Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Prepare a 5% starch solution by heating 250 mL of distilled or deionized water to boiling. Remove the water from the heat and slowly sprinkle in 12.5 g of soluble starch while stirring. Heat for a few more minutes, with continued stirring until the starch is dissolved.
2. Test the tap water in your school to determine if it tests positive with silver nitrate for the presence of Cl^– ions. If your water tests positive, use distilled water or bottled water for the experiment. If it tests negatively (most will) then use tap water for the students’ experimentation.
3. Cut the dialysis tubing into 6" pieces and place in water to soak for about 15 minutes before use. Extra dialysis tubing is included to accommodate those students who cannot properly tie the dialysis tubing.

Safety Precautions

Silver nitrate solution is corrosive and can cause burns; avoid contact with eyes and skin. Iodine and salt solutions are irritating to eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. 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. Silver nitrate can be disposed of following Flinn Suggested Disposal Method #11. All other materials can be disposed of following Flinn Suggested Disposal Method #26b.

Teacher Tips

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. The laboratory can reasonably be completed in one 50-minute class period.

• Tying the string tightly on the dialysis tubing and rinsing the outside of the bag are the key steps in the student procedure. Emphasize the importance of these steps with students.
• Sodium chloride and silver nitrate react to form silver chloride, a milky-white precipitate.

  NaCl(aq) + AgNO₃(aq) → AgCl(s) + NaNO₃(aq)

  Iodine reacts with starch to form a purple-blue complex.

Sample Data

Chart I. Starch/Salt Tests

Chart II. Testing Beaker Water

Answers to Questions

1. What is a positive test for the presence of starch?
   

   Dark purple or blue color when tested with iodine.
   

2. What is a positive test for the presence of salt (NaCl)?
   A white milky precipitate forms when tested with silver nitrate solution.
3. Why was the water tested for starch and salt before the experiment?
   

   As a control to be sure there was no starch or salt present in the beaker water before the diffusion period.
   

4. What if anything diffused through the dialysis testing? What evidence supports this claim?
   

   Salt diffused out of the dialysis bag into the surrounding water, as evidenced by the positive test with silver nitrate. Water diffusing into the dialysis tubing may cause it to swell in volume.
   

5. What apparently did not diffuse through the dialysis tubing? What evidence supports this claim?
   Starch apparently did not diffuse through the membrane, as evidenced by the negative starch test in the surrounding water.

Introduction

How do the membranes around cells help to regulate the internal makeup of the cell? How does a semipermeable membrane work? What is diffusion?

Concepts

• Semipermeable membrane

• Diffusion
• Concentration gradient

Background

Diffusion (the random movement of molecules) is one of the key processes involved in the movement of materials throughout living systems. Diffusion can be effectively demonstrated by observing the action of a drop of dye in a glass of water. The water molecules in the water are in constant, random motion. If a drop of blue food dye is added to the glass, the dye begins to slowly diffuse throughout the water. The individual dye molecules disperse and, compelled by collisions with the moving water molecules, eventually become evenly distributed throughout the glass. The water in the glass would now appear to be a uniform shade of lighter blue. The movement of individual molecules in this system is indeed random. The net movement of the dye molecules is directional, in the sense that they (initially) move from a region where they are highly concentrated (the dye droplet) to where they are less concentrated (the surrounding water). Thus diffusion (the term usually refers to the net movement of molecules) is said to occur down a concentration gradient—from a region of high concentration to a region of lower concentration. When the dye has become evenly distributed, and the dye and the water are at equilibrium, the net movement of the dye molecules appears to have stopped. It is important, however, to realize that the random movement of individual dye and water molecules continues even when “equilibrium” is reached.

Cells are enclosed within membranes that regulate the movement of materials into and out of the cell. Some molecules are small enough to pass freely in and out of the cell through the membrane. Other molecules cannot diffuse through freely. Thus, cell membranes are said to be selectively permeable or semipermeable. If we envision the membrane as being porous then it is easy to imagine that some molecules are small enough to fit through the pores while others are too large. Dialysis is the separation of smaller molecules from larger molecules in solution by selective diffusion through a semipermeable membrane.

Dialysis tubing will be used in this laboratory to simulate a cell membrane. It is made of a selectively permeable cellulose tube perforated with microscopic pores. The pores are small enough such that the tubing can be used to model the behavior of the cell membrane with respect to the sizes of molecules that will (or will not) diffuse through them.

Materials

Safety Precautions

Silver nitrate solution is corrosive and can cause burns; it will stain skin and clothing; avoid all contact with eyes and skin. Iodine and salt solutions are irritating to eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

1. Obtain a 6" length of pre-soaked dialysis tubing. The tubing is soaked in order to soften it for handling and to open up the hollow tube. Twist the tubing between your thumb and index finger to make sure the tubing can be opened. Keep the tubing moist throughout the experiment.
2. Close one end of the tube by tying a tight knot with a short piece of string about 1 cm from one end of the tubing. Be sure it is tied very tight with a secure knot so that nothing can leak from the tube.
3. Carefully pour 10 mL of starch solution into the dialysis bag using a 10-mL graduated cylinder. Thoroughly rinse the graduated cylinder. Then pour 10 mL of salt solution into the dialysis bag with the same 10-mL graduated cylinder.
4. Use another short piece of string to close the open end of the dialysis tube. Tie it tight enough to prevent leakage.
5. Rinse the dialysis tube bag thoroughly under running water so that there is no starch or salt solution on the outside of the dialysis bag.
6. Place approximately 200 mL of salt-free water into a clean 250-mL beaker. Place the dialysis bag into the beaker of water (see Figure 1).

7. Set the beaker in a place where it can sit undisturbed for 10–15 minutes. Complete step 8 while waiting.
8. Label four test tubes 1–4 in a test tube rack and prepare them as described. Clean and dry the 10-mL graduated cylinder between steps. Use a different Beral-type pipet for the iodine solution and the silver nitrate solution. Observe the tubes before and after the drops of test solution are added and record the results in Chart I on the Dialysis Worksheet.

Test tube 1—5 mL water + 3 drops iodine solution
Test tube 2—5 mL water + 3 drops silver nitrate solution
Test tube 3—5 mL starch solution + 3 drops iodine solution
Test tube 4—5 mL salt solution + 3 drops silver nitrate solution

9. After 10–15 minutes, use two clean test tubes to test the water in the beaker outside of the dialysis tube as follows:

Test tube 1—5 mL beaker water + 3 drops of iodine solution
Test tube 2—5 mL beaker water + 3 drops of silver nitrate solution

Record the test results in Chart II on the Dialysis Worksheet.

10. Answer the Post-Lab Questions on the Dialysis Worksheet.
11. Consult your instructor for proper disposal procedures.

Student Worksheet PDF

10333_Student1.pdf