To prepare *V* mL of a phosphate buffer with *C"M of NaH2PO4 and *D*M of Na2HPO4 that produces a pH 5.8, you will need the following:
$$ [H^+] = 10^{−5.8} = 1.58 × 10^{−6} M$$
Where [H+] is the desired hydrogen ion concentration.
We can use the Henderson-Hasselbalch equation to calculate the ratio of NaH2PO4 to Na2HPO4 required to achieve this pH:
$$pH = pKa + log \left ( \frac{[A^-]}{[HA]} \right )$$
Where:
* *Ka* is the acid dissociation constant of H2PO4- (the conjugate acid of NaH2PO4)
* [A-] is the concentration of Na2HPO4 (the conjugate base of NaH2PO4)
* [HA] is the concentration of NaH2PO4 (the acid form)
The *Ka* of H2PO4- at 25°C is *2.2 *10-7*. Therefore, substituting the given values into the Henderson-Hasselbalch equation, we get:
$$5.8 = 7.2 + log \left ( \frac{[Na2HPO4]}{[NaH2PO4]} \right )$$
Solving for [Na2HPO4] / [NaH2PO4], we get:
$$\frac {[Na2HPO4]}{[NaH2PO4]} = 0.398$$
Therefore, the ratio of [NaH2PO4] to [Na2HPO4] should be 1:0.398 in order to achieve a pH of 5.8.
Step 2: Prepare the NaH2PO4 and Na2HPO4 solutions
- Weigh out the required amount of NaH2PO4 (0.2 M) and Na2HPO4 (0.1 M) using an analytical balance.
- Dissolve each compound in Milli-Q water in separate containers.
- Make up the volume of each solution to 500 mL using Milli-Q water.
*Step 3: Mix the NaH2PO4 and Na2HPO4 solutions
- Mix the two solutions in the appropriate ratio (in this case, 1 volume of the 0.2 M NaH2PO4 solution to 0.398 volume of the 0.1 M Na2HPO4 solution).
- Add DI water to bring the total volume to 1 liter.
- Check the pH of the buffer using a pH meter. If necessary, adjust the pH by adding small amounts of either the 0.2 M NaH2PO4 or the 0.1 M Na2HPO4 solution.
