Materials Included In Kit

Additional Materials Required

Prelab Preparation

Preparation of aniline HCl: Using a 5 mL serological pipette, draw up all the liquid from the bottle and transfer to a 250 mL glass bottle. Add 155 mL of 0.2 M hydrochloric acid and mix. Aniline does not solubilize very well, so some precipitate is normal. Make sure the bottle is agitated vigorously before each student obtains their sample.

Preparation of ammonium persulfate: Transfer the 10 g of ammonium persulfate to a 250 mL glass bottle. Add 175 mL of deionized water and mix. Agitate the bottle before dispensing.

Safety Precautions

Ammonium persulfate is an oxidizing solid with acute toxicity. It is considered harmful if swallowed or comes in contact with skin. Use in a well-ventilated area. Aniline is highly toxic by ingestion inhalation, and skin absorption. Severe eye irritant and mild sensitizer. Combustible. Use only in a hood or well-ventilated area. Hydrochloric acid solution is toxic by ingestion or inhalation and is severely corrosive to skin and eyes. Ammonium hydroxide solution is a corrosive liquid and is extremely irritating to the eyes and respiratory tract. It is toxic by ingestion and inhalation. Work with ammonium hydroxide in the hood or in a well-ventilated lab only. The polyaniline produced in this lab is nonhazardous. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information. Please follow all laboratory safety guidelines.

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. Aniline solution may be disposed of according to Flinn Suggested Disposal Method #5. Ammonium persulfate solid may be disposed of according to Flinn Suggested Disposal Method #12a. Excess hydrochloric acid may be disposed of according to Flinn Suggested Disposal Method #24b. Excess ammonium hydroxide may be disposed of according to Flinn Suggested Disposal Method #10. Polyaniline can be disposed of according to Flinn Suggested Disposal Method #26a. 

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. The laboratory work for this experiment can be completed in one 50-minute class period. The prelaboratory assignment may be completed before coming to lab, and the data compilation and calculations may be completed the day after the lab.
• While the starting components are hazardous, the final product is nontoxic. We do not recommend letting students take the chemicals home.
• The reaction is slow to start, and students will be concerned the polymerization is not taking place. It will stay clear for several minutes before suddenly changing and becoming a very deep green color.
• The final polymer solution will look grainy; this is normal. Polyaniline is very stable and can keep indefinitely.
• The resistance measured by the multimeter will fluctuate and not come to a steady value. The purpose is for the students to observe a range and to see how that range changes when the sensor is exposed to either acid or base vapors.

Teacher Tips

• After the lab is complete, mix leftover aniline and ammonium persulfate solutions together. Synthesized polyaniline does not require special disposal (see Disposal section) or storage.

Further Extensions

• This lab can be extended by having the students use their conducting polymer in a circuit containing an LED bulb and a variable power source. The students can observe and increase the power until the bulb is lit.
• Students can also investigate different concentrations of the acid and base dopant to see how much or how little is needed to see a desired effect.
• An investigation into different concentrations of the polyaniline solution can also be done by diluting it with 1 M HCl. Students can experiment with different amounts on their sensors to see how the resistance changes.
• The polyaniline can also be turned into an ink and used to draw circuits. An extension of this lab into the topic of electrochemistry is to add gum arabic (acacia) to the polymer to thicken it into an ink. This can then be transferred into a pen cartridge.
• Using concentrated ammonia and hydrochloric acid should give stronger results as they off gas more. However, these should only be used in a fume hood or well-ventilated area.

Answers to Prelab Questions

1. Match the definitions with the category they best fit.

Natural Polymer: Are often water-based, Easily degradable, Shorter polymer chains
Synthetic Polymer: Derived from petroleum oil, Hard to degrade, Longer polymer chains

2. Match the polymers with the correct category.

Natural: Amber, Wool, Silk
Synthetic: Nylon, Kevlar, Silicone

3. Match the forms of polyaniline with their respective properties.

Leucoemeraldine—Colorless—Nonconducting
Emeraldine salt—Green—Conducting
Emeraldine base—Blue—Nonconducting
Pernigraniline—Purple—Nonconducting

Sample Data

A. Synthesis of Polyaniline

Observations
The solution stays colorless for several minutes, then slowly darkens and in almost an instant, turns a deep dark green. Very murky and hard to see. When agitated, the solution appears grainy.

C. Testing Conductivity

Observations
When the first sensor was held over the acid beaker, the polyaniline changed to a slightly lighter green.
When the second sensor was held over the base beaker, the polyaniline changed to a very deep blue, almost black. The same color changes occurred when each sensor was held over the opposite beaker.

Answers to Questions

1. The resistance measured by the multimeter is inversely related to conductivity.

1. When the acid sensor was held over the hydrochloric acid beaker, what happened to the conductivity?

   The resistance decreased, which means there was an increase in conductivity.

2. Which form of polyaniline is likely present (leucoemeraldine, pernigranaline, emeraldine salt, emeraldine base)? Why? List all that apply.

   Emeraldine salt because it is the only conductive form of polyaniline.

2. 1. When the base sensor was held over the ammonium hydroxide beaker, what happened to the conductivity?

      The resistance increased, which means there was a decrease in conductivity.

   2. Which form of polyaniline is likely present (leucoemeraldine, pernigranaline, emeraldine salt, emeraldine base)? Why? List all that apply.

      Leucoemeraldine, emeraldine or pernigraniline because they are all non-conducting forms and cannot be easily differentiated from each other in this lab.

3. How did the conductivity of the sensors change when held over the opposite beakers?

When the acid sensor was placed over the beaker with ammonium hydroxide, there was an increase in resistance and the sensor changed from a dark green to more of a dark blue color. This means the conductivity decreased.

When the base sensor was placed over the beaker with hydrochloric acid, there was a decrease in the resistance and the sensor changed from a dark blue to a dark green/blue. This means there was an increase in conductivity.

References

Enlow, J., Marin, D. and Walter, M., Using Polymer Semiconductors and a 3-in-1 Plastic Electronics STEM Education Kit to Engage Students in Hands-on Polymer Inquiry Activities. J. Chem. Educ., 2017, 94, 1714-1720.

Introduction

Polymers are an indispensable part of modern life. The word polymer is derived from two Greek words, polys (many) and meros (part). Polymers can be formed into fibers, drawn out into thin films or molded into a variety of solid objects. In this lab, you will be making a polymer sensor with conductive properties dependent on exposure to acidic or basic environments.

Concepts

• Polymers
• Acid–base
• Conductivity
• Oxidation–reduction

Background

Natural polymers include a wide range of biological molecules and materials, including DNA, proteins, starch, cellulose and wood. Synthetic polymers or plastics are incredibly useful modern materials, finding use in cellphones, computers, contact lenses, artificial joints, bike helmets and bullet proof vests. Polymers are large, chain-like molecules composed of multiple repeating units of smaller molecules (monomers) that have been joined together using chemical reactions. 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.

In this lab, you will examine the conductive properties of polyaniline, a conductive polymer synthesized from aniline. This was one of the first polymers discovered, and it remains one of most studied today. Its popularity is due in part to the fact that it is an electrochromic polymer. These types of substances can reversibly change color when an electrical field is applied. Conductive polymers are a great replacement for toxic metals and can be made more easily on a large scale. Polyaniline has a very low manufacturing cost and is nontoxic. It has the added benefits of being stable in harsh chemical environments and has high thermal stability.

Polyaniline exists in three oxidative states, giving different colors depending on the type of doping applied. Doping involves the introduction of impurities into a solution that will affect its properties—in this case its conductivity. There are two common types of doping used on polymers—oxidation–reduction (redox) and acid–base. This lab will utilize acid–base doping because the undoped form of polyaniline is sensitive to air. It can be synthesized in the lab by the oxidative polymerization of aniline (see Figure 1).

Aniline hydrochloride and ammonium persulfate react to form polyaniline. This synthetic method yields the acidic form, known as an emeraldine salt. It has a green color and is conductive. When the solution is basic, it forms an emeraldine base, which is blue in color and nonconductive. The fully reduced form is known as leucoemeraldine and is electrically insulating (nonconducting). When fully oxidized, it becomes pernigraniline, a form of polyaniline with a deep purple color which is also nonconducting. The different structures can be seen in Figure 2.

Acid–base doping can be used to reversibly turn the conductivity of polyaniline on and off. The utility of this feature is in the production of chemiresistors, also known as chemical vapor sensors. Depending on the type of sensor, it can be designed to detect the presence of either acidic or basic vapors. On their own, polyaniline sensors lack sensitivity since they cannot distinguish between specific chemical vapors, but other polymers can be added to increase selectivity and target specific compounds. A similar application of this technology can be found in carbon monoxide detectors.

Experiment Overview

In this lab, you will be making the conductive polymer polyaniline and examining its conductivity as a function of pH through acid–base doping.

Materials

Prelab Questions

1. Match the definitions with the category they best fit.

Categories
Natural Polymer
Synthetic Polymer

Definitions
Derived from petroleum oil
Often water-based
Easily degradable
Hard to degrade
Shorter polymer chains
Longer polymer chains

2. Match the polymers with the correct category.

Categories
Natural Polymer
Synthetic Polymer

Polymers
Amber
Kevlar
Silk
Nylon
Silicone
Wool

3. Match the forms of polyaniline with their respective properties.

Safety Precautions

Ammonium persulfate is an oxidizing solid with acute toxicity. It is considered harmful if swallowed or comes in contact with skin. Use in a well-ventilated area. Aniline is highly toxic by ingestion, inhalation and skin absorption. Severe eye irritant and mild sensitizer. Combustible. Use only in a hood or well-ventilated area. Hydrochloric acid solution is toxic by ingestion or inhalation and is severely corrosive to skin and eyes. Ammonium hydroxide solution is a corrosive liquid and is extremely irritating to the eyes and respiratory tract. It is toxic by ingestion and inhalation. Work with ammonium hydroxide in the hood or in a well-ventilated lab only. The polyaniline produced in this lab is non-hazardous. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information. Please follow all laboratory safety guidelines.

Procedure

A. Synthesis of Polyaniline

1. From your teacher, obtain 2.5 mL each of the aniline HCl and ammonium persulfate solutions, and place in separate 15 mL conical tubes.
2. Shake each tube immediately prior to mixing. Then quickly pour the ammonium persulfate solution into the tube containing the aniline HCl solution. Immediately cap and shake vigorously.
3. Shake the tube for about 15–20 seconds then allow the solution to settle.
4. Polymerization will usually take between 5 and 10 minutes to occur. Write down any observations during this process.
5. When the polymerization is complete, the solution will go from clear to a deep green color.

B. Creating Polyaniline Sensor

6. Cut your chromatography strip evenly into three even pieces (approximately 5 cm).
7. On both ends of each strip, use a graphite pencil to color a 1 cm electrode. Do this for all three strips. Make sure the graphite heavily coats the ends of strips with no gaps.
8. Test for any latent conductivity with the multimeter. Touch the leads to the electrodes at each end of the first strip. There should be no change in the reading. If there is, you will need to create a new electrode strip. (Note: It does not matter what specific value you choose on the multimeter, just that it is set to Ω [resistance].)
9. Repeat step 8 for the other two strips.
10. Shake the tube of polyaniline to resuspend the polymer in solution.
11. Place a few drops of the polyaniline along the space between the electrodes. Then use a brush to spread the polymer evenly between them. The solution must touch and slightly overlap the electrodes to complete the circuit.
12. Repeat step 11 for the other two sensors, and set them aside to dry completely.

C. Testing Conductivity

Note: The resistance measurement will not be a steady value. On your data sheet, record the average value of the range measured. Do not forget to include units for your measurements. Conductivity is inversely related to resistance. As resistance increases, conductivity decreases and vice versa.

13. You now have three sensors: control, acid (doped) and base (de-doped). Each sensor will have a different starting resistance due to the varying amount of polyaniline on each strip.
14. Using the multimeter, place the probes on each electrode of the control sensor. Record the average resistance value in your data table.
15. Obtain 10 mL of 6 M hydrochloric acid from your teacher. Measure the starting resistance of this sensor. Then carefully hold the sensor over the acid beaker and allow the fumes to diffuse over it for about 30 seconds.
16. Measure the resistance again, and record the average value from the multimeter on your data table along with any observations.
17. Obtain 10 mL of 6 M ammonium hydroxide from your teacher. Measure the starting resistance of this sensor. Then carefully hold the sensor over the base beaker and allow the fumes to diffuse over it for about 30 seconds.
18. Measure the resistance again, and record the average value from the multimeter on your data table along with any observations.
19. Measure the starting resistance of the acid doped sensor. Then take the acid sensor and hold it over the ammonia beaker. Record the resistance and any observations on your data sheet.
20. Repeat step 18 with the base sensor and the hydrochloric acid beaker.

Student Worksheet PDF

14150_Student1.pdf