, and the pH. The calculator applies a Van Slyke form and reports β in mol/L per pH unit, with an option to include the water contribution at 25°C.
The buffer-pair term is largest near pH = pKa. For 0.1 mol/L total buffer at pH 4.76 with pKa 4.76, that term is about 0.057575 mol/L per pH unit. This does not mean any 0.057575-mole addition will always change one litre by exactly one pH unit; β is a local differential measure around the entered state.
Read the Van Slyke relationship
The pair contribution is 2.303CtKa[H+]/(Ka+[H+])². The calculator obtains Ka from 10−pKa and hydrogen-ion concentration from 10−pH. Total buffer concentration Ct is the sum of weak acid and conjugate base in the simple model. The factor 2.303 converts the natural-logarithm derivative to a base-10 pH scale.
When pH equals pKa, Ka and [H+] are equal and the expression simplifies to 2.303Ct/4. Moving far above or below pKa lowers the pair contribution because one form dominates. Raising total buffer concentration raises capacity proportionally when pH and pKa stay fixed. These trends help check whether an entered decimal point or concentration unit is plausible.
Choose whether to include water
The optional water term is 2.303([H+]+Kw/[H+]) with Kw = 1×10−14 at 25°C. Near neutral pH it is small compared with many practical buffers, but it grows at very acidic or basic conditions. The result separates pair and water contributions so the selected assumption remains visible instead of silently changing the answer.
Exclude water when a course exercise requests only the weak-acid pair expression. Include it when the named model includes solvent contributions under the stated 25°C assumption. Other temperatures change Kw. This option does not add capacity from extra acids, bases, salts, polyprotic steps, precipitation, or reactions that may exist in a real formulation.
Interpret and check the result
Capacity is positive and carries a per-pH-unit meaning. Doubling Ct should double the pair term, and matching pH to pKa should maximize that term for fixed Ct. Compare the separate contributions and make sure the displayed scale fits the concentration entered. Use a full equilibrium or titration calculation when the proposed acid or base addition is not small.
Concentrations replace activities in this teaching model, so ionic strength and non-ideal behaviour are not resolved. A measured buffer may also change volume, temperature, or composition during addition. Treat β as a local estimate useful for learning and comparison, not a guarantee that the pH remains fixed under an arbitrary dose.
Model limits and private calculation
The calculator covers one monoprotic weak-acid/conjugate-base pair. It does not select a buffer, calculate component recipes, solve polyprotic overlap, model biological matrices, or establish product stability. Laboratory and process decisions should use validated equilibrium data, temperature, ionic strength, and the actual formulation when those effects matter.
The values are calculated in the current browser tab. NexaCurrent does not upload the quantities, formulas, chemical names, element choices, or other information entered here, and no account is needed. Copying is a deliberate action after the answer and working are visible. Refreshing or leaving the page clears the current calculation, so keep a copied result if it is needed for later study.