Retention Mechanism of Phosphate Anion on an Anion Exchange Column
磷酸根离子在阴离子交换树脂上的保留行为及其机理探讨

The retention mechanism of phosphate anion on an anion exchange column was studied. As a common consideratoin, an analyte ion was retained on ion exchanger as the form existed in the mobile phase. However, two peaks for phosphate anion were observed. The measured charges corresponding to the two peaks were 1.06 and 1.87 respectively. Obviously, the first peak is the monovalent H 2PO - 4 and the second peak is the divalent HPO 2- 4. By using EDTA eluent with pH 6.0, the phosphate anion can be retained on the same column as its divalent anion HPO 2- 4 but not H 2PO - 4, because the measured charge for the peak is 1.86. At pH 6.0, the dissociation of H 2PO - 4 is very weak, existing in mobile phase as monovalent anion H 2PO - 4 mainly. This means that for the weak inorganic acids with multi stepwise dissociation constant, the retention valence on ion exchanger depends not only on the existing forms in the mobile phase, but also on the change of exsiting of analyte on the stationary phase. We consider that in the ion exchange process of H 2PO - 4 with ion exchange group on the stationary phase, the secondary dissociation of H 2PO - 4 was made up due to the electrostatic interaction with ion exchange group on stationary phase and eluent ion. At the system using PyA as eluent, the further dissociation of H 2PO - 4 produced on the statonary phase resulting from the electrostatic interaction between H 2PO - 4 and anion exchange sites, and that between H 2PO - 4 and eluent ion. Because the capacity factors of H 2PO - 4 and HPO 2- 4 are different, two separated peaks were observed though only H 2PO - 4 was injected into the mobile phase.