1. What is Sodium Phosphate Buffer?

Sodium phosphate buffer is a commonly used buffer system in biochemistry and molecular biology. It consists of a mixture of monobasic (NaH₂PO₄) and dibasic (Na₂HPO₄) sodium phosphate that maintains stable pH in the range of 5.8 to 8.0.

2. How Does the Calculator Work?

The calculator uses the Henderson-Hasselbalch equation:

Where:

• \( pH \) — Desired pH of the buffer
• \( pKa \) — Acid dissociation constant (7.21 for phosphate buffer at 25°C)
• \( [HPO_4^{2-}] \) — Concentration of dibasic phosphate
• \( [H_2PO_4^-] \) — Concentration of monobasic phosphate

Explanation: The equation calculates the ratio of dibasic to monobasic phosphate needed to achieve the desired pH.

3. Importance of Phosphate Buffer

Details: Phosphate buffers are widely used in biological research because they are nontoxic, compatible with most biological molecules, and effective in physiological pH ranges.

4. Using the Calculator

Tips: Enter desired pH (typically between 6.0-8.0), pKa (7.21 for phosphate), total buffer concentration, and desired volume. The calculator will determine the amounts of each salt needed.

5. Frequently Asked Questions (FAQ)

Q1: What's the effective pH range for phosphate buffer?
A: Phosphate buffer works best between pH 5.8-8.0. Outside this range, its buffering capacity decreases significantly.

Q2: Should I use anhydrous or hydrated salts?
A: This calculator assumes anhydrous salts. If using hydrated forms (e.g., Na₂HPO₄·7H₂O), adjust the molecular weight accordingly.

Q3: How does temperature affect the buffer?
A: The pKa of phosphate buffer changes with temperature (about -0.0028 units per °C). For precise work, adjust pKa for your temperature.

Q4: Can I use potassium phosphate instead?
A: Yes, but you'll need to use different molecular weights (K₂HPO₄ and KH₂PO₄).

Q5: How should I prepare the buffer?
A: Dissolve the calculated amounts in about 80% of the final volume, adjust pH if needed, then bring to final volume.