In ion-exclusion chromatography, ions are separated from nonelectrolytes by an ion-exchange column. Nonelectrolytes penetrate the stationary phase, whereas ions of the same charge as the resin are repelled by the fixed charges. Because co-ions have access to less of the column volume, electrolytes are eluted before nonelectrolytes. The chromatogram shows the separation of trichloroacetic acid (TCA,${\mathbf{pK}}_{\mathbf{a}}\mathbf{=}\mathbf{-}\mathbf{0}\mathbf{.}\mathbf{5}$), dichloroacetic acid (DCA,), and monochloroacetic acid (MCA,${\mathbf{pK}}_{\mathbf{a}}\mathbf{=}\mathbf{2}\mathbf{.}\mathbf{86}$) 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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