Highlights Acids/Buffers II

1. The Henderson-Hasselbalch equation (pH = pKa + log [A-]/[HA] (where A- is what I called the 'salt' and HA is the acid) allows one to measure the pH if one knows the pKa and the amount of salt and acid. It also allows one to determine the amount of salt and acid if one knows the pH and pKa. This is a very important equation for understanding how buffers work.

2. This is important - for every proton added by a strong acid, one HA is created by the buffer and one A- is lost. Conversely, for every proton removed by NaOH, one A- is created and one HA is lost. If this is not clear to you, please come to see me.

3. When the amount of salt equals the amount of acid in the Henderson-Hasselbalch equation, the log term equals zero. Thus, when salt = acid for a buffer, pH = pKa. When the pH is less than the pKa, there will be more acid than salt. When the pH is greater than the pKa, there will be more salt than acid.

4. Note for a buffer that when [salt] = [acid], the maximum capacity of the buffer is reached. That is, at this point, the buffer will resist changes in pH greater than at any other point.

5. Buffers are effective when the pH of the solution in which they are found is within about 1 pH unit of the pKa of that buffer. We shall assume that when the pH of a solution is more than one pH unit above the pKa, the buffer contains essentially totally the A- form (no HA). Conversely, when the pH is a solution is more than one pH unit below the pHka, the buffer contains essentially totally the HA form (no A-).

6. When the pH of a solution is below that of the pKa, there is more HA than A-. Conversely, when the pH of a solution is above that of the pKa, there is more A- than HA.

7. You should be able to use the Henderson Hasselbalch equation to predict things about solutions without using a calculator.

8. You should also be able to draw and/or interpret titration curves as have been described in class.

9. What the Henderson Hasselbalch equation illustrates to us is that as the pH of a solution changes, the charges of the weak acids in that solution change, as well. For proteins, this is critical, because changes in charge of the weak acids in the amino acids cause the charge of the protein to change and protein charge changes lead to protein structure changes, which can increase or decrease the action of a protein. We'll talk more about this later.