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About this tool
Calculate buffer pH using the Henderson-Hasselbalch equation.
The Buffer pH Calculator applies the Henderson–Hasselbalch equation, pH = pKa + log10([A⁻]/[HA]), to return the estimated pH of a buffer from the acid's pKa and the amounts of weak acid and conjugate base you enter. It reports the pH to six decimals along with the base-to-acid ratio and the mole fraction of each species, and accepts either concentrations or moles because the equation depends only on their ratio. It is an approximation: activity coefficients, ionic strength, dilution, competing equilibria and the temperature dependence of pKa are not modelled, so the temperature field is recorded as a note rather than used in the calculation.
Open Buffer pH Calculator on AltFTool — it loads instantly in your browser.
Enter Acid pKa, Acid concentration or moles and Conjugate-base concentration or moles, and add a Temperature note (°C) if you want it recorded.
Press the Equal acetate buffer example under Examples to load pKa 4.76 with 0.1 and 0.1, or Reset to restore the defaults.
Read the estimated pH to six decimals with the Base / acid ratio, Acid fraction and Base fraction rows, then use Copy or Download.
Only the base-to-acid ratio enters the equation, so molarity and mole quantities give the same pH without conversion.
Seeing base/acid alongside the acid and base mole fractions makes it obvious how far from the 1:1 midpoint your buffer sits.
The note names the assumptions — no activity coefficients, no ionic-strength correction, no temperature-adjusted pKa — so results are read as an estimate to verify with a meter.
pH = pKa + log10([conjugate base] / [weak acid]). When base and acid are equal the log term is zero and the pH equals the pKa exactly, which is why an equimolar acetate buffer sits at about 4.76.
Rearrange to ratio = 10^(pH − pKa). For a target pH of 5.00 with a pKa of 4.76 the ratio is 10^0.24, about 1.74 parts conjugate base to 1 part acid.
Roughly pKa ± 1, which corresponds to base-to-acid ratios between 1:10 and 10:1. Outside that window one component is nearly exhausted and the buffer capacity to resist added acid or base falls away quickly.
Because Henderson–Hasselbalch takes temperature effects through the pKa, not as a separate term, and the calculator uses the pKa exactly as you enter it. If your buffer is used at 37 °C rather than 25 °C, look up the pKa at that temperature and enter it — for Tris the shift is substantial, while for acetate it is small.
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