A hypothetical diprotic acid has pKa1=5 and pka2=7. Could a buffer at pH6 be prepared with this acid? If so how?
Using H2CO3 as an example, the pH at the first equivalence point where
H2CO3 ==>H^+ + HCO3^-
H+ at equivalence point is sqrt(k1k2) or
pH = 1/2(pk1+pk2).
Do the same thing with H2A. Simply weigh out the NaHA salt (half neutralized.
pH = 1/2(pK1 + pK2) = 6
To determine if a buffer at pH 6 can be prepared with a diprotic acid, we need to compare the pKa values with the desired pH.
The pKa1 value (5 in this case) represents the pH at which half of the acid is dissociated into its conjugate base. The pKa2 value (7 in this case) represents the pH at which half of the remaining acid (conjugate base from pKa1) is dissociated into its conjugate base.
For a buffer to be effective, the pH needs to be within about ±1 unit of the pKa value. This is because at the pKa value, the acid and its conjugate base are present in approximately equal amounts, allowing the buffer to resist changes in pH.
In this case, the pKa1 is 5, and the pKa2 is 7. A buffer at pH 6 falls between these two pKa values, which means it lies within the effective buffering range.
To prepare the buffer at pH 6 with this diprotic acid:
1. Start with a solution containing the diprotic acid.
2. Adjust the pH of the solution to the desired pH (6 in this case) using a strong acid or a strong base.
- If the pH is above 6, add a small amount of a strong acid to lower the pH to 6.
- If the pH is below 6, add a small amount of a strong base to raise the pH to 6.
3. Make sure to use small amounts of the strong acid or base and check the pH after each addition to avoid overshooting the desired pH.
By adjusting the pH of the solution to 6, maintaining the acid and conjugate base in roughly equal amounts, a buffer at pH 6 can be prepared with this hypothetical diprotic acid.