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pH at Half-Equivalence Point:
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The pH at half-equivalence point in a titration curve is equal to the pKa (acid dissociation constant) of the weak acid being titrated. This relationship provides a simple method for determining the pKa of an acid from its titration curve.
The calculation uses the fundamental relationship:
Where:
Explanation: At the half-equivalence point in a weak acid-strong base titration, exactly half of the weak acid has been neutralized, creating a buffer solution where [HA] = [A⁻], making pH = pKa.
Details: Determining pH at various points in a titration curve is essential for understanding acid-base chemistry, identifying equivalence points, and calculating the pKa values of weak acids, which is crucial in pharmaceutical, biochemical, and analytical applications.
Tips: Enter the pKa value of the weak acid. The calculator will return the pH at the half-equivalence point, which is numerically equal to the pKa value entered.
Q1: Why is pH equal to pKa at half-equivalence point?
A: At half-equivalence, [HA] = [A⁻], making the Henderson-Hasselbalch equation simplify to pH = pKa + log(1) = pKa.
Q2: Does this relationship hold for all titration types?
A: This relationship is specific to weak acid-strong base titrations. For weak base-strong acid titrations, a similar relationship exists where pOH = pKb at half-equivalence.
Q3: How accurate is this method for determining pKa?
A: This method provides a reasonably accurate estimate of pKa, typically within ±0.1 units, when the titration is performed carefully and the equivalence point is clearly identifiable.
Q4: What factors can affect the accuracy of this calculation?
A: Concentration errors, temperature variations, ionic strength effects, and imprecise determination of the equivalence point can all affect accuracy.
Q5: Can this calculator be used for polyprotic acids?
A: For polyprotic acids, each dissociation has its own half-equivalence point where pH equals the corresponding pKa for that dissociation step.