Suppose drug A and drug B are to be separated by anion-exchange chromatography.Drug A is an acid with a pKₐ of 8.0, and drug B is an acid with a pKₐ of 4.0.Following the proper steps to place the mixture of drugs onto the anion-exchange phase, the pH of the mobile phase should be adjusted to ______ to elute drug A only.Then the pH of the mobile phase should be adjusted to _____ to elute drug B from the stationary phase.

Question

Quizplus · Accepted Answer

An acidic drug is found to be completely ionized at a pH 2 units above its pKₐ.Likewise, an acidic drug is completely nonionized at a pH 2 units below its pKₐ.The mixture of drugs A and B were likely placed in a pH 10.0 or higher buffer to add to the anion-exchange phase, because at a pH of 10 or higher, both drugs will be completely ionized and thus bind to the stationary phase.To elute only drug A, the pH of the mobile phase should be set to 6.0.This pH is 2 units below the pKₐ of drug A, causing drug A to become nonionized, allowing elution from the anion-exchange phase.At pH 6.0, drug B is still completely ionized.To elute drug B, the pH of the mobile phase should be changed to 2.0 or below.

Suppose Drug a and Drug B Are to Be Separated