Nathalie Dagmang
Group 8
Co-workers: Annjaneth Briones and groups 5, 6,
Date Performed: February 3, 2011
7 and 9 Results and Discussion Report 11: Determination of Total Ion Concentration Using Ion Exchange Chromatography 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 resins 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:
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. 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 1
The world's largest digital library
Try Scribd FREE for 30 days to access over 125 million titles without ads or interruptions! Start Free Trial Cancel Anytime.
The world's largest digital library
Try Scribd FREE for 30 days to access over 125 million titles without ads or interruptions! Start Free Trial Cancel Anytime.
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 r esin (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 ionexchange 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:
-3
In the experiment, the Cu2+ concentration was found out to be of an average of 2.2 x 1 0 M. The tendency of a resin to attract ions is affected by the nature of the resin, i.ons and the
The world's largest digital library
Try Scribd FREE for 30 days to access over 125 million titles without ads or interruptions! Start Free Trial Cancel Anytime.
The mass law of the reaction also explains this effect: (where R=concentration in the resin surface)
Rearranging:
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 cationexchange resin, like the one used in the experiment, the value of K for divalent cations in decreasing order is:
Hence, if the unknown sample was contaminated with ions other than Cu 2+
of the Cu
2+
with greater K, most
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
