A2Q for E12

Quantitative Determination of Total Ion Concentration by Ion Exchange Chromatography

Ion exchange chromatography is a chromatographic science concerned with the separation of various types of charged species in a system. This system is usually comprised of separate phases. This method is founded on the concept of ion exchange between two principal sites – the mobile and the stationary phase. In the experiment, resin was used as the stationary phase. Unknown solution is passed through the resin to allow the cation to replace the H+/OH- ions bound in the binding sites.

Several factors can affect ion-exchange. These factors include the surface area of the stationary phase (resin bead size), density of exchange sites on the stationary phase surface (cross-linkage), flow rate of the mobile phase (resin bead size and column geometry; system pressure in high-pressure chromatography, and the chemistry of the mobile phase (ionic strength of the sample solution, concentration of mobile phase).

The resin was soaked in water for an hour before introducing to the ion-exchange column (improvised burette) in order to dissolve water-soluble impurities and ionize the functional groups of the resin. Presence of such impurities can block off sites for exchange which will cause a major problem. Through immersion in water, the resin was allowed to swell leading to further exposure of the exchange sites. Dry resin can’t be used because the exchange sites are still not open.

Water level was kept above the top of the resin to prevent the formation of air pockets which may restrict access of the ions.

Strong acid was needed to replace the bound ions in the resin. Hydrogen ion supply was also replenished through the process.

It was crucial that the rate flow be maintained at 30 drops per minute to ensure that all copper (II) ions bind to the exchange sites. If the rate is too fast, an incomplete exchange process will result and less Cu2+ will bind to the sites. If it is too slow, however, the process is deemed inefficient due to the increased amount of time needed to complete the procedure.

The column was washed until the eluate’s pH equals that of the distilled water in order to make sure that the H+ concentration is not affected with the constant addition of water in the experiment proper.

The balanced reaction for the experiment is:

Possible sources of errors include inadequate regeneration with H+ ions which may result to lesser than actual cation concentration. Incomplete exchange caused by faster flow or the trapped air pockets will also yield lower concentrations. Failure to satisfy the pH equality at the elution phase will yield to increase in cation concentration while failure to satisfy pH equality in titration phase will result to a decrease in cation concentration.

REFERENCES:  

Dudas, F.O. 2011. Ion Exchange Chromatography.Massachusetts Institute of Technology. ocw.mit.edu. Web

Dyer A., Hudson MJ.,et al. 1993. Ion  Exchange Processes: Advances and Applications.  TheRoyal Society of Chemistry.

Skoog D., West D., et al. 1996. Fundamentals of Analytical Chemistry, 7th Edition. Saunders College Publishing