1. What is the pH Calculation for Diprotic Acids?

The formula pH ≈ ½ (pKa1 + pKa2) is used for diprotic acids at the intermediate point where the concentrations of H₂A and HA²⁻ are approximately equal. This represents a special case in acid-base chemistry for polyprotic acids.

2. How Does the Calculator Work?

The calculator uses the diprotic acid pH formula:

Where:

• \( pH \) — Negative logarithm of hydrogen ion concentration (dimensionless)
• \( pKa1 \) — Negative logarithm of first acid dissociation constant (dimensionless)
• \( pKa2 \) — Negative logarithm of second acid dissociation constant (dimensionless)

Explanation: This formula applies specifically when [H₂A] ≈ [HA²⁻], which occurs at the intermediate point between the two equivalence points in the titration of a diprotic acid.

3. Importance of pH Calculation

Details: Accurate pH calculation is crucial for understanding acid-base equilibria, predicting chemical behavior in solutions, and designing buffer systems for various chemical and biological applications.

4. Using the Calculator

Tips: Enter both pKa1 and pKa2 values as dimensionless numbers. The calculator will compute the approximate pH at the intermediate point where [H₂A] ≈ [HA²⁻].

5. Frequently Asked Questions (FAQ)

Q1: When does this formula apply exactly?
A: This formula gives the exact pH when [H₂A] = [HA²⁻] and provides a good approximation near this point for most diprotic acids.

Q2: What are typical pKa values for common diprotic acids?
A: Common examples include carbonic acid (pKa1=6.35, pKa2=10.33) and sulfuric acid (pKa1=-3, pKa2=1.99).

Q3: Can this formula be used for triprotic acids?
A: No, this specific formula applies only to diprotic acids. Triprotic acids have additional pKa values and more complex pH relationships.

Q4: What are the limitations of this approximation?
A: The approximation becomes less accurate when the two pKa values are very close together (ΔpKa < 2) or when dealing with very dilute solutions.

Q5: How does temperature affect this calculation?
A: Temperature affects both pKa values, so the calculated pH will vary with temperature. The relationship is complex and depends on the specific acid.