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Calculate components needed for preparing 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 mixture of monobasic sodium phosphate (NaH2PO4) and dibasic sodium phosphate (Na2HPO4). Here's a breakdown:
1. Determine the pKa: The pKa value for the phosphate buffer system closest to the desired pH (7.5) is 7.21 (for the H2PO4-/HPO42- equilibrium).
2. Use the Henderson-Hasselbalch Equation: pH = pKa + log([A-]/[HA])
pH = 7.5
pKa = 7.21
Solve for the ratio [A-]/[HA] (which represents the ratio of dibasic to monobasic phosphate)
3. Calculate the molar amounts:
You need a total of 25 mM (0.025 M) phosphate concentration.
Use the ratio from step 2 and the total concentration to calculate the individual concentrations of NaH2PO4 and Na2HPO4.
4. Convert to grams:
Multiply the calculated molar amounts by the molecular weights of NaH2PO4 (119.98 g/mol) and Na2HPO4 (141.96 g/mol) to find the grams needed for each component.
5. Dissolve in water:
Dissolve the calculated grams of each component in a small amount of water separately.
Combine the solutions and adjust the final volume to 0.5 L with water.
6. Verify pH:
Use a pH meter to check the final pH and make any minor adjustments with dilute solutions of acid or base if needed.
Note: You can use online calculators or software to simplify the calculations for this buffer preparation.
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