Materials Included In Kit

Additional Materials Required

Prelab Preparation

Sodium Alginate Solution, 2%: Measure 5.0 g of sodium alginate into a 600-mL beaker. Add 250 mL of distilled or deionized water and a stir bar. Stir on a magnetic stirrer for about one hour or until the solid dissolves. For best results, allow the mixture to sit overnight to give a uniform solution.

Sodium Alginate Test Solutions: To 50 mL aliquots of the 2% alginate solution, add 1 mL of the following solutions:

1. 0.04% bromphenol blue indicator solution
2. 0.1% Congo red indicator solution
3. Iodine solution
4. 0.5% starch solution

Sodium bisulfate solution, 1 M: Measure 250 mL of distilled or deionized water in a 250-mL graduated cylinder. Carefully add this water to the bottle containing 35 g of sodium bisulfate. Cap and mix thoroughly.

Safety Precautions

Acetic acid solution is corrosive to skin and eyes; slightly toxic by ingestion and inhalation. Sodium bisulfate is a body tissue irritant and moderately toxic; its solution is corrosive to skin and eyes and is slightly toxic. Iodine solutions are irritating to eyes, irritating and mildly corrosive to skin and toxic by ingestion. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. 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. Excess iodine—potassium iodide solution may be reduced according to Flinn Suggested Disposal Method #12a. Polymer gel products obtained in this demonstration may be disposed of in the trash according to Flinn Suggested Disposal Method #26a. Excess acetic acid solution and sodium bisulfate solution may be neutralized according to Flinn Suggested Disposal Method #24b. Excess calcium chloride, indicator, and starch solutions may be rinsed down the drain with plenty of excess water according to Flinn Suggested Disposal Method #26b.

Teacher Tips

• Enough materials are supplied for a class of 30 students working in pairs
• Nano-encapsulated drugs work by diffusion into the capsule and subsequently out of the capsule (which is mimicked on the macro scale in the student lab.) The drug Abraxane^® is supplied as a white to yellow, sterile, lyophilized powder for reconstitution with 20 mL of 0.9% Sodium Chloride Injection, USP prior to intravenous infusion. Each single-use vial contains 100 mg of paclitaxel and approximately 900 mg of human albumin. Each milliliter (mL) of reconstituted suspension contains 5 mg paclitaxel
• A polymer gel is a type of colloidal solution consisting of a cross-linked polymer network swollen in a liquid medium such as water. The properties of a gel depend on the interaction of these two components. The liquid component, or solvent, prevents the polymer matrix from collapsing into a hard, insoluble mass. The polymer matrix helps to retain the solvent.
• Sodium alginate will precipitate with most polyvalent metal ions and gives colored gels with many transition metal cations, including Fe³⁺ or Co²⁺ salts.
• Seaweeds have been tested to see if they can be used in water treatment to remove metal ion contaminants from water. Will sodium alginate decolorize a solution of copper(II) chloride?

Answers to Prelab Questions

1. What are the pH ranges for the indicators Congo red and bromphenol blue? That is, at what pH values do each of the indicators change color and what are the colors?
   Congo red changes from blue to red between the pH values of 3.0–5.0.
   Bromphenol blue changes from yellow to purple between the pH values of 3.0–4.6.
2. Two acid solutions, 1 M acetic acid and 1 M sodium bisulfate, are included in the laboratory. Determine the pH of each solution.
   pK_a of acetic acid, CH₃CO₂H, is 4.76
   pK_a of the bisulfate ion, HSO₄^–, is 1.99
   pK_a = –logK_a
   for acetic acid, K_a = 10^–4.76 = 1.74 x 10^–5
   for sodium bisulfate, K_a = 10^–1.99 = 1.02 x 10^–2
   for a weak acid where [H₃O⁺] at equilibrium is less than [HA] initial,
   
   {11953_Answers_Equation_1}
   In the case of acetic acid,
   
   {11953_Answers_Equation_2}
   pH = –log(4.17) + 3 = –0.62 + 3 = 2.38
   In the case of sodium bisulfate,
   
   {11953_Answers_Equation_3}
   pH ≈ –log(0.10) = 1.0

Sample Data

Part 1. Encapsulation and Acid Strength

Part 2. Diffusion

Answers to Questions

1. Explain the observations when the capsules containing acid–base indicators were added to the acid solutions.
   Since the solutions have a greater concentration of H⁺(aq) ions, these ions diffuse into the spheres, bringing the encapsulated indicator molecules to their acid colors.
2. Why are the rates of color change different between the acetic acid solution and the sodium bisulfate solution?
   The sodium bisulfate solution has a higher concentration of H⁺(aq) ions. The rate of diffusion of H⁺(aq) ions should therefore be greater than that for the acetic acid solution.
3. Do the indicators diffuse through the capsules? Propose a reason why they would or would not.
   No. Their size would prevent this.
4. Based on your observations in Part 2, do the starch molecules diffuse through the capsule? Do the iodine molecules? Explain.
   Only the iodine molecules diffuse through the capsule. When iodine-encapsulated yellow spheres were placed in a solution of starch, the solution became blue around the outside of the spheres, indicating the diffusion of iodine out of the capsule. When starch-encapsulated colorless spheres are placed in a solution of iodine, the spheres become blue, indicating the diffusion of iodine into the capsule.

References

Special thanks to Brett Criswell, Central Columbia High School, Bloomsburg, PA, for providing the idea and the instructions for this activity to Flinn Scientific.

Introduction

Nanotechnology is the development of processes that occur at the atomic, molecular or macromolecular range of approximately 1–100 nanometers. It is used to create structures, devices, and systems that have novel properties. New breakthroughs have allowed the transport of medicines to treatment areas in the body using the nanotechnology of encapsulating drugs in molecular “cages” that are on the order of 100–150 nm in size.

In this experiment, you will model this technology on a macro scale using a naturally occurring polymer extracted from kelp—sodium alginate.

Concepts

• Cross-linking
• Nanotechnology
• Polymers and polymer gels
• Encapsulation

Background

Sodium alginate is a natural polymer obtained from kelp and seaweed or brown algae belonging to the phylum Phaeophyta. The polymer is a principal component of the cell wall in brown algae, comprising up to 40% of the dry weight of large species such as giant kelp.

Sodium alginate is a polysaccharide composed of thousands of oxidized sugar “units” joined together to form an ionic polymer. The repeating units are six-membered rings containing negatively charged –CO₂^– groups. The C-1 carbon atom of one ring is connected via an oxygen atom to the C-4 carbon atom of the next ring in the polymer chain (see Figure 1).

The presence of ionic –CO₂^– side chains, as well as numerous –OH groups, make this natural polymer extremely hydrophilic or “water-loving.” The resulting solution is thick, viscous and smooth. Sodium alginate is used as a “thickening agent” in many processed foods, including ice cream, yogurt, cheese products, cake mixes and artificial fruit snacks. The nontoxic food additive absorbs water, helps to emulsify oil and water components and gives foods a smooth texture.

Replacing the sodium ions in sodium alginate with calcium ions leads to cross-linking between the polymer chains and produces an insoluble gel, calcium alginate. Each Ca²⁺ ion can bind to at least two carboxylate groups in the polymer. If the two –CO₂^– groups are on different (adjacent) polymer molecules, then the effect of adding divalent cations is to tie together or cross-link individual polymer molecules into a large, three-dimensional network. The cross-linked polymer swells up in contact with water to form an insoluble gel. Studies have shown that the polymer behaves like a giant chelating ligand (similar to EDTA), and that each Ca²⁺ ion is bound to four –CO₂^– groups.

When a concentrated solution of sodium alginate is added dropwise to a dilute solution of calcium chloride, insoluble spheres are formed as the calcium ions cross-link the alginate ions, forming a semipermeable capsule. These alginate capsules and their properties will be used to model a fascinating new use of nanotechnology.

Experiment Overview

The purpose of this laboratory is to investigate the the formation of insoluble spheres from sodium alginate. These spheres will be studied to see if they will encapsulate other compounds. The “capsules” will be observed for any diffusion of molecules or ions through the capsules when they are placed in solutions of varying acidity and then in solutions of iodine and starch.

Materials

Prelab Questions

1. What are the pH ranges for the indicators Congo red and bromphenol blue? That is, at what pH values do each of the indicators change color and what are the colors?
2. Two acid solutions, 1 M acetic acid and 1 M sodium bisulfate, are included in the laboratory. Determine the pH of each solution.
   pKa of acetic acid, CH₃CO₂H, is 4.76
   pK_a of the bisulfate ion, HSO₄^–, is 1.99

Safety Precautions

Acetic acid solution is corrosive to skin and eyes; slightly toxic by ingestion and inhalation. Sodium bisulfate is a body tissue irritant and moderately toxic; its solution is corrosive to skin and eyes and is slightly toxic. Iodine solutions are irritating to eyes, irritating and mildly corrosive to skin and toxic by ingestion. 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. Encapsulation and Acid Strength

1. Obtain a 6-well reaction plate, a 5" x 8" index card, and four microspatulas.
2. Obtain approximately 30 mL of 0.3 M calcium chloride solution, 15 mL of 1 M acetic acid, and 15 mL of 1 M sodium bisulfate solution in separate 50-mL beakers.
3. Fill one disposable pipet with the Congo red/sodium alginate solution, and another pipet with the bromphenol blue/ sodium alginate solution. Record the initial colors for these sodium alginate–indicator solutions on the Modeling Nanotechnology Worksheet.
   {11953_Procedure_Figure_2}
4. Pour the 0.3 M calcium chloride solution into the left-most well in each rows (wells A1, B1) until each well is about ¾ full.
5. Repeat step 4 adding 1 M acetic acid in the middle wells A2 and B2 and then 1 M sodium bisulfate in the far-right wells A3 and B3. Place the index card underneath the reaction plate.
6. Draw some of the Congo red/sodium alginate solution up into a clean pipet. Hold the pipet with the Congo red/sodium alginate solution a few centimeters above the surface of the CaCl₂ solution in the row 1 well. Release ~ 12 drops into the CaCl₂ solution. Record all observations about the spheres produced. What, if anything, is trapped in the spheres?
7. Using a clean microspatula, carefully pick up a sphere from the CaCl₂ solution A1 and place it in the middle well A2 filled with 1 M acetic acid. Add five more spheres to this well in the same manner. Record observations of any changes that occur with the spheres.
8. Repeat step 7, adding spheres from well A1 to the far-right well A3 containing 1 M sodium bisulfate solution. Record observations of any changes that occur in the spheres.
9. Repeat steps 6–8, this time using the bromphenol blue/sodium alginate solution and the three wells in row B (B1, B2, B3).

Part 2. Diffusion

10. Empty the contents of the 6-well reaction plate and the beakers containing the alginate solutions. Thoroughly rinse the wells and beakers with distilled or deionized water.
11. Fill one disposable pipet with the iodine—potassium iodide/sodium alginate solution, another pipet with the starch/sodium alginate solution. Obtain also a pipet-full of iodine solution and a pipet full of 0.5% starch solution.
    {11953_Procedure_Figure_2}
12. Pour the 0.3 M calcium chloride solution into the left-most well in each row (wells A1, B1) until each well is about ¾ full.
13. Add distilled water to each of the other four wells. Fill to about ¾ full.
14. Add 8 drops of the starch solution to the middle wells A2 and B2. Stir to mix.
15. Add 10 drops of the iodine solution to the far-right wells A3 and B3. Stir to mix.
16. Hold the pipet filled with the iodine/sodium alginate solution a few centimeters above the surface of the CaCl₂ solution in the well A1. Release ~ 12 drops into the CaCl₂ solution. Record all observations about the spheres produced. What, if anything, is trapped in the spheres?
17. Using a clean microspatula, carefully pick up a sphere from the CaCl₂ solution and place it in the middle well A2 filled with dilute starch solution. Add five more spheres to this well in the same manner. Record observations of any changes that occur with the spheres.
18. Repeat step 17, adding spheres from well A1 to the far-right well A3 containing dilute iodine solution. Record observations of any changes that occur in the spheres.
19. Repeat steps 16–18, this time using the starch/ sodium alginate solution and the three wells in row B (B1, B2, B3).
20. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

11953_Student1.pdf