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Ka Calculation Formula:
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The Ka (acid dissociation constant) calculation from the midpoint of a titration curve is a fundamental method in analytical chemistry for determining the strength of weak acids. At the half-equivalence point, pH equals pKa, providing a direct relationship for calculating the acid dissociation constant.
The calculator 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 converted to its conjugate base. At this point, [HA] = [A⁻], making pH = pKa according to the Henderson-Hasselbalch equation.
Details: Accurate Ka determination is crucial for understanding acid strength, predicting buffer capacity, designing pharmaceutical formulations, and studying chemical equilibrium in biological systems.
Tips: Enter the pH value at the half-equivalence point from your titration data. The value should be between 0 and 14. The calculator will automatically compute both Ka and pKa values.
Q1: Why is pH = pKa at the half-equivalence point?
A: According to the Henderson-Hasselbalch equation (pH = pKa + log([A⁻]/[HA])), when [A⁻] = [HA] at half-equivalence, log(1) = 0, so pH = pKa.
Q2: What are typical Ka values for common weak acids?
A: Acetic acid: ~1.8×10⁻⁵ M (pKa 4.76), Formic acid: ~1.8×10⁻⁴ M (pKa 3.75), Carbonic acid: ~4.3×10⁻⁷ M (pKa 6.37).
Q3: How accurate is this method for Ka determination?
A: This method provides good accuracy for monoprotic weak acids with clear inflection points in their titration curves.
Q4: Can this method be used for polyprotic acids?
A: For polyprotic acids, each equivalence point corresponds to a different proton dissociation, so multiple Ka values can be determined from different midpoints.
Q5: What factors can affect the accuracy of Ka determination?
A: Temperature, ionic strength, electrode calibration, and the presence of interfering substances can all affect measurement accuracy.