Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Empty Tea Bags: Use scissors to carefully cut the edge of the tea bags. Empty the contents into the waste.
2. Iron(II) chloride solution, 0.10 M: Mass 19.88 g of iron(II) chloride and transfer this amount to a 500-mL volumetric flask. Add distilled or deionized water to the mark. Cap and mix thoroughly.
3. Copper(II) chloride solution, 0.05 M: Use a 250-mL graduated cylinder to add 250 mL of the 0.1 M copper(II) chloride solution to a clean 500-mL volumetric flask. Add distilled or deionized water to the mark. Cap and mix thoroughly. This solution is used in Part 2.
4. Copper(II) Chloride Reference Solutions

1. 0.05 M CuCl—Use a 50-mL pipet to transfer 50 mL of the 0.10 CuCl₂ stock solution to a clean 100-mL volumetric flask. Dilute to the mark with distilled or deionized water. Cap and mix thoroughly.
2. 0.0 4 M CuCl₂—Use a 20-mL pipet to transfer 20 mL of the 0.10 CuCl₂ stock solution to a clean 100-mL volumetric flask. Repeat this with another 20 mL portion. Dilute to the mark with distilled or deionized water. Cap and mix thoroughly.
3. 0.03 M CuCl₂—Use a 20-mL pipet to transfer 20 mL of the 0.10 CuCl₂ stock solution to a clean 100-mL volumetric flask. Use a 10-mL pipet to transfer 10 mL of the 0.10 CuCl₂ stock solution to the same 100-mL volumetric flask. Dilute to the mark with distilled or deionized water. Cap and mix thoroughly.
4. 0.02 M CuCl₂—Use a 20-mL pipet to transfer 20 mL of the 0.10 CuCl₂ stock solution to a clean 100-mL volumetric flask. Dilute to the mark with distilled or deionized water. Cap and mix thoroughly.

Safety Precautions

Iron(II) chloride and copper(II) chloride solutions are skin and tissue irritants. They are also moderately toxic by ingestion. Avoid all body tissue contact when working with these chemicals. Sodium polyacrylate is irritating to the eyes and to nasal membranes if inhaled. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Sodium polyacrylate and the gelled material, along with the iron(II) chloride and copper(II) chloride solutions, can be disposed of in the trash according to Flinn Suggested Disposal Method #26a. Do not put sodium polyacrylate down the sink!

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs, or for 15 groups of students. Part 1 of this laboratory activity can reasonably be completed in one 50-minute class period. Part 2 can also be completed in the same lab period if run concurrently. The prelaboratory assignment may be completed before coming to lab, and the data compilation and calculations may be completed the day after the lab.
• A 2-point calibration for the copper(II) absorption can be used if time runs short
• Other ions that can be tested are cobalt and iron(III).
• Each M²⁺ ion binds to two carboxylate groups in the polymer networks. These additional cross-links constrict the expansion of the polymer chains, decreasing the water-absorbing ability of the polymer chain.

Teacher Tips

• Seaweeds, containing a naturally occurring gel forming polymer, sodium alginate, have been tested to see if they can be used in water treatment to remove metal ion contaminants from water.

Answers to Prelab Questions

Read the experimental Procedure, then answer the questions that follow.

1. A 0.76 g sample of sodium polyacrylate was placed in 100 mL of a 0.10 M cobalt chloride solution. A small portion of the solution was removed every five minutes, placed in a cuvet, and its absorbance at 510 nm was recorded. This process was repeated every 5 minutes until the absorbance reading reached a steady state value.
   
   The sodium polyacrylate was removed from solution and washed with 3 portions of 30 mL of distilled water. These washings and the remaining solution of cobalt chloride were combined and diluted to 200 mL.
   
   Reference solutions of the cobalt(II) ion were prepared and a resultant calibration curved was generated for the absorption of light at 510 nm as shown in the graph.

1. Determine the [Co²⁺] of the diluted sample from the calibration curve.

   From the graph, 0.103A corresponds to 0.023 mol Co²⁺/L.

2. How many moles of Co²⁺ ions are present in the diluted sample?

   M x V = moles (0.023 mol/L) x 0.200 L = 0.0046 moles Co²⁺

3. How many moles of Co²⁺ ions were present in the original sample?

   M x V = moles (0.1 mol/L) x 0.100 L = 0.010 moles Co²⁺

4. How many moles of Co²⁺ ions were removed by 0.76 g of sodium polyacrylate?

   (0.0100 – 0.046) moles Co²⁺ = 0.054 moles of Co²⁺ were removed

Sample Data

Part 1

Answers to Questions

1. Use graph paper or graphing program to construct the copper(II) ion calibration curve.
   {11997_Answers_Figure_4}
2. Determine the [Cu²⁺] of the diluted sample from the calibration curve.

   0.0095 moles Cu²⁺/L

3. How many moles of Cu²⁺ ions are present in the diluted sample?

   M x V = moles (0.0095 mol/L) x 0.100 L = 0.00095 moles Cu²⁺

4. How many moles of Cu²⁺ ions were present in the original sample?

   M x V = moles (0.05 mol/L) x 0.050 L = 0.0025 moles Cu²⁺

5. How many moles of Cu²⁺ ions were removed by sample of sodium polyacrylate?

   (0.0025 – 0.00095) moles Co²⁺ = 0.00155 moles of Cu²⁺ were removed

6. Based on your results, calculate the copper(II) ion removal capacity of sodium polyacrylate, that is, mol Cu²⁺(aq)/gram sodium polyacrylate.

   0.00155 mol Cu²⁺ removed/0.42 g sodium polyacrylate
   (0.00155 mol Cu²⁺)(63.55g Cu/mol Cu²⁺) removed/0.42 g sodium polyacrylate
   0.23 g Cu removed/g sodium polyacrylate

Introduction

From diapers to artificial snow, sodium polyacrylate’s ability to absorb water finds many uses. Study one more of its properties, its ability to exchange ions and eliminate heavy metals from solution

Concepts

• Ion exchange
• Superabsorbent polymer
• Absorbance in spectroscopy

Background

The word polymer is derived from two Greek words, polys (many) and meros (part). Polymers are large, chain-like molecules that contain many copies of one or two “repeating units,” called monomers, which have been joined together by a chemical reaction. It is not unusual to have thousands of monomer units in a single polymer molecule. Because of the enormous size of polymer molecules and the flexibility of polymer chains, many polymers have unique and useful properties. Polymers can be formed into fibers, drawn out into thin films, or molded into a variety of solid objects. Many polymers will swell up in contact with water to give gels, with properties that appear to be intermediate between those of a solid and a liquid. The properties of a polymer depend on the chemical nature of the monomer, the length of the polymer chain, and how the monomers are joined together.

Sodium polyacrylate is an example of a superabsorbent polymer. Superabsorbents operate on the principle of osmosis—the passage of water through a membrane permeable only to the water. Here, osmotic pressure results from the difference in sodium ion concentration between the inside of the polymer and the solution in which it is immersed. This osmotic pressure forces water into the solid polymer lattice in an attempt to equilibrate sodium ion concentration inside and outside the polymer. The electrolyte concentration of the water will affect the osmotic pressure, subsequently affecting the amount of water absorbed by the polymer.

For example, sodium polyacrylate will absorb approximately 800 times its own weight in distilled water, but will only absorb about 300 times its own weight in tap water, due to the high ion concentration of tap water. Sodium polyacrylate is manufactured by the free-radical polymerization of a mixture of sodium acrylate and acrylic acid and a cross linker such as trimethylol propanetriacrylate shown.

Experiment Overview

In this experiment, you will color sodium polyacrylate by exchanging the sodium ions of the polyacrylate with various divalent metal ions, and then determine the ion-exchange capacity of the polymer with copper(II) cations.

Materials

Prelab Questions

Read the experimental Procedure, then answer the questions that follow.

A 0.76 g sample of sodium polyacrylate was placed in 100 mL of a 0.10 M cobalt chloride solution. A small portion of the solution was removed every five minutes, placed in a cuvet, and its absorbance at 510 nm was recorded. This process was repeated every 5 minutes until the absorbance reading reached a steady state value.

The sodium polyacrylate was removed from solution and washed with 3 portions of 30 mL of distilled water. These washings and the remaining solution of cobalt chloride were combined and diluted to 200 mL.

Reference solutions of the cobalt(II) ion were prepared and a resultant calibration curved was generated for the absorption of light at 510 nm as shown in the graph.

1. Determine the [Co²⁺] of the diluted sample from the calibration curve.
2. How many moles of Co²⁺ ions are present in the diluted sample?
3. How many moles of Co²⁺ ions were present in the original sample?
4. How many moles of Co²⁺ ions were removed by 0.76 g of sodium polyacrylate?

Safety Precautions

Iron(II) chloride and copper(II) chloride solutions are skin and tissue irritants. They are also moderately toxic by ingestion. Avoid all body tissue contact when working with these chemicals. Sodium polyacrylate is irritating to the eyes and to nasal membranes if inhaled. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Procedure

Part 1. Coloring Superabsorbent Polyacrylate

1. Weigh out approximately 0.8 g of sodium polyacrylate in a plastic weighing dish.
2. Place the sodium polyacrylate in an empty tea bag (5.5 x 5.5 cm²).
3. Place the open edge of the tea bag on a preheated hot plate and seal the tea bag by pressing the edge with a stirring rod.
4. In a 125-mL Erlenmeyer flask, soak the tea bag in 50 mL of either 0.1 M copper(II) chloride solution or iron(II) chloride solution. Swirl the flask from time to time for about 10 minutes.
5. Remove the tea bag from the solution and wash it thoroughly with 30 mL of distilled water three times.
6. Oven dry the tea bag containing metal(II) polyacrylates at 110 °C for 40 minutes or until completely dry.
7. Remove the metal(II) polyacrylates from the tea bag. Observe and record the color of the metal(II) polyacrylate.
8. Hand in the resulted metal(II) polyacrylates for disposal.

1. Tare a plastic weighing dish on a balance sensitive to 0.01g.
2. Mass approximately 0.4 g of sodium polyacrylate. Record this mass in the data table to the nearest 0.01 g.
3. Place the sodium polyacrylate in an empty tea bag (5.5 x 5.5 cm²).
4. Place the open edge of the tea bag on a preheated hot plate and seal the tea bag by pressing the edge with a stirring rod.
5. Obtain the four reference CuCl₂ solutions—concentrations 0.05 M, 0.04 M, 0.03 M and 0.02 M.
6. Set up the spectrophotometer. Follow the procedure for your colorimetric measurements of the solution as directed by the instructor. Generally, spectrophotometers are used as follows:
   1. Turn the instrument on and allow it to warm up for 15 minutes. Set the wavelength at 810 nm.
   2. With no light passing through the instrument to the phototube, set the percent transmittance to zero with the “zero” control.
   3. Handle cuvets at the top so no fingerprints are in the light path. Polish cuvets with a tissue. Place a cuvet which is about ⅔ full of distilled water into the sample holder and set the percent transmittance to 100% with the appropriate control (not the zero control).
   4. Fill a cuvet about ⅔ full of a test solution, place it in the spectrophotometer and read the absorbance.
   5. Measure the absorbance of each of the reference solutions at 810 nm, using distilled water as the zero absorbance reference in the spectrophotometer.
   6. Record the absorbance value for each reference solution used in the Reference Solutions Data Table.
   7. Dispose of the contents of the cuvets and of the remaining test solutions as directed
7. In a 100-mL beaker, add 50 mL of 0.05 M CuCl₂ solution. Place the beaker on a stir plate.
8. Put the tea bag in the solution with gentle stirring by a magnet stirrer.
9. Measure the absorbance of the solution every 5 minutes, and return every aliquot to the beaker to maintain the total volume of the solution.
10. Stop the ion-exchange after the absorbance of the solution reaches a steady state.
11. Decant the solution containing residual Cu²⁺ into a 100-mL volumetric flask.
12. Wash the tea bag with 10 mL of distilled water three times, and rinse the cuvet and pipet used for the absorbance measurements with water.
13. Collect the washing solutions in the volumetric flask with the solution containing residual Cu²⁺. Add water to the mark, and measure the absorbance of the resulting solution.

Student Worksheet PDF

11997_Student1.pdf