Sodium Phosphate Buffer Calculator Ph 8

Henderson-Hasselbalch Equation:

\[ pH = pKa2 + \log\left(\frac{[HPO_4^{2-}]}{[H_2PO_4^-]}\right) \]

Calculated pH:

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1. What is the Sodium Phosphate Buffer System?

The sodium phosphate buffer system is a commonly used biological buffer based on the equilibrium between dihydrogen phosphate (H₂PO₄⁻) and monohydrogen phosphate (HPO₄²⁻) ions. It is particularly effective in the physiological pH range around 7.2.

2. How Does the Calculator Work?

The calculator uses the Henderson-Hasselbalch equation:

\[ pH = pKa2 + \log\left(\frac{[HPO_4^{2-}]}{[H_2PO_4^-]}\right) \]

Where:

\( pH \) — Calculated pH value (dimensionless)
\( pKa2 \) — Acid dissociation constant for H₂PO₄⁻ ⇌ H⁺ + HPO₄²⁻ (typically 7.21 at 25°C)
\( [HPO_4^{2-}] \) — Concentration of monohydrogen phosphate (mol/L)
\( [H_2PO_4^-] \) — Concentration of dihydrogen phosphate (mol/L)

Explanation: The equation calculates the pH of a phosphate buffer solution based on the ratio of the conjugate base to weak acid concentrations.

3. Importance of pH Calculation

Details: Accurate pH calculation is crucial for preparing biological buffers, maintaining enzyme activity, cell culture conditions, and various biochemical experiments where precise pH control is essential.

4. Using the Calculator

Tips: Enter concentrations of HPO₄²⁻ and H₂PO₄⁻ in mol/L, and the pKa2 value (default is 7.21). All concentration values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: Why is pKa2 typically 7.21?
A: The pKa2 value of 7.21 is the standard value for the phosphate buffer system at 25°C. This value may vary slightly with temperature and ionic strength.

Q2: What is the effective buffer range for phosphate buffers?
A: Phosphate buffers are most effective in the pH range of approximately 6.2-8.2, with optimal buffering capacity near pKa2 (7.21).

Q3: How does temperature affect the calculation?
A: Temperature affects the pKa2 value. The pKa2 decreases by approximately 0.0028 units per degree Celsius increase in temperature.

Q4: Are there limitations to this equation?
A: The Henderson-Hasselbalch equation assumes ideal behavior and may be less accurate at very high concentrations or extreme pH values where activity coefficients deviate significantly from 1.

Q5: Can this calculator be used for other buffer systems?
A: While the equation form is general, this calculator is specifically designed for the phosphate buffer system with pKa2 around 7.21.