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How can i prepare 0.5 lit phosphate buffer 25 mm ph 7.5
Accepted Answer
To prepare a 0.5 L phosphate buffer at 25 mM and pH 7.5, you'll need to use a combination of monobasic sodium phosphate (NaH2PO4) and dibasic sodium phosphate (Na2HPO4). Here's a general procedure:
1. Calculate the required masses:
- Use the Henderson-Hasselbalch equation (pH = pKa + log([A-]/[HA])) to determine the ratio of Na2HPO4 to NaH2PO4 needed for pH 7.5. The pKa for the phosphate buffer system is 7.21.
- Calculate the moles of total phosphate required (25 mM x 0.5 L = 0.0125 moles).
- Based on the ratio, calculate the mass of each salt required.
2. Dissolve the salts:
- Dissolve the calculated mass of each salt in approximately 400 mL of distilled water.
3. Adjust the pH:
- Use a pH meter and carefully add either dilute HCl or NaOH solution dropwise to adjust the pH to 7.5.
4. Make up to final volume:
- Once the desired pH is reached, add distilled water to bring the final volume to 0.5 L.
Important considerations:
- Use analytical grade salts for accuracy.
- Prepare the buffer in a clean, graduated container.
- Double-check your calculations and ensure the pH is accurate using a calibrated pH meter.
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