How can i prepare a 100ml, 0.1M phosphate buffer solution (pH = 5) out of Na2HPO4 and NaH2PO4
To prepare a 100ml, 0.1M phosphate buffer solution with a pH of 5 using Na2HPO4 and NaH2PO4, you need to calculate the quantities of each salt required based on their respective pKa values.
Phosphate buffer consists of a mixture of acid (H2PO4^-) and conjugate base (HPO4^2-) species. To achieve a desired pH, you need to choose the proper ratio of these two components.
Here are the steps you can follow to prepare the buffer solution:
1. Determine the ratio of Na2HPO4 (dibasic sodium phosphate) to NaH2PO4 (monobasic sodium phosphate) required to achieve the desired pH. The pKa values for phosphoric acid (H3PO4) are 2.15, 7.21, and 12.32. In this case, you are targeting a pH of 5, which is between the second and third pKa values.
2. Use the Henderson-Hasselbalch equation to calculate the ratio of the salt components:
pH = pKa + log([base]/[acid])
In this case, we want pH = 5, so we can use either the second or third pKa value. Let's use the third pKa value, which corresponds to the HPO4^2-/H2PO4^- system.
5 = 12.32 + log([HPO4^2-]/[H2PO4^-])
Rearranging the equation:
log([HPO4^2-]/[H2PO4^-]) = 5 - 12.32
log([HPO4^2-]/[H2PO4^-]) = -7.32
[HPO4^2-]/[H2PO4^-] = 10^(-7.32)
[HPO4^2-]/[H2PO4^-] ≈ 2.76 x 10^(-8) (approximately)
This means that the ratio of Na2HPO4 to NaH2PO4 should be approximately 2.76 x 10^(-8) to 1.
3. Calculate the amount of each salt required:
Since we are preparing a 0.1M solution, we need to consider the molar masses of Na2HPO4 and NaH2PO4.
- The molar mass of Na2HPO4 = 141.96 g/mol
- The molar mass of NaH2PO4 = 119.98 g/mol
Let's assume x represents the amount of Na2HPO4 required in grams. Then NaH2PO4 will be approximately 2.76 x 10^(-8) times the amount of Na2HPO4 required.
Therefore:
(x grams Na2HPO4) + [(2.76 x 10^(-8) x) grams NaH2PO4] ≈ (0.1 mol/L) x (0.1 L)
Simplifying the equation:
x + (2.76 x 10^(-8) x) ≈ (0.1 mol/L) x (0.1 L)
x + (2.76 x 10^(-8) x) ≈ 0.01
x(1 + 2.76 x 10^(-8)) ≈ 0.01
x ≈ 0.01 / (1 + 2.76 x 10^(-8))
Calculate the value of x using this equation to find the mass of Na2HPO4 required. Then, multiply this value by 2.76 x 10^(-8) to find the approximate mass of NaH2PO4 required.
4. Once you have determined the masses of Na2HPO4 and NaH2PO4 required, measure them accurately using a weighing balance and add them to a container.
5. Add distilled water gradually to the container, while stirring, until the total volume reaches 100ml.
6. Measure the pH using a pH meter or pH indicator paper and adjust it, if necessary, by adding small amounts of either Na2HPO4 or NaH2PO4 until the desired pH of 5 is achieved.
By following these steps, you should be able to prepare a 100ml, 0.1M phosphate buffer solution with a pH of 5 using Na2HPO4 and NaH2PO4.