Half Equivalence Point From pH
A half equivalence point is a key point in many acid base titrations. At this point, exactly half of the original weak acid or weak base has reacted. The buffer pair has equal amounts of conjugate forms. That makes the pH very useful.
For a weak acid titrated with a strong base, the pH equals pKa at half equivalence. For a weak base titrated with a strong acid, the pOH equals pKb. The pH also equals the pKa of the conjugate acid when the usual pKw relationship is used. This calculator uses that relationship and shows the linked constants.
Why This Matters
The half point helps identify acid strength, buffer behavior, and unknown samples. It is often easier to read than the exact equivalence point. The curve is flatter near the buffer zone. Small volume errors usually have less effect there. That is why many lab reports use the half point for pKa estimation.
The calculator accepts pH, titration type, pKw, volume data, and concentration data. It can return Ka, Kb, pKa, pKb, half volume, equivalence volume, moles, and estimated sample concentration. Optional fields let you work backward from a measured half volume. They also let you estimate the expected endpoint from a known sample.
Formula Used
The main weak acid rule is pH = pKa at half equivalence. Therefore, Ka = 10^-pKa. For weak base work, pOH = pKw - pH. At half equivalence, pOH = pKb. Therefore, Kb = 10^-pKb. The conjugate relationship is pKa + pKb = pKw. When an equivalence volume is known, the half equivalence volume is Ve / 2. When the half volume is known, Ve = 2Vhalf.
How To Use This Calculator
Enter the pH reading taken at the half equivalence point. Choose whether the analyte is a weak acid or weak base. Keep pKw at 14.00 for room temperature water, or enter another value if needed. Add equivalence volume or half volume when available. Add titrant strength and sample volume for concentration estimates. Press calculate. Review the result, formula steps, and warnings. Then export the record as CSV or PDF. Use calibrated meters and consistent units. Record temperature with every run. These habits make repeated calculations cleaner and easier to compare during classroom or plant work.