ION EXCHANGE

PURPOSE

DESCRIPTION

TIME REQUIRED

MATERIALS

Chemicals

6.0 M HCl solution (S00 mL of concentrated HCl diluted to 1000 mL with distilled or deionized water)*

1.0 M ammonia solution (66 mL of concentrated ammonia solution diluted to 1000 mL with water)*

0.1 M CaCl₂ solution (dissolve 11.1 grams CaCl₂ in water and dilute to 1000 mL)

0.1 M CuSO₄·5H₂Osolution (dissolve 25.0 grams of CuSO₄·5H₂₀ in water and dilute to 1000 mL)*

0.1 M FeSO₄·7H₂O solution (dissolve 28 grams FeSO₄·7H₂Oin water and dilute to 1000 mL)*

0.1 M NiCl₂·6H₂₀ solution (dissolve 24 grams NiCl₂·6H₂O in water and dilute to 1000 mL)

0.1 M Na₂CO₃ solution (dissolve 10.6 grams of Na₂CO₃ in water and dilute to 1000 mL)*

0.1 M KSCN solution (dissolve 10.0 grams of KSCN in water and dilute to 1000 mL)

1.0% dimethylglyoxime solution (dissolve 1.0 gram of dimethylglyoxime in 99 mL of ethanol)*

cation exchange resin*

Equipment

Pasteur pipet *

test tubes

100-mL beaker*

250-mL beaker*

clothespins (to hold pipet)

small funnel

rubber tubing (to fit funnel and pipet)

glass stirring rod (smaller dia. than pipet)

dropping bottles

modeling clay (to close off pipet)

cotton

HAZARDS

MODIFICATIONS/SUBSTITUTIONS

1. HCl is available as muriatic acid, 28%, at the hardware store.
2. Clear household ammonia is available at most grocery stores.
3. Calcium chloride is available as sidewalk deicer at hardware and grocery stores.
4. Copper sulfate is available as root killer at garden supply stores.
5. Iron(II) sulfate is available at health food or drug stores as ferrous sulfate.
6. Sodium carbonate is available as washing soda at grocery stores.
7. Rubbing alcohol containing denatured ethanol is available at drugstores.
8. Cation exchange resins are available from companies selling water softening system; e.g., "Cullex" from Culligan dealers.
9. Plastic soda straws could replace Pasteur pipets.
10. Any suitable open glass container may be used in place of beakers

PROCEDURE

1. Obtain a Pasteur pipet, sufficient cotton to fill about 0.5 cm at the bottom of the pipet, sufficient synthetic resin to fill the pipet, and approximately 52 mL each of solutions containing the following ions: Cu²⁺, Fe³⁺, Ni²⁺ and Ca²⁺. Also obtain the following solutions: 50 mL 6 M HCl, 5 mL 1 M NH₃, 5 mL 0.1 M KSCN, 5 mL 0.1 M Na₂CO₃, and dropping bottle with 1% dimethyglyoxime.
2. Pack the bottom of the Pasteur pipet with cotton by tamping it down with a stirring rod. Fill pipet with distilled water. Add some distilled water to about one spoonful of resin in a small beaker. Carefully and slowly pour the resin into the pipet while holding the pipet over a larger beaker. Some resin may spill during the filling process. It may be necessary to push the resin down past the constriction. Pack the column with the stirring rod. Attach a piece of white paper to the back of the column for a color comparison background.
3. Test for each of the ions before they are passed through the column.
   1. For copper(II), add a few drops of household ammonia to a few drops of CuSO₄ solution and observe the blue complex.
   2. For calcium ion, add a few drops of Na₂CO₃ solution to some CaCl₂ solution and observe the white precipitate.
   3. For iron (II), add some KSCN solution to FeSO₄ solution and observe the red complex.
   4. To test for nickel ion, add some ammonia and some alcoholic dimethylglyoxime reagent to the NiCl₂ solution and observe the brilliant red precipitate.
4. Add a few drops of distilled water to the Pasteur pipet to "saturate" it. Keep this column wet at all times to insure a constant flow. Put a small piece of modeling clay on the bottom to keep the column wet while you prepare for step 5.
5. Remove the clay and add 10-20 drops of copper solution to the pipet and follow it up with distilled water. Collect the eluant in a beaker or test tube and do the test for the copper ion. Judge how well the copper ion is removed by the ion exchange resin.
6. Test the other individual solutions in the same manner in the same pipet. Notice the coloration of the resin. Remember to test for each ion to confirm that the exchange has taken place. Also remember to keep the column wet.
7. Attach a supported funnel and a rubber tube to the top of the pipet. Send 5 mL 6 _ HCl through the pipet via the funnel in order to exchange hydrogen ions for the cations. Notice the color change of the resin.
8. Prepare a mixture of the cation solutions and send 10-20 drops of the mixture through the pipet. Test the resulting eluant for each of the ions.
9. Record all observations in tabular form.
10. Recharge the column with 5 mL 6 M HCl.

DISPOSAL

DISCUSSION

Ca²⁺ + 2 Na⁺|-| |-| Ca²⁺|-| + 2 Na⁺

The K_eq for this reaction is usually 10 or less. It is necessary to run the hard water through a large amount of exchanger to remove all of the hardness ions. Regeneration of the zeolite occurs when brine or concentrated NaCl solution is pushed through the column. Many water softeners operate on this principle.

Today, synthetic copies of this zeolite are made. These are the resins used in this experiment. They are much less expensive than zeolites and also superior because they can exchange hydrogen for other cations. The resins are giant hydrocarbon frameworks that have negative charges due to covalently bonded sulfite groups. Such a resin will exchange cations.

Reactions for step 3 of the procedure of this experiment are:

1. Ca²⁺(aq) + NH₃(aq) Cu(NH₃)₄²⁺(aq) [deep blue]

2. Ca²⁺(aq) + CO₃^2-(aq) CaCO₃(s) [white}

3. Fe³⁺(aq) + SCN^-(aq) FeSCN₂⁺(aq) [red]

4. Ni²⁺(aq) + 2 dimethlglyoximes Ni - complex [red]

TIPS

1. The pipet is best packed when it is filled with water.
2. Many dealers who sell ion exchange water softeners will give small quantities of cation exchange resins to schools.
3. The availability of specific resin varies from one geographic area to another.

REFERENCES

Westmeyer, P., Successful Devices in Teaching Chemistry, J. Weston Walch, Portland, ME, p. 213.

Submitted by Diana Doepkin, Marie Fieldler, John Hnatow, Tanya Phillips, and Doug Rickard

Woodrow Wilson Leadership Program in Chemistry lpt@www.woodrow.org
The Woodrow Wilson National Fellowship Foundation webmaster@woodrow.org
CN 5281, Princeton NJ 08543-5281 Tel:(609)452-7007 Fax:(609)452-0066