Materials:
- Distilled water
- Sodium dihydrogen phosphate (NaH2PO4)
- Sodium hydrogen phosphate (Na2HPO4)
- pH meter
- Magnetic stirrer
- Graduated cylinder
- 1L Volumetric flask
- Chemicals must be of analytical-reagent grade.
Procedure:
1. Calculate the required amounts:
Determine the amounts of NaH2PO4 and Na2HPO4 needed using the Henderson-Hasselbalch equation:
pH = pKa + log ([A-]/[HA])
where:
- pH is the desired pH (6.8)
- pKa is the acid dissociation constant of H2PO4- (7.21)
- [A-] is the concentration of the base (Na2HPO4)
- [HA] is the concentration of the acid (NaH2PO4)
Rearranging the equation, you get:
[Na2HPO4]/[NaH2PO4] = 10^(pH - pKa)
Plug in the values:
[Na2HPO4]/[NaH2PO4] = 10^(6.8 - 7.21) = 0.1995
2. Prepare the stock solutions:
a) NaH2PO4 stock solution (0.1M):
Weigh 1.1998 g of NaH2PO4 and dissolve it in distilled water in a 100 mL volumetric flask. Mix thoroughly to obtain a 0.1M NaH2PO4 stock solution.
b) Na2HPO4 stock solution (0.1M):
Weigh 1.4196 g of Na2HPO4 and dissolve it in distilled water in a 100 mL volumetric flask. Mix thoroughly to obtain a 0.1M Na2HPO4 stock solution.
3. Prepare the phosphate buffer:
Using a graduated cylinder, transfer 10 mL of the 0.1M NaH2PO4 stock solution and 19.95 mL of the 0.1M Na2HPO4 stock solution into a 1L volumetric flask. Ensure the solutions are at room temperature.
4. Adjust the pH:
Use a magnetic stirrer to mix the solution in the flask thoroughly. While stirring, use a pH meter to measure the pH of the buffer solution. If necessary, add small volumes of either the NaH2PO4 or Na2HPO4 stock solutions until the desired pH of 6.8 is achieved.
5. Make up to the mark:
Once the pH is adjusted to 6.8, add distilled water to the 1L mark on the volumetric flask. Mix well to ensure a homogeneous solution.
6. Recheck the pH:
Finally, recheck the pH of the prepared phosphate buffer to ensure it is within the desired range of 6.8 ± 0.05.
Note:
To ensure an accurate pH, calibrate the pH meter before use and store the phosphate buffer at an appropriate temperature (4°C) to maintain its stability.
