Equivalence Point Calculator
Determine the equivalence point volume and the pH expected there.
About Equivalence Point Calculator
Titrating a weak acid with a strong base gives an equivalence pH above 7, because the conjugate base hydrolyses. The indicator must change colour across that pH, which is why phenolphthalein suits weak-acid titrations and methyl red does not. Assuming equivalence occurs at pH 7 leads to the wrong indicator and a systematic titration error.
Formula
Equivalence Point
Equivalence pH for a weak acid comes from hydrolysis of its conjugate base at the diluted concentration
- equivalence point
- Stoichiometric completion, not necessarily pH 7
- indicator
- Chosen so its colour change spans the equivalence pH
Worked example
With the default values below, Equivalence Point Calculator returns Equivalence Volume of 25.0000. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Analyte volumeminimum 0 mL
- 25 mL
- Analyte concentrationminimum 0 mol/L
- 0.1 mol/L
- Titrant concentrationminimum 0 mol/L
- 0.1 mol/L
- pKa of the weak analyteaccepts -2–16
- 4.76
Results
- Equivalence Volume
- 25.0000 mL
- pH at the Equivalence Point
- 8.7295
- Salt Concentration at Equivalence
- 0.050000 mol/L
- Suggested Indicator
- Phenolphthalein — colour change 8.2 to 10
- pH at Half Equivalence
- 4.7600
Step by step
- 1Values used
- 2Equivalence Point
- 3Equivalence Volume= 25.0000 mL
- 4pH at the Equivalence Point= 8.7295
- 5Salt Concentration at Equivalence= 0.050000 mol/L
- 6Suggested Indicator= Phenolphthalein — colour change 8.2 to 10
- 7pH at Half Equivalence= 4.7600
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 6 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Equivalence Point calculated?
Equivalence pH for a weak acid comes from hydrolysis of its conjugate base at the diluted concentration Titrating a weak acid with a strong base gives an equivalence pH above 7, because the conjugate base hydrolyses. The indicator must change colour across that pH, which is why phenolphthalein suits weak-acid titrations and methyl red does not.
Why does Equivalence Point matter?
Assuming equivalence occurs at pH 7 leads to the wrong indicator and a systematic titration error.
What values do I need to enter?
This calculator takes 4 inputs: Analyte volume, Analyte concentration, Titrant concentration, pKa of the weak analyte. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.