Question

Ion-Exchange Column:

You condition the column with 6 M HCl. Before you transfer the unknown to the ion-exchange column you need to wash the column until pH is above 6. Why?

Answer 1

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Ion exchange resins (the material inside the Ion-Exchange Column) consist of high molecular polymer products, high in molecular weight and insoluble. These polymers contain positive or negatively charged functional groups capable of exchanging ions with a solution. Generalizing the definition, we can consider the ion exchange resins as acids or organic bases, of general formula R - H and R - OH wherein R represents an organic molecular structure. They remain in the surface of the polymer molecule formed during the condensation or polymerization. H⁺ or OH^- ions are present in the solution according to the type of exchangers. Cation exchangers release H⁺ and exchange it with cation that we want to remove according to the following equation:

Cation exchange resins saturate after being used to interchange cations so the process of regeneration requires that the original acid group be regenerated. Hydrochloric acid is more potent than sulfuric acid in the process of regenerating cation exchange resin. 6M solution of HCl will do the job. After regeneration, there is excess of acid remaining around the resin. So it is necessary to wash away this excess. If you don’t, the resin will not work properly. When you put a solution of the cation you want to remove in the presence of an acidic media the excess of H+ ions will shift the exchange equilibrium to the left, preventing the exchange to happen.

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