Chemlab: Chemistry 3/5

Natural Salt Solutions 1: Ion Exchange


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Techniques used this week include using an ion exchange column, testing pH, and using an analytical balance. Your TA will demonstrate the packing of an ion exchange column. Key points to remember when using a column include:

• Keep the column wet at all times. You may cap the bottom if you are not putting solution through it, but it should not be allowed to run dry.

• Always rinse the column thoroughly and test the effluent carefully for ions and/or pH.

• Think carefully about the chemistry of each step. You will be less likely to make a mistake if you write a chemical equation for each reaction occurring on the column.

Image 1 Universal indicator pH paper is a modern version of the more familiar litmus paper. It is impregnated with a combination of colored compounds that are pH sensitive. pH paper should be used to categorize solutions as strongly acidic (red), neutral (no significant color change), or strongly basic (blue). The pH test should not be carried out by directly dipping pH paper into a solution since contamination by the indicating compounds would result. A good procedure is to transfer a drop of solution with a clean stirring rod to a piece of pH paper for the test. Economize by breaking the pH paper into 3 or 4 pieces. Placing the pH paper on a watch glass for the testing will keep your lab bench clean and dry.

Finally you will use an analytical balance this week. Use of this instrument is described in the Guide to Equipment and Instruments section of this manual, in the booklet at your lab bench, and in the Techniques section of the ChemLab website. Your TA will demonstrate the use of the analytical balance for you.

Before you leave lab this week you must weigh the three samples of potassium hydrogen phthalate required for next week's lab. See the end of the procedure section for details.

Column Preparation
The required volume of resin used to load the column can be measured conveniently by water displacement. Put a few milliliters of purified water in a 10 mL graduated cylinder. (In all procedures in which water is a reagent, distilled water is assumed. A supply should be kept in your polyethylene wash bottle.) Using a scoop or spatula, transfer enough resin into the cylinder so that the water level rises by about 1.3 mL.
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Remove the plastic stoppers from the top and bottom of the column, clamp it to a ring stand, and place a small beaker underneath. Swirl the graduated cylinder to put the resin into a slurry and quickly pour it into the empty column.
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Quantitative, or complete, transfer of the resin is unnecessary, but the cylinder should be rinsed a couple of times with purified water and the rinses added to the column to insure that nearly all the resin is transferred. Once the water level is near the top of the resin bed, the inside of the column should be rinsed to wash down resin that adheres to the side. Repeat the rinsing, if necessary. A glass rod can be used for rough leveling of the top of the resin bed as long as there is a layer of water over the resin. When the water level reaches the top of the resin, add a small amount of sand, enough to make a layer approximately 1/2 cm thick.
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The sand should be carefully washed down into a uniform layer. The sand layer protects the top of the resin bed from severe disturbance when solutions are added to the column. Even with the sand in place, some care must be exercised not to add solutions too vigorously.

When you are finished using a column at the end of the lab period, the resin and sand should be dumped out the top and put in the waste resin container for recycling. It may be helpful to push the resin out by squirting water from a squeeze bottle through the bottom of the column

Procedure for Qualitative Experiments
Many of the properties of the ion exchange resin can be illustrated using a solution of copper sulfate. Copper sulfate solutions contain blue Cu2+ cations and colorless SO42- anions. The location of the blue Cu2+ ions can be easily followed visually. A convenient test for SO42- is provided by its reaction with Ba2+. The white solid BaSO4 that forms is extremely insoluble in water. (Ba2+ + SO42- BaSO4(s)). This test is similar to the precipitation tests done in qualitative analysis. Follow the steps below, using the column prepared above. Write down anything you do that differs from your prelab procedure, record all observations carefully, and think about their interpretation in terms of the properties of cation exchangers. Record the results of the pH and SO42- tests and also the behavior of the blue Cu2+ ions. Use chemical equations to describe your observations, as well as words.

Image 8 Rinse a few milliliters of purified water through a freshly packed column and test the pH of the effluent with universal indicator paper. Pour 8 mL of the 0.015 M CuSO4 onto the column.

After charging is complete, test drops of effluent for pH and SO42-.
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Image 11 Use a ruler to measure the thickness of the blue layer and the total column height for later use in determining the column capacity. Then rinse (see the glossary!) the column with 3 mL of purified water.

Check the pH of a drop of the effluent flowing from the column every milliliter or so until the effluent is neutral.
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If the effluent is not neutral after the 3 mL ellutes, repeat the rinse with a second 3 mL batch of purified water.

Pour 8 mL of 2 M HCl through the column to regenerate it. Each time a new charging solution is used, a well-rinsed collecting vessel (e.g. 30 mL beaker or test tube) should be used to catch the effluent. Test drops of the effluent for pH and SO42-. Note any changes in the blue Cu2+ ions visible in the column. Rinse the column with two consecutive 3 mL purified water rinses. If the effluent flowing from the column at the end of these rinses is not neutral, continue rinsing until neutrality is obtained. All the copper must be washed off the column before the NaOH is added in the next step, or the column will turn black.

Using consecutive additions of 1/2 to 1 mL, pour up to 10 mL of 0.4 M NaOH onto the column. Test the pH of the effluent at intervals of several drops, using a stirring rod to remove a drop as it elutes. Test the individual drop on a fresh piece of pH paper on a watch glass. Keep track of the total volume added and add NaOH until the column is saturated with Na+ ions and the capacity is reached. How will you tell that it is saturated? Rinse the column neutral with two or more 3 mL purified water rinses.

After repeated rinsing, charge the column saturated with Na+ ions with 8 mL of 0.015 M CuSO4 and test the effluent for pH and SO42-. Record your observations in your lab notebook. Can Cu2+ ions displace Na+ ions on the exchange resin? Write a chemical equation to describe your observations.

The column must be cleaned out before you leave. Use a wash bottle to flush water backward through the column and remove the resin. Place the used resin in the receptacles provided so it can be recycled. Fill the empty column with distilled water and return it to the bench where you got it.

Weighing samples for next week
On an analytical balance, accurately weigh three samples of 0.18-0.22 g potassium hydrogen phthalate into three clean, dry 8-dram snap-cap vials, recording the actual amount to the nearest 0.1 mg (0.0001 g). The vials should be capped, labeled, and stored in your lab section's labeled box for use next week. Potassium hydrogen phthalate is also called potassium biphthalate and is often abbreviated 'KHP'. This abbreviation emphasizes the H+ donor role of this material. It will be used next week as a standard acid for titration.
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