Dow/NSTA Summer Workshop Lesson Plan

Activity: 

Agricultural Chemistry: Selectivity and Toxicity

Grade Level: 

 9-12  

Prepared By: 

 Al DeGennaro
(aldegenn@sun-link.com)
Westminster High School
Westminster, Maryland		 Cindy Roepcke
(CBR_hs@pickerington.k12.oh.us)
Pickerington High School
Pickerington, Ohio


What We'll Study...

Herbicides are an important classification of organic substances that effectively control vegetation. Some herbicides are selective for controlling certain types of vegetation, while others are nonselective. It is important for consumers, growers, and chemical manufacturers to understand the specific use and application of the variety of herbicides available. Understanding material safety data sheets (MSDSs) and product labels enable the manufacturer, grower, and consumer to evaluate the risks and benefits when developing and using an agriculture chemical.

Did You Know...

Herbicides are a group of compounds that can control unwanted plants. In ancient times, salt was used to control weeds. Later, inorganic salts such as sodium chlorate, copper salts, and arsenic compounds were used, but large quantities of these compounds were required per acre. Herbicides developed before World War II not only killed weeds but also desirable crops. In 1945, 2,4-D (2,4-dichlorophenoxyacetic acid) was introduced as one of the first selective herbicides. At that time many cash crops were grass-like plants. This herbicide was unique because it effectively killed broadleaf plants typical of many "weeds" but not grass-like plants. This was a significant breakthrough.

The chemistry of a large number of herbicides is dependent upon halogenated hydrocarbons. Several of these herbicides function as plant hormones, accelerating growth abnormally. The herbicide 2,4-D is an example of this type of herbicide. Another important herbicide, atrazine, prevents photosynthesis by binding to a protein that is part of the chloroplast. Atrazine is used as a selective herbicide in corn fields because the corn plant can deactivate atrazine but weeds cannot. Glyphosate is a nonselective herbicide that was introduced in the 1970s. It contains a surfactant and inhibits the normal production of a specific plant protein.

Today's growers and consumers need to make informed choices about which herbicides are used, when they are used, and the safe handling of these chemicals. Herbicides are used in agriculture to control weeds among crops to improve yield, by homeowners to control weeds in lawns and gardens, and by communities and utilities to control brush and weeds along railways, roadsides, and power-line rights of way. Depending on the herbicide, the time and manner of application will vary. To be effective, some herbicides need to be applied directly to the plant after it has started to germinate. Other herbicides need to be applied to the soil before seeds are even planted in the ground. The choice of herbicide can affect plant rotation or the ability to alternate crops from year to year (such as corn one year and wheat the next). Some selective herbicides remain active in the soil for several years, limiting crop rotation possibilities. MSDSs and product labels contain pertinent safety information and are available for herbicides, as they are for other chemicals.

Research chemists and biologists are constantly trying to reduce the environmental impact of herbicides. One factor is the amount of chemical required in an application. Other environmental characteristics that are considered include environmental persistence (measured in half-life degradability), soil mobility, volatility, and bioaccumulation. New methods of plant control also include bioengineering strains of crop seeds that are more resistant to the herbicide used to control weeds. Increased food production needs as we move into the year 2000, along with concern for the environment, will continue to be important issues.

 

OBJECTIVES

Students will:

 

SAFETY AND ENVIRONMENTAL CONSIDERATIONS

Students should wear safety goggles for all experiments using reagent chemicals in Part I and any other safety or protective clothing or equipment indicated on the MSDS or label of the chemicals used in this experiment. The instructions provided have the teacher dispensing the acid, base, and silver nitrate solutions. Depending upon the sophistication of the class (such as upper-level chemistry classes), students might be able to dispense their own. If students are responsible for dispensing concentrated acids or bases, the teacher should caution students about the use of concentrated acids or bases.

 

MATERIALS (PER LAB STATION)

Part I: Using MSDSs To Evaluate Safety and Appropriate Use of Chemicals

Part II: Toxicity Testing with Daphnia

Each group of 3 to 4 students could be responsible for one of the chemical concentrations or this can be done as a demonstration. Note: This is a modification of a classic high school biology experiment. It is a good opportunity for cross-curricular laboratory experience.

Advance Preparation

Prepare the following concentrations of the chemicals to be tested using volumetric flasks or Erlenmeyer flasks:

Part III: Selectivity of Herbicides

*Trademark of Dow AgroSciences

 

PROCEDURE

Part I: Using MSDSs To Evaluate Safety and Appropriate Use of Chemicals

Discuss and define the following terms that might appear on MSDSs, which describe health hazards and toxicity:

Experimental Procedure

  1. In the first row of a 24-well microplate, place a small square of cotton cheesecloth in the first well, a small sample of unflavored gelled gelatin in the second well, and egg white in the third well. Add one to two drops of water to the cheesecloth. Prepare exactly the same arrangement in the last row of the microplate to use as a control.
  2. Have your instructor apply one to two drops of concentrated sulfuric acid to each of the samples. Observe and record differences or similarities.
  3. In the second row of the microplate, prepare the same arrangement of cheesecloth, gelatin, and egg whites. The cheesecloth will not need water added to it. Have the instructor add one to two drops of undiluted household ammonia to each sample. Observe and record differences or similarities.
  4. Repeat the above procedure, this time adding several drops of 0.1M silver nitrate to each sample.
  5. Repeat the procedure, adding five to ten drops of ethanol to each sample.
  6. Dispose of all materials in the trash. Rinse and dry your microplate.
  7. Evaluate several MSDSs (hydrochloric acid, nitric acid, methanol, silver nitrate) for health hazards using the terminology defined and investigated. Include MSDSs or label information from a variety of herbicides such as Crossbow, Roundup, and other locally available herbicides.

Part II: Toxicity Testing with Daphnia

  1. Obtain seven small beakers.
  2. Place 20 mL of herbicide solution in one beaker, 20 mL of caffeine solution in another, and 20 mL of nicotine solution in a third.
  3. In a fourth beaker, add about 20 mL of pond water. Separate an eyedropper and reverse the glass in the bulb so that the wide end can be placed in the Daphnia culture. Transfer four live Daphnia to the beaker containing the pond water, but do not transfer any more water than is necessary. This will be your control beaker and will not have any chemicals added.
  4. In the last three beakers, transfer four live Daphnia to each, but do not transfer any more water than is necessary.
  5. Line up the three beakers containing the Daphnia and the control beaker on the countertop, along with the beakers containing the three chemicals. Make sure you can see all the Daphnia in each one. You might need to place the beakers containing the Daphnia on a white sheet of paper or on the overhead projector.
  6. One student will be the timer and will use a stopwatch or clock with a second hand. When the signal is given, pour each beaker containing a chemical into a beaker containing the Daphnia. Do not add chemicals to the control beaker.
  7. After 1 minute, check each beaker and record the number of Daphnia that are still moving.
  8. Continue taking measurements every 2 or 3 minutes. The control Daphnia should be alive and well through the whole experiment. Stop taking measurements when all the Daphnia are dead (except for the control). Record the total time after mixing in your data chart.
  9. Repeat the experiment with different concentrations of the chemicals used in the experiment and compile the results.

Part III: Selectivity of Herbicides

Procedure for Group of 3 to 4 Students

  1. Add potting soil to at least three different small germinating pots for each seed variety you select to grow.
  2. Plant 10 to 15 seeds from one or more varieties from each of the plant categories: broadleaf and narrowleaf (grass-like). Label the containers. Note: It may be more feasible to bring in a seedling of a woody plant from the above list since germination of these seeds takes additional time.
  3. Allow each seed type to germinate and grow until the plant is approximately 10 to 15 cm tall. Set the plants in indirect sunlight near a window or under a sun lamp. Allow approximately 1 week for germination to occur. Record observations about the germination of the seeds (time to germinate, color, height).
  4. Prepare a 1% solution of Crossbow and Roundup (or use commercially prepared concentration). Note: Other herbicides can be used to compare selectivity of herbicides depending upon locality and crop selection as indicated in the ANSWERS section.
  5. Spray each seed type with the same amount of herbicide, applying it directly to the foliage of the plant. At least one pot of each seed type should remain as a control.
  6. Return each group of plants to adequate sunlight or under a sun lamp.
  7. Observe for the next 7 to 10 days. Note which type of plant is most susceptible to which type of herbicide and the length of time that it takes for the herbicide to kill the plant.

Note: Houseplants can be substituted for seedlings if you wish to start this project immediately without waiting for the seedlings to sprout. Pick houseplants that would represent the different plant types.

 

DATA AND OBSERVATIONS

Part I: Using MSDSs To Evaluate Safety and Appropriate Use of Chemicals

Data Table
  Chemical  Cheesecloth
(Cellulose)		  Clear Gelatin
(Protein)		  Egg White
(Protein)
Concentrated sulfuric acid      
Household ammonia      
Ethanol      
Silver nitrate      

Part II: Toxicity Testing with Daphnia

Data Table
 Time
(min)		  Pond Water  Nicotine  Caffeine  1% Stock
Herbicide		  2% Stock
Herbicide		  12% Stock
Herbicide
0            
1            
3            
7            
15            

Part III: Selectivity of Herbicides

Data Table
 Plant Name  Plant Category  Crossbow
Herbicide		  Roundup
Herbicide		  Other Herbicide(s)
Buckwheat        
Rye grass        
Jade plant        
Corn        
Soybeans        
Multiflora rose        

 

QUESTIONS

Part I: Using MSDSs To Evaluate Safety and Appropriate Use of Chemicals

  1. The observations recorded using the sulfuric acid and ammonia provide examples of how caustic or corrosive substances act. How do you think you should best handle these types of substances in the laboratory?
  2. Summarize the effect of an organic substance (ethanol) on a biological substance.
  3. Summarize the effect of a heavy metal on biological substances.
  4. Which parts of your body might be susceptible to each of the above types of chemicals tested?
  5. What are the possible sites of the human body where a chemical substance can enter?
  6. Evaluate the types of protective equipment and methods for handling the chemicals used.
  7. Evaluate the health hazards of the chemicals listed on the MSDSs provided. Rank them in order of personal health hazard.

Part II: Toxicity Testing with Daphnia

  1. Compare the results over time of each type of chemical.
  2. What is the effect on the Daphnia as concentration of a chemical increases?
  3. Which of the chemicals proved to be the most toxic? Does this necessarily mean that it should be banned?
  4. If the Daphnia did not die when they contacted the chemicals, does that mean it is safe? How else might they be harmed?
  5. What other tests might be needed before a new chemical could be considered safe for people or other animals?
  6. Using a Merck Index or Flinn catalog, look up the LD50 values of the chemicals used and interpret the meaning of these numbers.

Part III: Selectivity of Herbicides

  1. Which herbicide appears to be nonselective? Which herbicide appears to be selective?
  2. Which plants did the selective herbicide kill? Which plants did the selective herbicide not affect?
  3. Against what type of plant(s) is Crossbow herbicide most effective?

 

OPTIONAL EXTENSIONS

  1. Try other concentrations for the nicotine, such as 1% and 0.5%.
  2. Replant new seeds in soil treated with Crossbow and soil treated with Roundup 2 weeks after treatment. Use seeds from at least one broadleaf plant and one narrowleaf plant. Compare the ability of each seed type to germinate again. This illustrates the half-life degradability of the herbicide in the soil.

 

REFERENCES

Books

  • Hart, H., Organic Chemistry: A Short Course, sixth edition, Houghton Mifflin Company, Boston, 1983, pp. 166­167.
  • Hill, J.W., Chemistry for Changing Times, fifth edition, Macmillan Publishing Company, New York, 1984, pp. 390­391.
  • Wilbraham, A.C., Staley, D.D., Simpson, C.J., and Matta, M.S., "Functional Groups and Organic Reactions," Chapter 26, in Chemistry: Issues in Chemical Technology, Addison-Wesley, Menlo Park, 1974, pp. 55­56.

Technical Literature

  • The MSDS-Your Guide to Chemical Safety, EMED Company, PO Box 369, Buffalo, NY 14240.
  • Pesticides: Information Pamphlet, American Chemical Society, Department of Government Relations and Science Policy, 1155 Sixteenth Street NW, Washington, DC 20036, 1987.
  • Roundup Herbicide Bulletin, No. 4, February 1991, Monsanto Co., 800 N. Lindbergh Blvd., St. Louis, MO 63167.
  • Ortho Weed Control Guide, available from Solaris, 995 Lavoy Dr., Murray, UT 84123.

Internet


NOTES TO THE TEACHER

Answers to Questions

Part I: Using MSDSs To Evaluate Safety and Appropriate Use of Chemicals

The activity and results should help students understand terms used to describe safety information in the laboratory (such as toxic, caustic, and corrosive), the difference between a spill on clothing versus the skin, and the necessity for wearing safety apparel such as aprons, gloves, and goggles.

Data Table
 Chemical  Cheesecloth
(Cellulose)		  Clear Gelatin
(Protein)		  Egg White
(Protein)
Concentrated sulfuric acid turns cloth brown/black bubbles, blisters the gelatin coagulates
Household ammonia no change gelatin becomes runny coagulates
Ethanol no change no change coagulates
Silver nitrate stains gray after 5 to 10 minutes white precipitate white coagulate

Part II: Toxicity Testing with Daphnia

Data Table
 Time
(min)		  Pond Water  Nicotine  Caffeine  1% Stocka
Herbicide		  2% Stock
Herbicide		  12% Stock
Herbicide
0  4 4 4 4 4 4
2 4 2 4 4 4 0
4  4 2 4 4 4 0
7 4 2 3 4 2 0
10 4 0 1 4 2 0
15 4 _b 0 4 0 _b

aConcentration recommended on label
bNo data

The trials are fairly reproducible, but it is difficult to see the Daphnia and even more difficult to tell when they are actually dead. In particular, nicotine appears to stun them, but some revive when observed later. If the beakers are projected on an overhead projector, the live Daphnia can be seen, but it is difficult to count them. On a concentration basis, 2,4-D is more toxic than nicotine. However, at the concentration recommended on the label, it has no detrimental effect on the Daphnia. These are important factors to be considered in the risk versus benefit decisions that must be made with chemicals. Thus, teachers may wish to use a more concentrated herbicide to show potential dangers and more dilute to show its relative safety.

Other sources of error are listed below. Rather than invalidating the experiment, they illustrate the difficulties in working with living organisms and interpreting the results.

  • Individual Daphnia have varying tolerances to the chemicals. This is partially compensated for by the use of four Daphnia in each test.
  • If not transferred quickly, some of the Daphnia will die from dehydration or other stress before the test begins.
  • Some pond water is collected with the Daphnia, making the total volume unpredictable and hence affecting the concentration.
  • This test only measures short-term lethal dose on one species, which is only one aspect of safety. Other environmental tests would be needed to measure long-term effects, possible reproductive damage, or mutation of offspring.

LD50 results are generally reported as mg of chemical/kg of test organism when 50% of the population dies. LD50 values are useful to help evaluate relative toxicity of chemicals, but teachers should note that they are specific for the test organism and method of administration (oral, skin absorption, injection).

Part III: Selectivity of Herbicides

Data Table
 Plant
Name		  Plant Category  Crossbow
Herbicide		  Roundup
Herbicide		  Ortho 2,4-D
Buckwheat broadleaf brown color, wilted about 4th day brown color, wilted about 7th day brown color about 5th day
Rye grass narrowleaf most of grass relatively unaffected turned brown after 1 week relatively unaffected
Jade plant woody brown leaf about 3rd day brown leaf brown leaf
Corn narrowleaf some color change, relatively unaffected turned yellow not much damage
Soybeans broadleaf yellow/brown color turned brown turned brown
Multiflora rose woody turned brown, brittle leaf on 6th day did not use no change

Back to Procedures

ADDITIONAL BACKGROUND

Some examples of herbicides developed since World War II include:
 Common
Name		  Chemical Name  Activity
2,4-D

2,4-dichlorophenoxyacetic acid

 kills broadleaf plants, used extensively in wheat
triclopyr

(3,5,6-trichloro-2-pyridinyloxy)acetic acid

 effective against unwanted trees and brush as well as broadleaf weeds on rangeland
atrazine

6-chloro-N2-ethyl-N4-isopropyl-1,3,5-
triazine-2,4-diamine

 used in corn, sorghum, and other crops
trifluralin

,,-trifluoro-2,6-dinitro-N,
N-dipropyl-p-toluidine

 used in cotton, lima bean, tomato, cantaloupe, sugarbeet fields
chlorsulfuron

2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-
triazin-2-yl)amino]carbonyl]-
benzenesulfonamide

 used to kill a wide variety of weeds in wheat, barley, oat fields
glyphosate N-(phosphonomethyl)glycine kills all types of vegetation, but treated area can be replanted within 1­2 weeks after application

Suppliers

  • Flinn Catalog, PO Box 219, Batavia, IL 60510.

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Important Note-The information contained herein is presented in good faith. Teachers should verify their own results prior to the use of these lesson plans in a classroom setting. Use of this document is beyond the control of The Dow Chemical Company ("Dow"), The Dow Chemical Company Foundation ("the Foundation"), the National Science Teachers Association ("NSTA"), and/or the authors. Consequently, Dow, the Foundation, NSTA, and/or the authors assume no obligation or liability for the use of these materials or the outcomes of any experiments and make no warranty, express or implied. Safety glasses or goggles should be worn at all times. Other protective clothing should be worn as instructed by the teacher. All materials should be properly disposed of as instructed by the teacher. The user of these materials is solely responsible for compliance with all applicable federal, state, and local law(s) concerning appropriate safety and disposal procedures.

 

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