Sodium Phosphate Buffer Calculator

Henderson-Hasselbalch Equation for Phosphate Buffer:

\[ pH = pKa2 + \log\left(\frac{[Na₂HPO₄]}{[NaH₂PO₄]}\right) \]

pKa2 Value:

(dimensionless)

[Na₂HPO₄] Concentration:

mol/L

[NaH₂PO₄] Concentration:

mol/L

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 dibasic (Na₂HPO₄) and monobasic (NaH₂PO₄) sodium phosphate. It is effective in the physiological pH range and widely used in biochemical and molecular biology applications.

2. How Does the Calculator Work?

The calculator uses the Henderson-Hasselbalch equation:

\[ pH = pKa2 + \log\left(\frac{[Na₂HPO₄]}{[NaH₂PO₄]}\right) \]

Where:

\( pH \) — Resulting pH of the buffer solution
\( pKa2 \) — Acid dissociation constant for H₂PO₄⁻ ⇌ H⁺ + HPO₄²⁻ (typically 7.2)
\( [Na₂HPO₄] \) — Concentration of dibasic sodium phosphate (mol/L)
\( [NaH₂PO₄] \) — Concentration of monobasic sodium phosphate (mol/L)

Explanation: The equation calculates the pH based on the ratio of the conjugate base (HPO₄²⁻) to weak acid (H₂PO₄⁻) concentrations.

3. Importance of pH Calculation

Details: Accurate pH calculation is crucial for preparing buffer solutions with desired properties for biological experiments, pharmaceutical formulations, and chemical processes where pH stability is essential.

4. Using the Calculator

Tips: Enter pKa2 value (typically 7.2 for phosphate buffer), concentrations of both sodium phosphate components in mol/L. All concentration values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What is the typical pKa2 value for phosphate buffer?
A: The pKa2 for the phosphate buffer system is typically 7.2 at 25°C, though it can vary slightly with temperature and ionic strength.

Q2: What is the effective buffering range of phosphate buffer?
A: Phosphate buffer is most effective in the pH range of 6.2-8.2, with optimal buffering capacity at pH 7.2.

Q3: Why use sodium phosphate instead of potassium phosphate?
A: Sodium phosphate is often preferred when sodium ions are less likely to interfere with biological systems compared to potassium ions.

Q4: Are there limitations to phosphate buffers?
A: Phosphate buffers can precipitate with calcium ions, affect enzymatic activities, and may not be suitable for systems sensitive to phosphate.

Q5: How does temperature affect the pKa2 value?
A: The pKa2 of phosphate decreases slightly with increasing temperature (approximately -0.0028 units per °C).