How do you work out the pKa of a substance from the titration curve that has two equivalence points?
To determine the pKa of a substance from a titration curve with two equivalence points, you need to consider the corresponding pH values at each equivalence point. The pKa is the negative logarithm (base 10) of the acid dissociation constant, Ka. Here is a step-by-step guide to find the pKa:
1. Identify the two equivalence points on the titration curve. The first equivalence point corresponds to the complete neutralization of the first acidic proton, while the second equivalence point corresponds to the complete neutralization of the second acidic proton.
2. Determine the pH value at each equivalence point. To do this, locate the steepest part of the titration curve around each equivalent point and find the corresponding pH values using the scale provided.
3. Calculate the concentration of the analyte at each equivalence point. At the first equivalence point, the concentration of the analyte will be equal to the concentration of the acid. At the second equivalence point, the concentration of the analyte will be twice the concentration of the acid.
4. Use the Henderson-Hasselbalch equation to find the pKa for each equivalence point. The Henderson-Hasselbalch equation is given as:
pKa = pH + log10 ([A-] / [HA])
Where [HA] is the concentration of the acidic form (unprotonated) and [A-] is the concentration of the conjugate base (protonated).
For the first equivalence point, [HA] is equal to the initial concentration of the analyte, and [A-] is zero. Therefore, the pKa can be calculated directly using the pH value at the first equivalence point.
For the second equivalence point, [HA] is equal to half the initial concentration of the analyte, while [A-] is equal to 0.5 times the initial concentration. Use these values and the pH at the second equivalence point to calculate the pKa.
5. Report the pKa values obtained at each equivalence point.
By following these steps, you can determine the pKa of a substance from a titration curve with two equivalence points.