Chemlab: Chemistry 3/5


Natural Salt Solutions 1: Ion Exchange

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Chemistry & Background
Attached to the carbon skeleton of the resin are the active groups that bind the exchangeable ions. Initially, the cation exchange resin used in this experiment is in the hydrogen ion form, in which H+ ions are bound to the active sites. These H+ ions can be removed either by dissociation in solution or by replacement with other positive ions. When an H+ ion is exchanged in aqueous solution, it combines with a water molecule to form H3O+, the hydronium ion. The resin's active sites have different attractive strength for different ions, and this selective attraction can serve as a means for ion separation and quantitative analysis.

In the resin used in this experiment, sulfonic acid groups bind the cations. Sulfonic acids are close relatives of the more familiar sulfuric acid. Schematic pictures of the atomic arrangements of these groups are given below.

sulfuric acid and sulfonate group structures

Sulfuric Acid Mathanesulfonic Acid Sulfonic acid group
bound to the carbon
skeleton of an ion
exchange resin

Like the hydrogens bound to the oxygens of sulfuric acid or methanesulfonic acid, the hydrogens on the sulfonic acid groups of the cation exchanger are acidic and can be readily lost through ionization.

resin sulfonate acid dissociation

When pure water is passed through a cation exchange column in the hydrogen form, only a minute fraction of the acidic hydrogens are lost from the active groups. The imbalance of positive and negative charges is never large enough to be a significant factor in the resin's stoichiometric composition. The charge imbalance remains small because any build-up of negative charge on the surface of the resin attracts and binds the positive hydrogen ions more and more strongly. This effect is an example of the general rule that macroscopic systems remain electrically neutral to a good stoichiometric approximation. Imbalances in the number of positive and negative charges are always small compared to the number of molecules present.

If a salt solution is passed through the ion exchanger, however, hydrogen ions can be easily lost into the solution in exchange for cations from the dissolved salt. When the exchange occurs, the electrical neutrality of the resin is maintained. This exchange process is shown in the following equation:

Na+ ion exchange

A convenient notation for representing the stoichiometry of ion exchange processes uses the notation RSO3H to represent a single active group, with R to representing the inert resin it is attached to.

Thus, the previous process becomes

RSO3H + Na+ + H2O (RSO3-)(Na+) + H3O+

If the absorbed cation has a +2 charge, two hydrogen ions are displaced. For a dipositive cation (such as Mg2+), we write

2 RSO3H + Mg2+ + 2 H2O (RSO3-)2(Mg2+) + 2 H3O+

Note that we write RSO3H for the hydrogen form, but we indicate the cation's charge by writing (RSO3-)(Na+) and (RSO3-)2(Mg2+). This is done to emphasize the exchanged cation, set in parentheses.

You can see from the above equations, that one H+ ion will be exchanged for each positive charge of a cation. For example, a cation with a 2+ charge, like Mg2+, will exchange with two H+ ions, forming two H3O+ ions. A convenient way to keep track of ions, particularly in a mixture, is to count the charges, rather than the number of ions. An equivalent is defined as a mole of charges. One equivalent of positive charge can be composed of one mole of Na+ ions or one-half mole of Mg2+ ions. Either will exchange with one mole of H+ ions on an ion exchange column, forming one mole of H3O+ ions in solution. You will see more about equivalents next week, when you analyze the mixture of cations in seawater.

The reactions between the hydrogen-form of the resin and cations in solution provide a method for exchanging all the cations in a salt solution for hydrogen ions. This will be exploited in next week's experiment to analyze seawater, a complex mixture of salts. Many of the cations such as Na+ and K+ present in important natural salt solutions are difficult to analyze quantitatively because of their low chemical reactivity. However, once they are exchanged for hydrogen ions, the number of moles of hydrogen ions displaced from the resin is easily determined by titration with base.

At the end of an analysis, the resin can be returned to the hydrogen form, or regenerated, by exposing it to the high concentration of hydrogen ions present in a solution of a strong acid. Note that sodium ions displace hydrogen ions when a resin in the hydrogen form is exposed to a sodium chloride solution, while the reverse is true when a resin in the sodium form is exposed to hydrochloric acid. The exchange is a reversible equilibrium:

Na+ exchange equation

The reaction can go in either direction from a particular starting point. The direction it proceeds depends on the relative concentrations of RSO3H and (RSO3-) (Na+) sites on the resin and the relative concentrations of Na+ and H3O+ ions in solution. Le Chatelier's principle allows you to predict which direction the reaction will proceed, under various conditions.

Synthetic ion exchange resins are produced in the form of small porous beads. In this experiment the beads have diameters of about 0.1 mm. An efficient approach to the problem of exchanging all the ions in a salt solution for resin hydrogen ions is to pack the beads into a glass tube to prepare an ion exchange column. The salt solution is then introduced at the top of the column and slowly passed through the resin. Cations not absorbed at the top of the column continually encounter fresh resin (in which all active groups still contain hydrogens) as they progress downward. If the flow rate is sufficiently slow for exchange to take place and the column contains an excess of exchangeable hydrogen ions over solution cations, exchange is complete. A small volume of pure water is passed through the column after the salt solution to insure that all the displaced hydrogen ions are rinsed off for analysis.

Below is a glossary of terms that will appear in the following sections:

capacity: the total amount of positive charge that a column can exchange, in equivalents or milliequivalents.

charging: the process in which ions from a salt solution are adsorbed onto the column.

effluent: the solution emerging from the bottom of a column.

eluant: the salt solution that is poured through the column to carry out an elution.

elution: the process of de-adsorbing one kind of ion from the resin and replacing it with another kind of ion.

ionic form: the identity of the ions that are adsorbed onto a column before use. The resin used in this experiment is initially supplied in the hydrogen form.

regeneration: the process of returning a column to its original ionic form after use. Regeneration is carried out by eluting the column with a concentrated solution of the desired ion.

rinse: The process of pouring purified water through the column. This should be done in "slugs" or aliquots; i.e. in each addition, add the largest possible volume of purified water possible, but don't cause the column to overflow. Subsequent additions of liquid to the column should not be made until the liquid from the previous addition has passed down into the column.
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