1. What is pKa?

pKa is the negative logarithm of the acid dissociation constant (Ka). It represents the pH at which half of the acid molecules are dissociated and half are undissociated. A lower pKa value indicates a stronger acid.

2. How to Determine pKa from Titration Curve

The pKa can be determined from a pH titration curve by identifying the pH value at the half-equivalence point:

Where:

• \( pKa \) — Acid dissociation constant (dimensionless)
• \( pH \) — pH value at half-equivalence point (dimensionless)

Explanation: At the half-equivalence point of a weak acid titration, exactly half of the acid has been neutralized by base. At this point, the concentrations of the acid and its conjugate base are equal, making pH = pKa.

3. Importance of pKa Calculation

Details: pKa values are crucial for understanding acid-base chemistry, predicting reaction outcomes, designing pharmaceutical compounds, and understanding biochemical processes. They help determine the extent of ionization of compounds at different pH levels.

4. Using the Calculator

Tips: Enter the pH value measured at the half-equivalence point of your titration curve. The value must be between 0-14 pH units. The calculator will return the pKa value which is numerically equal to the pH at this point.

5. Frequently Asked Questions (FAQ)

Q1: Why is pKa equal to pH at half-equivalence point?
A: According to the Henderson-Hasselbalch equation, when [HA] = [A⁻], the log term becomes zero, making pH = pKa.

Q2: What is the typical range of pKa values?
A: pKa values typically range from -10 to 50, with most common acids falling between 0-14. Strong acids have negative pKa values, while weak acids have positive values.

Q3: Can this method be used for polyprotic acids?
A: Yes, for polyprotic acids, each equivalence point corresponds to a different pKa value. The first half-equivalence point gives pKa₁, the second gives pKa₂, etc.

Q4: What factors affect pKa determination accuracy?
A: Temperature, ionic strength, and measurement precision can affect the accuracy. The titration should be performed under controlled conditions for best results.

Q5: How does pKa relate to buffer capacity?
A: Maximum buffer capacity occurs when pH = pKa. Buffers are most effective within ±1 pH unit of their pKa value.