Sodium Phosphate Buffer Preparation Calculator

Sodium Phosphate Buffer Equation:

\[ \frac{[HPO_4^{2-}]}{[H_2PO_4^-]} = 10^{(pH - pKa_2)} \]

[HPO₄^2-] (dibasic) concentration:

[H₂PO₄^-] (monobasic) concentration:

Ratio (dibasic/monobasic):

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1. What is Sodium Phosphate Buffer?

Sodium phosphate buffer is a commonly used buffer system in biological and chemical applications. It consists of a mixture of monobasic (NaH₂PO₄) and dibasic (Na₂HPO₄) sodium phosphate, which act as weak acid and conjugate base respectively.

2. How Does the Calculator Work?

The calculator uses the Henderson-Hasselbalch equation for the second dissociation of phosphoric acid:

\[ \frac{[HPO_4^{2-}]}{[H_2PO_4^-]} = 10^{(pH - pKa_2)} \]

Where:

\( [HPO_4^{2-}] \) — Dibasic phosphate concentration (mol/L)
\( [H_2PO_4^-] \) — Monobasic phosphate concentration (mol/L)
\( pH \) — Desired pH of the buffer
\( pKa_2 \) — Second dissociation constant of phosphoric acid (typically 7.21 at 25°C)

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

3. Importance of Phosphate Buffer

Details: Phosphate buffers are widely used in biological research, medical diagnostics, and pharmaceutical applications due to their good buffering capacity in the physiological pH range (6.5-8.0).

4. Using the Calculator

Tips: Enter your desired pH (typically between 6.0-8.0), the pKa₂ value (default is 7.21), and the total phosphate concentration you want to prepare. The calculator will determine the required amounts of monobasic and dibasic phosphate.

5. Frequently Asked Questions (FAQ)

Q1: What is the effective buffering range of phosphate buffer?
A: Phosphate buffer is most effective in the pH range of 6.5-8.0, centered around its pKa₂ of 7.21.

Q2: How does temperature affect the pKa₂?
A: The pKa₂ decreases slightly with increasing temperature (about -0.0028 units per °C).

Q3: What are typical concentrations for biological buffers?
A: Common concentrations range from 0.01 M to 0.1 M, depending on the application.

Q4: Can I use potassium phosphate instead of sodium phosphate?
A: Yes, the calculations are the same, but potassium salts may be preferred in some applications.

Q5: How should I prepare the buffer solution?
A: Dissolve the calculated amounts in water, check pH with a calibrated pH meter, and adjust if necessary.