Biochem help needed! What would happen to the pH of a buffer solution if you added a small amount?

First, it is always helpful to think about the Henderson-Haselbalch equation when dealing with buffer solutions:

pH = pKa + log [base]/[acid]

Now, if you initially have a buffer with pH = pKa, then you know that [base] must equal [acid].

Now, if you add a small amount of HCl to this buffer solution, some of the base form of the buffer will be converted (stoichiometrically to the amount of HCl added) into the acid form of the buffer. So, the ratio of [base]/[acid] will change by becoming smaller than 1. The log of that ratio will then be a small negative number, which lowers the pH of the buffer solution slightly.

Now, if you have a buffer where pH = pKa - 1, you can tell that [base] is much less than [acid], and actually that [base] = 1/10 of the acid concentration.

Now, if you add the same small amount of HCl to this buffer, the [base]/[acid] ratio will change more significantly because the original [base] is much lower than in the first case. That will lead to a larger decrease in the pH of the buffer solution.

Hope that helps...