DETERMINATION OF TOTAL ION CONCENTRATION USING ION EXCHANGE CHROMATOGRAPHY A.O. DE L A SERNA OLLEGE OF S CIENCE CIENCE I NSTITUTE OF BIOLOGY , C OLLEGE NIVERSITY OF THE P HILIPPINES HILIPPINES ITY HILIPPINES U , DILIMAN , Q , QUEZON C ITY , P HILIPPINES OCTOBER 1, 2010
ABSTRACT This affirms the importance of the employment of ion exchange chromatography, including itsconcept of isocracy and electroneutrality, resin polymerization and sulfonation, stoichiometric ratio, hydration and particulates, in chemical analysis and determination of total ion concentration of Cu II in samples provided by instructor. This report discusses basic mechanisms and materials of specifically cation exchange columnar chromatography. Dowex 50 cation exchange resin was the stationary phase and the solvent containing the analyte ion was the mobile phase. IC conditions were kept paramount during the duration of the experiment. 10 mL of the analyte sample was then eluted to the preconditioned column for three trials. Per trial, the eluate was allowed to flow to a fresh Erlenmeyer flask until its pH = 5. This fresh pool was then titrated with previously standardized sodium hydroxide NaOH against a primary standard KHP. Cu 2+ concentration in ppm was then calculated u sing the titrimetric titrimetric +, data. The actual concentration was 1250 ppm Cu2 the entire class reported a substantially accurate experimental experimental value ² 1260 2+ ppm Cu of 0.8% error. The highly accurate value affirms the feasibility of IC as a means of determining total ion concentration.
INTRODUCTION
The classical titrimetric and gravimetric methods have long been solely tantamount to analytical chemistry until the early Twentieth century. Chemical analysis is such a trending pursuit that analytical methodologies have been advancing exponentially.[1] Hence, instrumental methods for analysis were developed. This report features ion exchange chromatography as a chromatographic instrumental method for analytical chemistry and as a separation science. Chromatography, as an instrumental method, separates, identifies, determines and quantifies the chemical components of a mixture. Methodologically, it can be planar using porous paper support or columnar using narrow tube support. It is composed of a stationary phase that is fixed in the support and the mobile phase that moves through the stationary phase.
Elution chromatography is one columnar chromatography technique involving washing the solute through a stationary phase by quantitative additions of the mobile phase.[2] In 1850, forty years prior to Arrhenius· proposal of the ionization theory, a primordial notion about ion exchange was discovered by two Englishmen, J.T. Way and H.S. Thompson. The concept was described by the liberation of Calcium and Magnesium ions by treating earths with ammonium sulfate. Ion exchange is a meristem henceforth. [3] The impeccable promise for analysis of the ion exchange concept underwent further harnessing. It was inculcated and formally tagged in chromatography in 1905 by M. Tswett. [3] The term ´ion chromatographyµ has then been shortened by the common usage of the acronym IC. It has become a formidable advancement after its formalization as a novel analytical method by
Small, Stevens and Bauman of the Dow Chemical Company. The technique was then commercialized in 1975 by the Dionex Corporation, the leader of ion-exchange equipment today.[4]
Be guided that the discussions give more emphasis on cation exchange columnar chromatography as it has been employed throughout the course of the experiment.
The experiment aimed to determine the concentration in ppm of Cu2+ in a given sample using IC as a separation technique. In the experiment, Dowex 50 cation exchange resin was the stationary phase and the solvent containing the analyte ion was the mobile phase. A column previously inserted with a wad of absorbent cotton for resin support and packed with sulfonated resin was kept hydrated with distilled water washings in an improvised IC burette tube. A flow rate of 15 drops per minute was maintained by precise manipulation of the stopcock. The mobile phase was allowed to flow until the pH of the eluate = pH distilled water = pH of 5. These particulates are discussed intently in the remainder of this report. 10 mL of the analyte sample was then eluted to the preconditioned column for three trials. Per trial, the eluate was allowed to flow to a fresh Erlenmeyer flask until its pH = 5. This fresh pool was then titrated with previously standardized sodium hydroxide NaOH against a primary standard KHP. Cu2+ concentration in ppm was then calculated using the titrimetric data.
RESULTS AND DISCUSSION
The copper cation was separated from the sample at par with the displacement of hydrogen protons from the column. Separation is made possible by the adsorption of charged analyte molecules to immobilized resin ion exchange function group of opposite charge. The displaced ions from the column exist in a stoichiometric ratio or factor with the analyte ion.[1] Hence, the displaced ion will be titrated and the calculated amount multiplied by the said propagated ratio or factor will be the amount of analyte constituents. There is an exchange of ions, hence the name, ´ion exchange chromatographyµ. An intent discussion about IC, its processes, chemical concepts, stoichiometric calculations for analysis, limitations, conditions, separation phases and other particulates are found in the remainder of this report.
As its name suggests, ion exchange chromatography is a chromatographic science concerned with the separation of various type of charged species in a system. This system is usually comprised of separate phases.Since it explicitly meddles with ion exchange, to maintain the ionic integrity of the system, a stoichiometric reaction must be paramount at all analyses.[1]
To further substantiate and affirm the importance of this stoichiometric ionic reaction, look intently to these chemical equations (a) nrSO3 ² H+ + Mn+ (rSO3 )nM + nH+ (b) 2rSO3 ² H+ + Cu2+ (rSO3 )2Cu + 2H+ (c) 2H+ + 2OH- 2H2O Equation (a) shows the general chemical reaction in cationic exchange. (Anionic exchange works analogously except that OH- species are involved, not H+ ). nrSO3 ² H+ refers to the sulfonatedcation exchanger resin, Mn+refers to the metal / cationanalyte species, and H+ refers to the displaced proton ion. Most importantly, n refers to that stoichiometric factor, the defining number in our calculation. Equation (b) is the particular reaction in the experiment. Dwelling on the stoichiometry of the equation, for every 1 mole of copper II species, 2 moles of the cation exchanger reacts, therefore, 2 moles of H+ are displaced. These displaced H+ are then titrated with any strong base, NaOH for example, and both species react with each other in one-to-one correspondence. - and Therefore, twice the amount of OH transitively twice the amount of H+ , is equal to one amount of Cu 2+
Analogously, thrice of an amount of H + is equal to one amount of Aluminum 3+. All of which can be explained by the interplay of ionic charges during the stoichiometric reaction.
liberation of H+. A modest rate of flow neutralizes the turbulence and high fluid pressure at the narrow end of the tube. [3]
Now grounded to the notion that the calculations are dependent on that stoichiometric ratio, this working equation for determination of the amount of Cu2+ concentration in a given sample is derived:
exchanger contain sulfonic acid groups nSO3 ² H+ attached / chelated to the aromatic ring of insoluble inorganic molecule nR.[5]Gel-based resins are produced by polymerizing with usually styrene-divinylbenzene S-DVB copolymer. The cross linkages with the copolymer give the resin physical strength to withstand subsequent ion reactions. The spherical shape narrows particle size allotment. The polymerization yields inactive resins and must be activated. [4]
(d)Ppm Cu2+ =
Now we have a stoichiometric foothold for the ion exchange process, we are now ready to look intently on the explanations of the materials and mechanism of IC at par with the experiment performed (cationic exchange columnar chromatography). Standard Conditions. Prior to conducting the IC, a
set of parameters must be paramount and maintained throughout the duration of the experiment. That is why a pH of 5 (the pH of distilled water must be observed. A pH of 5 means that the system is isocratic and therefore is highly deprotonated from H+ after the cation exchanger has fully reacted with the cationanalyte (all H+ have been displaced and are now ready for titration with strong base). [4] Isocracy or electroneutrality. The stationary adsorbent
phase or the site of exchange, the column of resins, must be solvated with deionized and distilled water. [4] Ultrapure Reagents should be used in cleaning the IC tube.
Impurities can accumulate on the column which reduce the ion exchange capacity and reacts drastically with the system chemicals. [4] Flow rate should occur between 0.7 and 4.5 mL or 15 drops per minute.Flow rate is crucial for the ion
exchange to take place. Water is the medium for reaction. Rapid flow will never assure the optimum completion of the reaction and the total
Resins. The resin used is Dowex ² 50.Cation
nrSO3 ².Inactive resins have hydrophobic spheres and must be introduced to sulfonic acid nrSO3 ² to expose the functional hydrophilic sites for exchange. As the resins react with the acid, water forms and swells the beads making it a suitable medium for reaction. Other acids like carboxylic acid and other bases like quaternary ammonium can functionalize resins. [4] Notice that a resin functional group is written with H+ written posteriorly (nrSO3 ² H+). This shows that the resin is activated and the H+ are now ready to be exchanged with another cation species. The resins are also perforated with microporesfor liquid pathways to increase surface area of adsorption. These pores are generated by poragens like toluene. Sulfonation
by
Regenration of cationic resin exchangers by 2M H- 2 SO4.Sulfuric acid is such a strong acid that it
prolongs the activity of the cation exchanger. Divalent and Trivalent organic and carboxylic are more strongly held making them a less activator for the resins. Their stronger cohesion prevents the resin ions from approaching the ion exchange sites.[4] Transition ions from the previous usage of the resin might still be left in the column. Sulfuric acid displaces these cations as explained by IC mechanism making the column isocratic and ready for another usage.
Addition of water. The resin must be hydrated at all
Statistics. The actual concentration per 10 mL
times. Water level should not fall below the resin level.Aside from maintaining the isocracy of the system, water solvates the system and serves as a suitable medium for reaction to take place. It also displaces entrapped gases. [4] The air pockets cause pulsations in the flow and may react with the resins forming altered canals. These canals cause apparent loss in column capacity and adsorptive power.
sample was 1250. Table 4 shows the percent error of the experimental ppm value per group.
Table 4.the percent error of the experimental ppm value per group.
the aforementioned discussion states that ion exchange explicitly depends on the ionic interactions, therefore, the ionic interactions are functions of ionic strength. Separation is possible due to the differences in ionic strength.Transition metals like Cu2+ has greater ionic affinity with the sulfonic acidic functional group of the cation exchanger than H+, therefore, H+ is eluted first. Liberation
of
H+.Since
Titration. As mentioned, the liberated H+ are
titrated with a strong base, NaOH to determine the concentration of Cu 2+. Table 1 shows the standardization data of titrant NaOH of all the groups. It is grounded on the stoichiometric ratio that 1 mole Cu2+ = 2 moles H+. Therefore, using the working equation (d), the concentration of Cu 2+ in ppm was determined per sample. See appendix B, table 2 for detailed titration data of trials 1, 2 and 3 per group. Table 3.Cu 2+ ppm per group
Each group has less than 10% error. This further affirms the feasibility and accuracy of IC in total ion concentration determination. Appendix C shows detailed data of statistics per group. Limitations of IC. Like any other instrumental
method and as a still growing chromatographic science, IC is assailed with a few limitations. Table 5 lists ions yet analyzable by IC Table 5.Ions yet analyzable by IC Not Yet Analyzable Isocyanite derivatives Silicate Large Phosphonates Nitrilotriacetic acid EDTA Ethylenediamine Toluenediamine
One notable disadvantage is that IC is costly. The class improvised a burette type IC tube. Amidst the greater accuracy than that of classical methods, IC, like any other instrumental titration, has many demands and conditions to be observed.
An IC setup may encounter chromatographic anomalies. An example is water dip, otherwise known as carbonate water dip. There are also minor shifts in retention and calibration curves. Temperature must also be kept constant since minor fluctuations can alter the ionic kinetic energy. There are times that to maintain the isocracy, a buffer is needed. However, buffer may react with the resin or even the solvent containing the analyte. Themethod provides no direct information on events occurring at the surface of the stationary phase, because the ion-exchange equilibrium is always determined by the balance between the H+ and analyte ion interaction with active sites of a resin. Impurities may also affect the data. These limitations may have lead to these statistical deviations. However, the standard deviations per group are very minimal and can still substantiate the feasibility of IC. Appendix C shows more statistical data including standard and relative standard deviations and confidence intervals. The standard deviations were pooled and the average ppm was computed. Table 6 shows the overall class statistics. Table 6 shows the overall class statistics.
SUMMARY AND CONCLUSION
Upon intent analysis of the data, it can be concluded that since the class has a very small amount of pooled standard deviation (9.86ppm), the class garnered a very precise result. The class got an accurate result basing on the substantially low percent error of 0.8% with the reported class value of 1260 ppm. It can be induced that the interval in which, taking into account the confidence level, the value of the Cu 2+ concentration from finite number of sources lies is in 1260±7.59 ppm. The veracity and feasibility of IC can be affirmed basing from the experiment results. It can be concluded that the total ion concentration can be accurately determined via IC, and that the mechanism of isocracy of ionic charges and stoichiometric ratio are of vital importance for calculations involving IC. The highly accurate value affirms the feasibility of IC as a means of determining total ion concentration. REFERENCES
[1] Dyer A., Hudson MJ.,et al. 1993. Ion Exchange Processes: Advances and Applicationsµ. The Royal Society of Chemistry. ´
[2] Skoog D., West D., et al. 1996. Fundamentals of Analytical Chemistry, 7 th Editionµ.Saunders College Publishing ´
[3] Walton, Harold. 1976. Ion Exchange Chromatographyµ.Dowen, Hutchinson & Ross, Inc. ´
[4] Smith, Frank. 1983. The Practice of Ion Chromatographyµ. Wiley-Interscience Publications ´
The class got a low percent error with respect to the actual concentration of 1250 ppm. The class garnered a value of 1260 ppm (0.8% error). The pooled sd is also substantially low, meaning, the class got precise measurements. The actual concentration of the Cu2+ lies in the interval 1260±7.59 ppm.
[5] Ballesteros J., CalejaHJ.,CarilloKJ.,et al. 2009 ed. Analytical Chemistry Laboratory Manualµ.University of the Philippines ² Diliman, Institute of Chemistry. ´
