Answer:
Buffer A: pH 5.25; 2.82 mol of acetate for every 1 mol of acetic acid
Buffer B: pH 9.25; 11.2 mol of bicarbonate for every 1 mol of carbonate
Explanation:
The pI of a protein is the pH at which it has a net charge of zero.
The protein has a positive charge below the pI and a negative charge above it.
Since pI = 7.25, let's make one buffer at pH = 5.25 and one at pH 9.25.
1. Preparation of pH 5.25 buffer
(a) Choose the conjugate pair
We should choose a buffer with pKₐ close to 5.25.
Acetic acid has pKₐ = 4.8, so let's make acetate buffer.
(b) Protocol for preparation
We can use the Henderson-Hasselbalch equation to get the acid/base ratio.
To make the buffer, mix the solutions to get a ratio of 2.82 mol of acetate for every 1 mol of acetic acid.
2. Preparation of pH 9.25 buffer
(a) Choose the conjugate pair
We should choose a buffer with pKₐ close to 9.25.
Bicarbonate has pKₐ = 10.3, so let's make a pH 9.25 bicarbonate buffer.
(b) Protocol for preparation
To make the buffer, mix the solutions to get a ratio of 11.2 mol of bicarbonate for every 1 mol of carbonate.