LabCalc

Buffer pH Calculator

pH = pKa + log₁₀([A⁻] / [HA])

Solve the Henderson–Hasselbalch equation for pH, pKa or the conjugate base to acid ratio, and get the amounts needed to prepare the buffer.

Inputs

Use the pKa of the relevant ionisation at your working temperature.

Preparation (optional)

Ct = [HA] + [A⁻]

Masses are only shown when you enter the molecular weight of the salt form you actually have. No molecular weight is assumed.

Result

[A−] / [HA] = 1.585

Acid 38.6863 mmol and conjugate base 61.3137 mmol in 1000 mL.

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Calculation

  1. Henderson-Hasselbalch

    pH = pKa + log10([A−] / [HA])

  2. Rearranged

    [A−] / [HA] = 10^(pH − pKa)

    ratio = 10^(7.4 − 7.2)

    [A−] / [HA] = 1.585

  3. Acid / base ratio

    [HA] / [A−] = 1 / R

    [HA] / [A−] = 0.631

  4. Split the total buffer concentration

    [HA] = Ct / (1 + R), [A−] = Ct × R / (1 + R)

    Ct = 0.1 mol/L, R = 1.585

    [HA] = 0.0386863 mol/L, [A−] = 0.0613137 mol/L

  5. Amount of each component

    n = c × V

    V = 1 L

    n(HA) = 38.6863 mmol, n(A−) = 61.3137 mmol

Inputs

pH
7.4
pKa
7.2
[A−] / [HA]
1.585
Total buffer concentration
0.1 mol/L
Final volume
1 L

The Henderson-Hasselbalch equation assumes a conjugate acid/base pair and ideal, concentration-based behaviour. It ignores activity coefficients, ionic strength and temperature effects, and it is inaccurate far from the pKa or at very low buffer concentrations.

No buffer system, pKa or molecular weight is chosen for you — every value shown was entered above.

Always verify and adjust the final pH with a calibrated pH meter.

About this calculator

The Henderson–Hasselbalch equation relates the pH of a buffer to the pKa of the weak acid and the ratio of conjugate base to acid. Solving for the ratio tells you how to split a chosen total buffer concentration between the two species.

This is an idealised, concentration-based treatment: activity coefficients and ionic strength are not accounted for, and no buffer system, pKa or molecular weight is ever filled in for you. Always check the pH of the prepared buffer with a calibrated meter at the working temperature.

LabCalc is provided as a calculation aid. Users remain responsible for verifying calculations, units, chemical compatibility, experimental procedures and applicable safety requirements before laboratory use.

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