In ion-exclusion chromatography, ions are separated from nonelectrolytes by an ion exchange column. Nonelectrolytes penetrate the stationary phase, whereas half of the ions are repelled by the fixed charges. Because electrolytes have access to less of the column volume, they are eluted before nonelectrolytes. The chromatogram here shows the separation of trichloroacetic acid (TCA, pKa 0.5), dichloroacetic acid (DCA, pKa 1.1), and monochloroacetic acid (MCA, pKa 2.86) by passage through a cation-exchange resin eluted with 0.01 M HCl. Explain why the three acids are separated and why they emerge in the order shown.

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TCA DCA MCA 0 10 20 30 Time (min)