Determination of total ion concentration using ion exchange chromatography Institute of Biology, College of Science University of the Philippines, Diliman, Quezon City, Philippines Date Submitted: May 14, 2013
ABSTRACT ABSTRACT Chemical analysis has long been the subject of interest for many. Different techniques were made especially that of separating substances. This experiment aims to show how Ion Exchange Chromatography, as one of the separation techniques employed in analytical chemistry, works. Calculations were also applied to obtain the concentration of the metal ion being studied. This experiment was done using a burette as an improvised ion exchange column. Conditions were kept paramount during the duration of the experiment. Dowex 50, a strong acid cation exchanger was used as the stationary phase. Ten mL of the analyte (unknown sample) was then pipetted pipetted out the and poured in to the column. Continuous Continuous washing ws done using using a distilled water until the pH obtained from the beaker containing he eluate is equal to pH 5. It was then titrated with 0.09805 M NaOH. The results show that the unknown sample contains a cation concentration of 1443 ppm Cu 2+ or 0.02270 M Cu 2+. The RSD obtained with a value of 69.4 and the confidence interval is from 1194-1691. However there is no guarantee that these values are accurate since during the process of titration, trials 1, 2, and 3 became overtitrated with NaOH. The degree of overtitration was not noted down and no further trials were made to correct the error.
RESULTS AND DISCUSSION The term ion exchange refers to the separation techniques in analytical chemistry that allows different ionic materials to be selectively retained on an ion exchange resin (epa.gov). Generally, it is understood to mean the exchange of ions of like sign between a solution and a solid highly insoluble body in contact with it (Jeffery et. al. 1989). This is achieved by ionexchange chromatography by which the stationary phase is an inert polymer that is derivatized with charged groups that can bind (exchange) ions of opposite charge (csun.edu). In general, chromatography can be planar, that uses a porous paper support, or columnar, that uses a narrow tube support. It has a stationary phase and a mobile phase. For the purpose of the experiment, elution chromatography, a type of columnar chromatography that involves washing the solute through the stationary phase by adding the mobile phase is used. As mentioned, a resin is used when doing an ion exchange chromatography. Resins are insoluble materials that are also known as polymers that are large compared to other compounds. It consists of
exchangeable ions located at different sites in the polymer chain. These sites are made up of functional groups that are usually sulfonic acids for cations and trialkylaminesfor anions and are attached to the polymer chain (epa.gov). Below is a table showing the different types of ion exchange materials used in ion chromatography. As shown, Dowex 50, the resin used in the experiment is a type of a strong acid cation exchanger. A cation resin has a stationary, functional group with a negative charge. This means that it will exchange positive ions with the eluent solution. solution.
Table 1. Comparable ion exchange exchange materials
liberation of H +. The displaced H+ was then titrated with standardized 0.09805 M NaOH. The equation below shows the reaction taking place in the ion exchange column. Equation 1. Ion Exchange
2 R SO3 Table taken from Vogel’s Textbook of Quantitative Chemical Analysis
In the experiment, an improvised column was made out of a burette. A wad of cotton that will serve as resin support was inserted at the bottom of the burette. The resin was then poured into the column allowing the acid along with it to flow out of the column. This was then washed with distilled water again and again until such time that the pH of the eluate obtained is equal to that of the pH of the distilled water (pH 5). However, the level of liquid in the column never falls below the resin level so as to avoid air pockets to form and remain inside the column, otherwise, the air pockets will cause an uneven flow and poor efficiency of ionexchange. The resin used was prepared earlier before the experiment started by soaking the dry resin in water. It was then soaked in concentrated acid afterwards. The purpose for this is to hydrate the resin spheres. This is important to create uniform resin bed spaces and to avoid compaction. Compaction blocks the flow hence, prohibiting effective ion separation. Additionally, soaking the resin to an acid gives it the needed H + ions to facilitate ion exchange (epa.gov). The analysis of the unknown sample then followed. The sample was first diluted with distilled water in a 100 mL volumetric flask. Ten mL of the sample was then obtained using a pipette. It was transferred in a beaker and was then poured into the column. The flow rate of the sample in the column was maintained at 15 drops per minute. The reason for this is to make sure that ion exchange will take place. After the sample was placed in the column, the column was washed twice with distilled water until such time that the pH of the eluate will equal to the pH of the distilled water. Since water is the medium of the reaction, rapid flow will hinder the completion of the reaction and the total
H
Cu
2
( RSO3 ) 2 Cu 2 H
Based on the stoichiometry of the exquation, the cation concentration, in ppm, was calculated using the formula below. M OHVOH 1 mol H / 1 mol OH
ppm Cu
1 mol Cu
2
/ 2 mol H
63.55g / mol 1000mg /1g
Volume of sample, L
To change this to molarity, simply convert the obtained ppm values to grams per liter and divide it by the molar mass of Cu2+. Below is the table showing the calculated concentration of Cu2+. The results show that the unknown sample has an average cation concentration of 1443 ppm or 0.02270 M. Based on the confidence interval at 95%, the real cation concentration is just within the range of 1194-1691 ppm or 0.019-0.027 M. However there is no guarantee that these values are accurate since during the process of titration, trials 1, 2, and 3 became overtitrated with NaOH. The degree of overtitration was not noted down and no further trials were made to correct the error. To compare this to the actual cation concentration in the sample which has a concentration of 1250 ppm, the value obtained from the experiment has a percent error of 15.44%. This error could have been minimized if the experiment was repeated since overtitration happened. Hence, the values obtained could have been more accurate and precise. Table 2. Calculated values
TRIAL 1 2 3 AVE RSD CONFIDENCE INTERVAL
MOLARITY OF NAOH 0.09785 0.1014 0.09491 0.09805 N/A
TOTAL CATION CONCENTRATION, ppm 1370 1557 1401 1443 69.44
N/A
1194.243-1691.757
in M 0.02155 0.02450 0.02204 0.02270 69.60 0.0190.027
There are a lot of factors that can affect the ionexchange. 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) (ocw.mit.edu). The effects of flow rate have been discussed already. As for the bead size, as resin bead size decreases, the surface area per unit volume increases hence, finegrained resins provide more numerous exchange sites and therefore usually perform better separations. Furthermore, in fine resin bead sizes, gravity is insufficient to induce flow. Additionally, the organic coating on the resin beads, during manufacture can be altered to provide more or fewer exchange sites per unit area and is called the cross linkage of the resin. Higher cross-linkages provides better separations.
REFERENCES Dudas, F.O. 2011. Ion Exchange Chromatography. Massachusetts Institute of Technology. ocw.mit.edu. Web. 13 May 2013 Ion Exchange Chromatography. 2012. California State University Northridge.csun.edu. Web. 12 May 2013 Jeffery, G.H., Bassett, J. , Mendham, J., and Denney, R.C. 1989. Vogel’s Textbook of Quantitative Chemical Analysis 5 th Edition. The School of Chemistry,Thames Polytechnic, London Tutorial 3.4 Ion exchange. United States Environmental Protective Agency. epa.gov. Web. 12 May 2013
