Buffer pH calculator
pH = pKa + log₁₀([A⁻] ÷ [HA]) — solve any one of the four values.
Conventional pKa values (25 °C)
Select a value to fill the pKa field. Polyprotic species are listed one pKa per row.
Fill the known values; leave the unknown blank and it is solved for you.
Formula and how to use it
Henderson–Hasselbalch relates a buffer's pH to its acid and base forms: pH = pKa + log₁₀([A⁻] ÷ [HA]), where [A⁻] is the conjugate-base concentration and [HA] the weak-acid concentration. The pKa is the acid's dissociation constant on a logarithmic scale, pKa = −log₁₀(Ka), and the ratio [A⁻]/[HA] sets how far the pH sits from the pKa.
Worked example: an acetate buffer with [HA] = 0.10 M and [A⁻] = 0.10 M has pH = 4.76 + log₁₀(1) = 4.76, the acetic acid pKa. A 10:1 base-to-acid ratio gives pH = 4.76 + 1.00 = 5.76. A buffer works best within about ±1 pH unit of its pKa, where the ratio stays between 0.1 and 10 and buffering capacity is greatest.
This calculator solves any one of pH, pKa, [A⁻], or [HA] from the other three, and includes a reference table of conventional pKa values at 25 °C. It applies the ideal Henderson–Hasselbalch equation; ionic-strength corrections and buffer-capacity modelling are not included.