Calculate acidic or basic buffer pH
Choose an acidic buffer made from a weak acid and conjugate base, or a basic buffer made from a weak base and conjugate acid. Enter pKa or pKb and positive matching amounts or concentrations. The calculator uses their dimensionless ratio and displays the logarithm, pH or pOH substitution, and final buffer pH.
Equal weak-acid and conjugate-base amounts give pH = pKa. Equal weak-base and conjugate-acid amounts give pOH = pKb, followed by pH = 14−pOH under the 25°C assumption. These equal-ratio cases are useful reference points and make it easy to notice when numerator and denominator have been reversed.
Use Henderson–Hasselbalch for an acid buffer
Acid mode uses pH = pKa + log10(base/acid). The numerator is the conjugate base and the denominator is the weak acid. More conjugate base raises the calculated pH; more weak acid lowers it. For pKa 4.76 and equal amounts, the logarithm of one is zero and the result is pH 4.76.
Amounts may be moles or concentrations when both entries share a unit and represent the same final solution basis. Do not divide millimoles by mol/L or grams by moles. If volumes differ before mixing, first determine final amounts or concentrations consistently. The calculator intentionally labels each role to reduce the common mistake of treating every acid/base pair with the same ratio order.
Use the basic-buffer relationship
Base mode uses pOH = pKb + log10(conjugate acid/weak base). It then converts pOH to pH with pKw = 14 at 25°C. For pKb 4.75 and equal conjugate-acid and base amounts, pOH is 4.75 and pH is 9.25. A larger conjugate-acid share raises pOH and lowers pH.
The fixed pKw assumption is appropriate only for the stated dilute-aqueous teaching context. Water autoionization changes with temperature, and thermodynamic pH depends on activity. If a source supplies a temperature-specific pKw, a rigorous calculation should use that value rather than applying 14 automatically. Keep the temperature premise beside any copied basic-buffer result.
Check whether the approximation is suitable
The conjugate-pair ratio must be positive and dimensionless. A ratio of ten changes pH or pOH by one unit; a ratio of one tenth changes it by minus one. Reversing the ratio should reflect the answer across pKa or pKb. These simple logarithm checks expose swapped fields and inconsistent powers of ten.
Henderson–Hasselbalch works best for a meaningful buffer containing appreciable quantities of both conjugate forms. Very extreme ratios, very dilute solutions, strong-acid or strong-base excess, activity effects, and coupled equilibria can make the approximation poor. A numerical answer is not proof that the entered mixture actually behaves as an effective buffer.
Scope and private use
This buffer pH calculator does not choose reagents, balance neutralization, determine post-mixing concentrations, or account for ionic strength, temperature-dependent constants, polyprotic species, precipitation, or biological reactions. It evaluates the stated conjugate-pair relationship. Use a full equilibrium model when the composition falls outside those assumptions.
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.