Materials Included In Kit

Additional Materials Required

Safety Precautions

Concentrated hydrochloric acid, and solid and aqueous sodium hydroxide are highly toxic by ingestion or inhalation and is severely corrosive to skin and eyes; can cause severe body tissue burns. Wear chemical splash goggles and chemical resistant gloves. Please consult the appropriate Safety Data Sheets for further 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.

Teacher Tips

• This experiment can be done in a more quantitative way by titrating the basic solution, as opposed to adding drops qualitatively. This experiment can therefore serve as a substitute to traditional acid-base titration labs. In addition, the use of a pH meter or probe can be substituted for pH paper.
• Green chemistry will likely be a new topic for students. Given that it is not a regular part of high school curricula nor most undergraduate curricula it may help to spend a bit of time on the 12 principles as part of your pre-lab lecture. The Beyond Benign website (www.beyondbenign.org) includes information on how green chemistry can be further integrated throughout a chemistry curriculum.
• The 1.4 M NaOH solution in 1:1 ethyl alcohol:water can be prepared ahead of the lab or prepared by students as part of the experiment. To prepare 100 mL of the solution, the following procedure should be followed: dispense 23.33 mL of 6M NaOH into a small beaker and add approximately 40 mL each of ethyl alcohol and distilled deionized water. It is not necessary to perform more quantitative measurements because the exact stoichiometry of the reaction is not a concern. That is, this preparation should give enough NaOH to promote base hydrolysis of the PLA cup.
• Have the students handle and cut up the PLA pieces before putting on their gloves. The pieces should be about the size of an adult thumbnail. Wait to hand out the NaOH until they have added the PLA pieces to the flask.

Sample Data

Answers to Questions

1. How much NaOH is required to completely react with the PLA.

   5 g PLA cup x (1 mole PLA monomer/72 g) x (2 mole NaOH/1 mole PLA monomer) = 0.1388 moles NaOH

2. Why is the Acidification step required?

   The acidification step converts sodium lactate to polylactic acid. In other words, the acidification step is taken to protonate the monomers following the depolymerization step.

3. Describe the appearance of the plastic before it is placed in solution and after it is placed in the solution.

   Prior to being placed in solution the plastic is clear and colorless. Once placed in solution the plastic retains its color but begins to dissolve.

4. Describe the appearance of the solution before the reaction starts and after the reaction is complete.

   Prior to the reaction beginning the solution is clear and colorless. Once the reaction is complete the solution is a pale yellow color.

5. What is the pH of the solution after the reaction stops and before any HCl is added?

   After the reaction stops and before the any HCl is added the solution is very basic. That is, the approximate pH is greater than 12. We know the pH is high prior to adding acid because even after the addition of 50 drops of 6 M HCl the pH is still in the basic regime, at around 11.

6. Does the plastic degrade under base hydrolysis conditions?

   The plastic doesn’t degrade (or decompose) under base hydrolysis conditions. Rather, the plastic depolymerizes. On a macroscopic scale, you will observe that the plastic dissolves in basic solution. Upon dissolution, the plastic, or polylactic acid polymer, breaks down into monomers of sodium lactate. Subsequently these sodium lactate monomers are protonated in an acidification step.

7. Explain how the conversion of polylactic acid cups into a cleaning agent is an example of green chemistry in action.

   Polylactic acid (PLA) is derived from a renewable feedstock (corn). Furthermore, it biodegrades to environmentally benign decomposition products on a short time scale. Its use in consumer products is therefore preferable to the use of petroleumbased plastics such as HDPE and LDPE. Moreover, the conversion of PLA cups that would otherwise be discarded to a landfill reduces environmental impact to an even greater degree.

References

Anastas, P.T.; Warner, J.C. Green Chemistry: Theory and Practice, Oxford University Press: New York, 1998.

Gurney, Rich. Hydrolysis of Post-Consumer Polylactic Acid Waste, Chmistry, Simmons College, 2008. 

Introduction

Biobased polymers are plastics derived from renewable biomass sources. This lab features polyactic acid, a polymer derived from corn. Several of the 12 principles of green chemistry are featured in this lab: the use of renewable feedstocks as starting material and pollution prevention by converting a waste cup into a usable cleaner. This lab demonstrates how to chemically convert plastic cups made from polylactic acid into household cleaning agents, and is an example of how green chemistry applies to consumer products.

Concepts

• Green chemistry
• Aqueous solution chemistry
• Acid–base chemistry
• Titrations
• Hydrolysis reactions
• Polymers

Background

Much of what makes this world modern is the result of the application of chemistry and chemical reactions. Oil and gasoline, prescription drugs, plastics, solvents, and fertilizers, to name a few, are all products of chemistry. Over time, many of the processes used to create these products were found to have unintended consequences and be quite harmful, whether to workers, the consumers or to the environment. In response to these pressing issues, green chemistry was developed as an approach to creating safer chemical products and processes from the initial design stage. The principles of green chemistry provide a framework for scientists to use when designing new materials, products, processes and systems. The principles focus on sustainable design criteria and provide tools for innovative solutions to environmental challenges.

• Prevention: It is better to prevent waste than to treat or clean up waste after it has been created.
• Atom Economy: Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product, leaving few or no atoms behind.
• Less Hazardous Chemical Syntheses: Synthetic methods should be designed to use and generate substances that possess little or no toxicity to human health and the environment.
• Designing Safer Chemicals: Chemical products should be designed to be fully effective while minimizing or eliminating their toxicity.
• Safer Solvents and Auxiliaries: Minimize the use of auxiliary substances (e.g., solvents, separation agents) wherever possible and make them innocuous when used.
• Design for Energy Efficiency: Energy requirements of chemical processes should be recognized for their environmental and economic impacts and should be minimized. If possible, synthetic methods should be conducted at ambient temperature and pressure.
• Use of Renewable Feedstocks: Renewable raw material or feedstock should be used whenever technically and economically possible.
• Reduce Derivatives: Unnecessary derivatization (use of blocking groups, protection/deprotection, temporary modification of physical/chemical processes) should be minimized or avoided if possible, because such steps require additional reagents and can generate additional waste.
• Catalysis: Catalytic reagents are superior to stoichiometric reagents.
• Design for Degradation: Chemical products should be designed so that at the end of their function they break down into innocuous products that do not persist in the environment.
• Real-Time Analysis for Pollution Prevention: Analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.
• Inherently Safer Chemistry for Accident Prevention: Substances and the form of a substance used in a chemical process should be chosen to minimize the potential for chemical accidents, including releases, explosions and fires.

Experiment Overview

In this lab you will convert a polylactic acid cup into a household surface cleaner. Polylactic acid is a polymer derived from corn, a renewable resource, in contrast to the majority of commercial plastics which are derived from petroleum, a non-renewable resource. In addition, polylactic acid biodegrades on a reasonable time scale (~180 days) relative to other plastic polymers such as polyethylene (HDPE/LDPE), polyethylene terephthalate (PET) and polystyrene (PS) which can persist in the environment for 500 to 1000 years without biodegrading. This is environmentally problematic because the world produces approximately 200 billion pounds of plastics and nearly half of this plastic winds up in landfills each year.

Polylactic acid (PLA) is derived from natural resources. Lactic acid, which is derived from corn, is the monomer used to create this polymer. PLA degrades under compost conditions into CO₂, H₂O, and humus, all benign components. Also, the CO₂ released during degradation returns the carbon to the atmosphere with no overall net gain. Since the polymer is made from a corn feedstock, the process uses significantly less petroleum than traditional polymers. It is categorized under plastic resin code #7 or #0 (“other plastics”). While it is recyclable, currently there are no municipal recycling facilities that accept PLA.

Instead of disposing PLA products into landfills or compost facilities, another option of handling them at the end of the products’ useful life is to reuse them as other products. One thing to do with PLA materials is to convert them into an antimicrobial cleaning solution. In order to do so the PLA polymer must be depolymerized via hydrolysis in basic solution, a reaction that results in the deprotonation of lactic acid monomers and formation of sodium lactate. Subsequently, sodium lactate monomers are protonated using hydrochloric acid to form lactic acid (see Figure 1). The reaction scheme describes the depolymerization of polylactic acid into sodium lactate and subsequent protonation of sodium lactate to form lactic acid.

Materials

Safety Precautions

Concentrated hydrochloric acid and aqueous sodium hydroxide are highly toxic by ingestion or inhalation and is severely corrosive to skin and eyes; can cause severe body tissue burns. Wear chemical splash goggles and chemical resistant gloves.

Procedure

1. Put on safety glasses, gloves and apron/lab coat.
2. Cut PLA cup into small pieces using scissors. The smaller the pieces, the faster the reaction. Do not use any green parts. (If there is ink writing on the cup do not use those pieces).
3. Place PLA pieces into a weigh boat.
4. Measure 5 g of PLA pieces on a balance.
5. Add 5 g of PLA pieces into the 250 mL Erlenmeyer flask using a funnel.
6. Using the graduated cylinder, measure 100 mL of the pre-made solution (1.4 M NaOH in 1:1 ethanol/water).
7. Add the solution and magnetic stir bar into the flask.
8. Place the flask onto the hot plate.
9. Turn on the heating function of the hot plate and heat the solution to 90°C (reduce the heat if the flask begins to vigorously boil).
10. Heat and stir the solution until the PLA pieces have completely dissolved. Use a stirring rod to stir the solution. Alternatively, the stir function on a stir-plate will safely stir the solution. Record observations as reaction proceeds in the data table provided.
11. After the PLA pieces have completely dissolved and the solution is pale yellow, turn off the hot plate. Temperature should be 80–90°C.
12. Use heat resistant gloves to remove flask from hot plate. Place the flask in an ice water bath and allow the solution to cool until it is below 60°C. This mixture is now called “hydrolyzed PLA”.
13. Using the plastic pipette, transfer 1–2 drops of the solution to the watch glass.
14. Test the pH of the hydrolyzed PLA by wetting a pH strip in the watch glass. Record the pH.
15. Slowly add 50 drops of 6 M HCl into the flask. Mix well.
16. Dip the glass stirring rod into the solution then touch it to the mouth of the flask to remove big drops of liquid. The wetted tip can then be touched to a strip of blue litmus paper to test its pH. Note whether the solution is acidic or basic.
17. Record whether the solution is acidic or basic after each test (thoroughly mix the solution after adding 6 M HCl before testing it).
18. Repeat steps 15–17 until an acidic pH is obtained. Each strip of litmus paper can be used several times.
19. Measure the final pH of the solution using a pH strip. Aim for a pH range of 4–5. The solution now contains lactic acid and sodium chloride (NaCl).
20. Using a funnel, transfer the lactic acid solution into the squirt bottle.
21. Spray the solution onto a dirty surface and wipe clean with a paper towel.

Student Worksheet PDF

14084_Student1.pdf