Nathalie Dagmang Group 8

Co-workers: Annjaneth Briones and groups 5, 6, Date Performed: February 3, 2011

Results and Discussion Report 11:

Determination of Total Ion Concentration Using Ion Exchange Chromatography

Ion-exchange resins consist of insoluble inorganic molecules, or also known as “high” polymers
which are relatively large compared to other compounds. These are capable of taking up ions from
solutions which are then exchanged for other ions from the resin that have the similar charge.

There are two kinds of ion-exchange resins: the anion-exchange resins or anion exchangers, and
the cation-exchange resins or cation exchangers. The anion exchangers are able to interchange
negatively charged ions, while the cation exchangers can do the same to positively charged ions.

This characteristic of the ion-exchange resin can be explained by its structure:

Figure 1. C-atoms chain Figure 2. C- and N-atoms chain

It is made of chains of Carbon atoms or both Carbon (Figure 1) and Nitrogen atoms (Figure 2) to
which are attached active groups, H + atoms or inactive groups such as methyl groups (-OH 3) and amine
groups (-NH2) which are attached to the carbons, and neighboring chains which creates its “3D”
structure. For cation-exchangers, the active groups are acidic, and for anion-exchangers, these are basic.

One example of ion-exchange resin is the anion exchanger, R-NH 2 with amine groups as its
inactive groups. This accepts protons, specifically H +, to form R-NH3+ and holds anions by electrostatic
forces.

The resin used in the experiment was the cation-exchanger Dowex 50, a resin with sulfonic acid
groups (-SO3H). When immersed in a solution with cations, some of the H + ions of the resin’s acidic
groups will be exchanged for the cations from the solution. The charge of the cation is equal to the
number of H+ ions liberated and the number of sulfonic acid groups required on the resin to be held.
This relationship is shown in the balanced equation of the reaction that happened in the experiment:
+¿ ¿
2+ ¿↔ ( RSO ) Cu+2H ¿
+¿+ Cu 32
¿
H
2 R−SO−¿ ¿
3

The negative charges of the active groups are always balanced by the positive ions it attracts,
either the H+ ions or the cations from the analyzed solution, making the resin as a whole, electrically
neutral.

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 Because an exchange resin can take up amounts of solute and solvent, it is subject to swelling
due to the osmotic effect. The osmotic effect results from the difference in solute concentration
between the resin and the solution passed through the column. Because the solute concentration is
higher in the pores of the resin, the solution tends to enter the resin and causes the swelling. This
swelling controlled by the skeleton of the resin (the neighboring chains which makes up its 3D structure)
using elastic forces. At equilibrium, these elastic forces are equal to the osmotic pressure, which may be
more than 1000 atm.

In the experiment, the resin was first soaked in water prior to preparing the ion-exchange
column to lessen its concentration and more solute from the concentrated acid which would be added
later would enter its pores. Resins are made of polymers with high molecular weight so it would not
dissolve in water.

The resin was soaked in concentrated acid in order to regenerate it with H + ions needed to
facilitate the ion-exchange. However, HNO 3 is not advisable to use for this purpose as it causes
significant gas evolution by oxidation, which may in effect eject the resin explosively from the burette
and impair the efficiency of the column.

In relation to that, the liquid level of the ion-exchange is kept above the resin level so as to
prevent air pockets to form and remain inside the column, otherwise, the air pockets will cause an
uneven flow and poor efficiency of ion-exchange. It may also hinder in the interaction of the solution
with the resin as it may prevent its contact with the resin.

The solution was also kept at a pH equal to that of the distilled water to be used later on. This is
done to make sure that the H+ concentration is not affected with the constant addition of water in the
experiment proper.

In the ion-exchange column, the solution is transferred into a burette containing the ion-
exchange resin. As the solution flows through the resin, it interchanges more of its Cu 2+ ions with the H+
ions of the resin and comes in contact with the resin that have lost fewer of its ions. To allow the
complete replacement of H+ ions of the resin, the solution should be given enough time to establish an
exchange equilibrium and to come in contact with all portions of the resin. Because of this, the rate of
the expelling the solution which have already come in contact with the resin, or the eluate, should be
kept at a slow rate of at most 15 drops per minute.

The eluate was then titrated with NaOH to get the concentration of H +, thus, also getting the
concentration of cations that the H+ ions was replaced for:
2+ ¿¿

2 mol H +¿=1 mo l Cu ¿

In the experiment, the Cu2+ concentration was found out to be of an average of 2.2 x 10 -3 M.

The tendency of a resin to attract ions is affected by the nature of the resin, i.ons and the
solvent. The preference of the resin for a specific kind of ion is evident in the speed of the flow of
solution down the column. If a resin prefers a given kind of ion, the solution containing these ions will

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react greatly with the resin and will take a longer time in the column to react with the resin. Hence, a
cation more preferred by a resin will travel slower down the ion-exchange column.

The mass law of the reaction also explains this effect: (where R=concentration in the resin
surface)

K ' =¿ ¿ ¿ ¿

Rearranging:

K ' ¿¿ ¿ ¿

It can be seen that the ion with the larger K will be stronger retained in the resin. Experiments
also show that polyvalent ions have larger Ks than univalent ions. The differences in K in a given charge
group also depends on the size of the ions and some of their properties. For the sulfonated cation-
exchange resin, like the one used in the experiment, the value of K for divalent cations in decreasing
order is:
2+¿ …¿
2+¿ >Cu ¿
2+ ¿>¿ 2+¿ >Cd ¿¿
2+¿ >Cd ¿
2+¿ >Ca2+¿ >¿ ¿¿
2+ ¿> Sr ¿
2+¿>Pb ¿
Ba

Hence, if the unknown sample was contaminated with ions other than Cu 2+ with greater K, most
of the Cu2+ ions will just flow out of the burette until all of the ions with the greater K have reacted with
the resin. This trend is often used as an advantage in separating ions in a solution.

In addition, if other solvents were added instead of water, the resin might be dissolved at some
extent that may affect the resulting H+ concentration of the eluent.

Also, inadequate regeneration with H + ions may affect the resulting H + concentration measured
in titration. With less H+ ions, some cations might flow out with the eluent and the resulting
concentration will be lesser than the actual cation concentration. An incomplete exchange caused by a
faster flow rate or trapped-air pockets will also produce the same results.

References:

Benjamin, W.A. Quantitative Analysis, 1964

Skoog, et al., Fundamentals of Analytical Chemistry, Eighth edition, 2004