QUANTITATIVE DETERMINATION OF TOTAL ION CONCENTRATION BY ION EXCHANGE CHROMATOGRAPHY

J. OLIVEROS1 and M. N. SALES11 INSTITUE OF BIOLOGY, COLLEGE OF SCIENCEUNIVERSITY OF THE PHILIPPINES, DILIMAN, QUEZON CITY 1101, PHILIPPINESDATE SUBMITTED: 6 MAY 2015DATE PERFORMED: 29 APRIL 2015

ABSTRACT

The purpose of the study was to discuss the principles behind ion-exchange chromatography and its use as a technique for separation. The study also asserts the importance of the ion exchange chromatography, including its concept of isocracy and electroneutrality, resin polymerization and sulfonation, stoichiometric ratio, hydration and particulates, in chemical analysis and determination of total ion concentration of Cu2+ from the sample. 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. Ion chromatography 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 = 6. 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. The theoretical concentration was 2500 ppm Cu2+, the calculated value was 671 ppm Cu2+ of 73.16% error, yielding a % purity of 26.48% Cu2+.

INTRODUCTION

Before, the only efficient and feasible method known in the purification of rare earth ions involved tens or hundreds of tedious fractional precipitation, which later on was revealed to still be relatively impure. A method therefore, that can separate one trace compound from another, even in the presence of a large excess adsorbable cation, is needed [1].

Ion exchange is probably the most frequently used chromatographic technique for the separation and purification of proteins, polypeptides, nucleic acids, polynucleotides, and other charged biomolecules. The reasons for the success of ion exchange are its widespread applicability, its high resolving power,

its high capacity, and the simplicity and controllability of the method [2].

Ion exchange refers to the separation techniques in analytical chemistry that allow different ionic materials to be selectively retained on an ion exchange resin.

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.Separation in ion exchange chromatography depends upon the reversible adsorption of charged solute molecules to immobilized ion exchange groups of opposite charge [2].

Separation is obtained since different substances have different degrees of interaction with the ion exchanger due to differences in their charges, charge densities and distribution of charge on their surfaces. These interactions can be controlled by varying conditions such as ionic strength and pH. The differences in charged properties of biological compounds are often considerable, and since ion exchange chromatography is capable of separating species with very minor differences in properties, e.g. two proteins differing by only one charged amino acid, it is a very powerful separation technique [2].

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.

Elution chromatography is a columnar chromatography technique involving washing the solute through a stationary phase by quantitative additions of the mobile phase.

Table 1. List of Ion Exchange Materials

Table 1 provides 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.

In ion exchange chromatography one can choose whether to bind the substances of interest and allow the contaminants to pass through the column, or to bind the contaminants and allow the substance of interest to pass through.

In general, chromatography can be planar; a porous paper support is used, or columnar; a narrow tube support is utilized. Also, it has a stationary phase and a mobile phase that are both essential for the set-up. 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, was used to achieve maximum output.

METHODOLOGY

The experiment aimed to determine the concentration in ppm of Cu2+ in a given sample using the Ion chromatography 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 Ion chromatography burette tube. A flow rate of 30 drops per minute was maintained by precise manipulation of the stopcock. The mobile phase was allowed to flow until the pH of the eluate is equal to the pH of distilled water, which was equal to 6. 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 = 6. 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.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. 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,

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.

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:

(IV) Ppm Cu2+ =M V (1 mol H + / 1 mol OH ) (1 mol Cu2+ / 2 mol H + ) (63.55g / mol ) (1000mg / 1g )hence the name, ion exchange chromatography.

ppm Cu =

OH OH

ptVolume of sample, L

RESULTS AND DISCUSSION

The set-up was concerned of separation of various types of charged species in a system for it is an ion exchange chromatography, thus the system is comprised of separate phases. A principal stoichiometric reaction was then applied at all analyses to maintain the ionic integrity of the system.

To attest to the importance of this stoichiometric ionic reaction, look intently to these chemical equations

I. nrSO3 H+ + Mn+ (rSO3)nM + nH+ II. 2rSO3 H+ + Cu2+ (rSO3)2Cu + 2H+ III. 2H+ + 2OH- 2H2O

Equation (I) shows the general chemical reaction in cationic exchange. nrSO3 H+ refers to the sulfonated cation exchanger resin, Mn+refers to the metal / cation analyte species, and H+ refers to the displaced proton ion. Also, n refers to the stoichiometric factor. [3]

Equation (II) 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. [3]

Therefore, twice the amount of OH- and twice the amount of H+ is equal to one amount of Cu 2+. Also,

The calculated ppm from the experiment was(mean); 671.4236668 ppm. The theoretical was2500ppm. It can be observed that even though there was a small deviation between each trial for the ppm, there is still a large difference between the calculated values and the theoretical, this can be explained by the errors and inaccuracies of the techniques used during the experiment

Principal Conditions: Before conducting the Ion Chromatography, a set of parameters must be paramount and maintained throughout the duration of the experiment. That is why a pH of 6 (the pH of distilled water must be observed. A pH of 6 means that the system is isocratic (not comparable) and therefore is highly deprotonated from H+ after the cation exchanger has fully reacted with the cation analyte (all H+ have been displaced and are now ready for titration with strong base). [4]

Electro-neutrality: The stationary adsorbent phase, the column of resins (site of ion exchange), must be solvated with deionized and distilled water. [4]

Flow rate should be 1.5 mL or 30 drops per minute: This is because flow rate is crucial for the ion exchange to take place. Water is the medium for reaction. Rapid flow or imprecise flow will never assure the optimum completion of the reaction and the total liberation of H+. A modest rate of flow neutralizes the turbulence and high fluid pressure at the narrow end of the tube. [4]Resins: The resin used was Dowex 50. Cation exchanger contain sulfonic acid groups nSO3 H+ attached / chelated to the aromatic ring of insoluble inorganic molecule nR.[5] Gel-based resins are manufactured/formed by polymerizing with usuallystyrene-divinylbenzene S-DVB copolymer. The

cation exchanger than H+, therefore, H+ is eluted first.

Table 1. Standardization of NaOH solution

Trial 1 2 3

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 resinsand must be activated. [5]

10 Standardweight, g

Net volumeNaOH, mL

.1008 .1270 .1002

3.85 5.2 3.9

Sulfonation.: Inactive resins have hydrophobic spheres and must be introduced to sulfonic acid nrSO3 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. [5]

Notice that a resin functional group is written with H+ written posteriorly (nrSO3 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 micropores for liquid pathways to increase surface area of adsorption. These pores are generated by poragens like toluene.

Water influence: The resin must be hydrated at all 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. [5] The air pockets/bubbles cause pulsations/disturbances in the flow and may react with the resins forming altered canals. These canals cause apparent loss in column capacity and adsorptive power.[4]

Liberation of H+: Since it was stated 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. Because transition metals like Cu2+ have greater ionic affinity with the sulfonic acidic functional group of the

M NaOH .1279 .1194 .1256Average M NaOH .1242

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 1mole Cu2+ = 2 moles H+. Therefore, using the working equation (IV), the concentration of Cu 2+ in ppm was determined per sample. See appendix table 2 for detailed titration data of all trials.

Disadvantages of IC: Like any other instrumental method and as a still growing chromatographic science, it is assailed with a few limitations. One notable disadvantage is that it is costly. The class improvised a burette type Ion Chromatography tube. Amidst the greater accuracy than that of classical methods, like any other instrumental titration, it has many demands and conditions to be observed.

Errors and Inaccuracies: The setup may encounter chromatographic anomalies. An example is water dip, otherwise known as carbonate water dip. [5] 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. The method provides no direct information on events occurring at the surface of the stationary phase, because the ion-exchange equilibrium is always determined by the balancebetween the H+ and analyte ion interaction with active sites of a resin. Impurities may also affect the data. These flaws may have lead to statistical deviations

SUMMARY AND CONCLUSIONS

The results show that the unknown sample has an average cation concentration of 671 ppm or0.01056 M. The theoretical value for the actual concentration was 2500 ppm. A large deviation can be observed when comparing the two values. To compare this to the actual cation concentration in the sample, the value obtained from the experiment has a percent error of 73.16%. This error could have been minimized if the experiment was done more carefully and with more meticulous methods while doing the techniques. Hence, the values obtained could have been more accurate and precise.

However, even if the feasibility of Ion Exchange Chromatography can be affirmed basing from the experiment results, there were still errors that greatly affected the veracity of this type of technique in analytical separation. Although, it can be concluded that the total ion concentration can be accurately determined via Ion Chromatography, and that the mechanism of isocracy of ionic charges and stoichiometric ratio are of vital importance for calculations involving the said technique. The highly accurate value affirms the feasibility of Ion exchange chromatography as a means of determining total ion concentration.

There are a lot of factors that can affect the 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) [6].

All of these factors plus additional ones which were discussed before are main elements that greatly affect the results calculated from the experiment, thus errors couldve been committed, may it be by method, instrument or even by indeterminate systems. All of these can be accounted and should be accounted during the experiment.

REFERENCES

[1] Fischer, R., Peters, D. 1968. Basic Theory and Practice of quantitative chemical analysis.

[2] Wurts, William. "WATER HARDNESS -- CALCIUM & MAGNESIUM." WATER HARDNESS -- CALCIUM& MAGNESIUM. Kentucky StateUniversity CEP at UK Research andEducation Center, 1 Jan. 2008. Web. 1Apr. 2015.

[3] Dyer A., Hudson MJ.,et al. 1993. Ion Exchange Processes: Advances and Applications. The Royal Society of Chemistry.

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

[5] Walton, Harold. 1976. Ion Exchange Chromatography. Dowen, Hutchinson & Ross, Inc.

[6] Smith, Frank. 1983. The Practice of Ion Chromatography. Wiley-Interscience PublicationsAPPENDIXTable 1. The water hardness scale. Water Hardness ppm CaCO3

Soft 0 20

Moderately Soft 20 60

Moderately Hard 61 120

Hard 121 180

Very Hard > 180

Table 2. Standardization of EDTA solution

Trial123

Working Ca(II)standard10 mL10 mL10 mL

Net Vol of EDTA8 mL8.5 mL8.3 mL

M of EDTA9.878915223x10 3M9.297802562x10 3M9.521845998x10 3M

Table 3. Hidden Spring Water Sample Analysis

Trial123

Volume of water sample, mL505050

Net Volume of EDTA, mL131313

Total Hardness, ppm CaCO3248.7248.7248.7

Average248.7

RSD0 ppt

Confidence Interval 248.7 +/- 0 Calculations

Standardization:

M of NaOH = (.1008 / 204.2g/mol x .998 ) = .1279601104 M ( 3.85mL/1000mL)

M of NaOH = (.1270/ 204.2g/mol x .998 ) = .1193644994 M ( 4.2mL/1000mL)

M of NaOH = (.1002 / 204.2g/mol x .998 ) = .125567938 M ( 3.9mL/1000mL)Average M of NaOH = .1279601104M + .1193644994M+ .125567938 M = .1242974345 M3

ppm Cu2+:

trial 1ppm of Cu2+

trial 2ppm of Cu2+

trial 3ppm of Cu2+

.1242974345 M x 1.6 mL / 1000 x 1 x 1/2 x 63.55 x 1000= = 631.928157 ppm( 10 mL /1000 mL).1242974345 M x 1.7 mL / 1000 x 1 x 1/2 x 63.55 x 1000= = 671.4236668 ppm( 10 mL /1000 mL).1242974345 M x 1.7 mL / 1000 x 1 x 1/2 x 63.55 x 1000= = 671.4236668 ppm( 10 mL /1000 mL)

trial 1. 2. 3Average ppm of Cu 2+

631.928157 + 671.4236668 + 671.4236668= = 658.2584969 ppm3

New Mean: 671.4236668 ppm

=SD =

n 2(X X)i = 1 n 1

2 2(671.4236668 671.4236668 ) + (671.4236668 671.4236668 ) 2 1= 0

tSDRSD = sd x1000 pptX 0

Confidence Limit = X n

(12.70) (0)=671.4236668

x1000 ppt

= 671.4236668 3= 0 = 671.4236668 0

% purity = 6 1

1 gram/liter [g/L] = 1000.000002 part/million [ppm]g / L = 671.4236668 = .671423666551000.000002M of Cu2+ = .67142366655 = .01056528191 M63.55% purity = .01056528191 M x 10/1000

trial 1ppm of Cu2+

trial 2ppm of Cu2+

trial 3ppm of Cu2+

.1242974345 M x 1.6 mL x 1 x 1/2 x 63.55= = 6319.28157 ppm( 10 mL /1000 mL).1242974345 M x 1.7 mL x 1 x 1/2 x 63.55= = 6714.236668 ppm( 10 mL /1000 mL).1242974345 M x 1.7 mL x 1 x 1/2 x 63.55= = 6714.236668 ppm( 10 mL /1000 mL)

trial 1. 2. 3Average ppm of Cu 2+

6319.28157 + 6714.236668 + 6714.236668= = 6582.584969 ppm3

New mean: 6714.236668 ppm

SD =

n 2(X X)i = 1 n 1

2 2

=(6714.236668 6714.236668 ) + (6714.236668 6714.236668 ) 2 1= 0

Confidence Limit = X tSDn

= 6714.236668

(12.70) (0)3

RSD = sd x1000 pptX 0 = x1000 ppt6714.236668= 6714.236668 0 = 0