Understanding the Basics
* Sodium Phosphate Buffer: This buffer is typically made using a mixture of monobasic sodium phosphate (NaH2PO4) and dibasic sodium phosphate (Na2HPO4). These two components act as a conjugate acid-base pair, allowing the solution to resist changes in pH.
* pH 6.9: The desired pH of your buffer. This will determine the ratio of NaH2PO4 to Na2HPO4 needed.
* 0.006 M NaCl: This is simply a salt solution added to the buffer. It doesn't directly influence the buffer's pH but might be used for specific experimental needs.
Steps to Prepare the Buffer
1. Calculate the Required Masses:
* Determine the pKa: The relevant pKa for phosphate buffer is 7.21 (the pKa of the H2PO4- / HPO42- equilibrium). This is close to your desired pH, making it a good choice.
* Use the Henderson-Hasselbalch equation:
* pH = pKa + log ([HPO42-] / [H2PO4-])
* 6.9 = 7.21 + log ([HPO42-] / [H2PO4-])
* Solve for the ratio: [HPO42-] / [H2PO4-] = 0.50
* Calculate the molarity of each phosphate component:
* Total phosphate molarity = 0.02 M
* [Na2HPO4] = 0.02 M * (0.50 / 1.50) = 0.0067 M (approximately)
* [NaH2PO4] = 0.02 M * (1.00 / 1.50) = 0.0133 M (approximately)
* Convert molarities to grams:
* You'll need the molecular weights of NaH2PO4 (119.98 g/mol) and Na2HPO4 (141.96 g/mol).
* Calculate the mass needed for each component using the following formula: Mass = (Molarity * Volume * Molecular weight) / 1000
* For example, for NaH2PO4, if you want to make 1 L of buffer:
* Mass = (0.0133 mol/L * 1 L * 119.98 g/mol) / 1000 = 1.59 g
* Calculate the mass of Na2HPO4 similarly.
2. Dissolve the Salts in Water:
* Weigh out the calculated amounts of NaH2PO4 and Na2HPO4.
* Dissolve them in about 800 mL of distilled water.
* Add the calculated mass of NaCl to the solution and stir to dissolve.
* Use a magnetic stirrer or stir bar to ensure thorough mixing.
3. Adjust the pH (optional):
* Use a pH meter to check the pH of the solution.
* If the pH is not exactly 6.9, you can adjust it slightly by adding small amounts of either 1 M HCl (to lower the pH) or 1 M NaOH (to raise the pH). Add these solutions dropwise while monitoring the pH with a meter.
4. Bring the Solution to the Final Volume:
* Once the pH is adjusted, carefully add distilled water to bring the total volume of the solution to 1 L.
5. Check and Store:
* Check the pH one more time to ensure it is correct.
* Store your buffer at room temperature, or in a refrigerator for longer storage.
Important Considerations
* Quality of Water: Use high-quality distilled water for the best results.
* Accuracy: Precisely weigh the salts to ensure the correct buffer concentration.
* pH Meter Calibration: Calibrate your pH meter properly before use for accurate measurements.
* Safety: Always wear appropriate safety gear (gloves, goggles) when handling chemicals.
This procedure should give you a 0.02 M sodium phosphate buffer solution at pH 6.9 with 0.006 M NaCl. Remember to adjust the volumes and calculations if you need to make a different volume of buffer.
