Dow/NSTA Summer Workshop Lesson Plan


Activity: 

Parts Per Million

Grade Level: 

9-12  

Prepared By: 

Susan Cooper
(coopers2@mail.firn.edu)
LaBelle High School
LaBelle, Florida
Steve Long
(slong@rps.nwsc.k12.ar.us)
Rogers High School
Rogers, Arkansas



What We'll Study...

The concept of parts per million (ppm) using a Texas Instruments Calculator-Based Laboratory (CBL) and colorimeter or a Spectronic 20 spectrometer.

Did You Know...

When we throw something away, wash it down the drain, or burn it, the elements present in the substance may become rearranged to form new substances, but the elements are still there. It is important for us to know not only what chemicals we are putting into the environment but also how much.

It is now possible for scientists to measure the amount of contaminants present in air or water samples in terms of parts per thousand (ppt), parts per million (ppm), parts per billion (ppb), parts per trillion, or even smaller amounts. These concentrations are very small, but they are important. The smallest amount that can be detected is the detection limit. During the past 50 years, the detection limit has become very small; in some cases, special instruments can detect just a few molecules.

A part per million is equal to:

A part per billion is equal to:

A part per quadrillion is equal to:

Many organic substances can biodegrade by bacterial action into water and carbon dioxide. Scientists often refer to these bacteria as "bugs." These bugs need nutrients to live, but excess nutrients containing nitrogen and phosphorous may be discharged to a receiving body of water. These nutrients promote algae and plant growth, which can affect the amount of dissolved oxygen (DO) available for fish as well as the amount of light reaching lower depths in the water. Most fish need at least 4 ppm of dissolved oxygen to survive, and some species such as bass and trout need much more.

One part per million is the same as 1 mg/L for water solutions. This can be shown as follows:

1 mg = 10-3 g; 1 L = 103 mL

The density of pure water is measured in g/mL, therefore 10-3 g/103 mL = 10-6, or 1 ppm. If the drinking water in your city is fluoridated at the water treatment plant, it is probably added at 1 ppm.

Substances such as acids and bases are neutralized to salts before being discharged to a large body of water such as a river. Depending on the risk that a chemical poses to the environment, dilution with water may be a safe way to dispose of it.

More than 50 percent of the people in the United States depend on groundwater for their drinking water. The best method for protecting this valuable resource is to prevent contaminants from ever entering our groundwater.

All waste treatment processes usually produce solids, which are transferred to a landfill or incinerator after treatment.

 

OBJECTIVES


Students will:

 

MATERIALS (PER LAB STATION)


Experiment Using CBL

Experiment Using Spectronic 20

 

SAFETY AND ENVIRONMENTAL CONSIDERATIONS


 

PROCEDURE


Part I

Experiment Using CBL

  1. Label six beakers or cups with numbers 1 through 6. Measure 90.0 mL of distilled water into each of the beakers.
  2. Fill a cuvette to the line with concentrated food coloring. This concentration will be called 1/1, one part per one.
  3. Into beaker #1, accurately measure 10.0 mL of concentrated food coloring and stir. This concentration is 1/10, one part per ten.
  4. Carefully measure 10.0 mL of beaker #1 and stir into beaker #2. This concentration is 1/100, one part per hundred.
  5. Continue until you have stirred 10.0 mL of beaker #5 into beaker #6. The last beaker has a concentration of one part per million.
  6. Before continuing with the CBL colorimeter, complete the data table except the percent transmittance column. Be sure to observe the color with a white background behind the beaker.

Experiment Using Spectronic 20

  1. Turn on the Spectronic 20 and allow it to warm up while preparing the solutions to be tested. Follow the instructions for the spectrometer. Most require 10 to 15 minutes to stabilize before use. To duplicate the experiment using CBL, set the wavelength to 470 nm if using red food coloring.
  2. Label six beakers or cups with numbers 1 through 6. Measure 9.0 mL of distilled water into each of the beakers.
  3. Fill a graduated pipet or 1-mL volumetric pipet to the line with concentrated food coloring. This concentration will be called 1/1, one part per one.
  4. Into beaker #1, accurately measure 1.0 mL of concentrated food coloring and stir. This concentration is 1/10, one part per ten.
  5. Carefully measure 1.0 mL of beaker #1 and stir it into beaker #2. This concentration is 1/100, one part per hundred.
  6. Continue until you have stirred 1.0 mL of beaker #5 into beaker #6. The last beaker has a concentration of one part per million.
  7. Before continuing with the spectrometer, complete the data table except the percent transmittance column. Be sure to observe the color of each solution with a white background behind the beaker.

Part II

Experiment Using CBL

  1. Connect the colorimeter to the CBL with a CBL-DIN adapter in Channel 1, and link the TI-83 calculator to the CBL with a link cable. Turn on the CBL and the calculator. Press the PRGM key on the calculator and select the CHEMBIO program. Follow the prompts on the calculator to collect data for the experiment.
  2. Perform a two-point calibration (0% and 100% transmittance) for the colorimeter and CBL with the TI-83. If using red food coloring, use the blue LED (470 nm).
a) First, close the lid of the colorimeter, set the colorimeter knob to 0% T, and allow the reading on the CBL to stabilize. Press the [Trigger] button on the CBL and enter 0 when asked to Enter Reference.
b) Set the knob on the colorimeter to 470 nm (if using red food coloring) and insert a blank cuvette (containing distilled water only). Allow the reading on the CBL to stabilize and press the [Trigger] button on the CBL. Enter 100 at the Enter Reference prompt.
  1. Fill each cuvette to the line with successive dilutions of food coloring that were prepared in Part I. To use the calculator to store the data, use the trigger/prompt mode on the calculator program and enter the concentration of each solution when requested. Note: The calculator will store concentration in List 1, absorbance in List 2, and percent transmittance in List 3.
  2. To view the graphs, use the Stat Plot key. Plot 1 should have stored List 1 and List 2 so that the graph will show absorbance vs. concentration. To view percent transmittance, turn off Plot 1 and turn on Plot 2, which should have List 1 and List 3. Examine each graph using the graph functions on the calculator.
  3. (Optional) Graph your results by downloading the information stored in the TI-83 to a computer using the Graph Link software and Vernier's Graphical Analysis program.

Experiment Using Spectronic 20

  1. Perform the proper calibration on the spectrometer using a blank of distilled water. Follow the procedures for setting 0% and 100% transmittance using the guidelines written for your instrument.
  2. Fill each cuvette with a different solution, keeping them in order for easy identification. Use good technique and carefully wipe the outside of the cuvette with soft tissues or paper towels before inserting the cuvette into the sample holder.
  3. Measure the percent transmittance for each of the solutions and record the information in your data table.
  4. (Optional) Graph your results using a graphing calculator or graph paper, or log the data into a graphing calculator and link to a computer to print data from the computer printer.

 

DATA AND OBSERVATIONS


Data Table
 Beaker #  Conc.
Fraction
 Conc.
Decimal
 Conc.
Exponent
 % Transmittance  Color
 1  1/10  0.1  10-1    
 2          
 3          
 4          
 5          
 6          

 

QUESTIONS


  1. At which concentration could you no longer detect the red food coloring with your eyes?
  2. Did the colorimeter or spectrometer enable you to detect a difference between each concentration? Explain.
  3. Sketch both graphs (absorbance vs. concentration and percent transmittance vs. concentration). Discuss the reason for the differences. Which is more linear?
  4. Give an example of a chemical pollutant that is not detectable by our senses but causes harm to people or the environment.

 

OPTIONAL EXTENSIONS


Disposal of Household Products

May these substances safely be poured down the drain when you are finished using them? If not, how should you dispose of them?

  1. Antifreeze (ethylene glycol)
  2. Weed killer or insecticides
  3. Used motor oil
  4. Detergents
  5. Vanish drain cleaner
  6. Woolite cold water wash
  7. Expired medicines
  8. Ammonia
  9. Soap
  10. Latex paint
  11. Paint thinner
  12. Hydrogen peroxide
  13. Nail polish remover
  14. Pepto-Bismol
  15. Fluoride treatment
  16. Rubbing alcohol
  17. Vegetable scraps (garbage disposal)
  18. Grease (from bacon or cooking)

Note: To properly dispose of any specific substance, call the county agency in charge of waste disposal in your area, check the label and other information provided by the manufacturer, or contact the manufacturer for more information.

Determining Optimum Wavelength (using Spectronic 20)

  1. Determine the optimum wavelength to record the percent transmittance data for the food coloring you are using. To do so, use a sample of solution from beaker #4.
  2. Set the spectrometer to 340 nm and calibrate the instrument at 0% and 100% transmittance as before. Insert the sample cuvette and record the transmittance.
  3. Remove the sample and reset the wavelength at 360 nm. Reset 0% and 100% transmittance, insert the sample cuvette, and record the reading.
  4. Continue moving the wavelength up by 20 nm increments to a maximum of 580 nm. Each time, you must reset 0% and 100% transmittance. Record your data after each reading.
  5. The optimum wavelength will be the reading that has the least transmittance (greatest absorbance). This wavelength may vary depending upon the color of food coloring used or the brand of food coloring used.

Evaluation of a Different Food Color (using Spectronic 20)

  1. Select a different color (or brand) of food coloring. Determine the proper (optimum) wavelength for recording the data. See "Determining Optimum Wavelength" discussed above.
  2. Perform the serial dilution in the lab procedure for the new food coloring. Test to see how the different coloring compares with the original coloring used.
  3. Propose hypotheses to explain any differences you may detect. Plan a procedure to test one of your hypotheses and have your instructor verify your procedure before use.

 

NOTES TO THE TEACHER


Answers to the Disposal Extension

  1. No; antifreeze (ethylene glycol) is biodegradable by bacterial action ("bugs"); however, it is very poisonous even in small amounts to pets and people, so it should be disposed of so that there is no danger to them. Read the label for instructions on proper disposal.
  2. No; weed killer and insecticides may be toxic to fish and may not be readily biodegradable. They also may kill the "bugs" (bacteria) at the wastewater treatment plant. Read the label for instructions on disposal. The safest disposal method is to take them to a hazardous waste collection center.
  3. No; used motor oil may not be disposed of down the drain or poured on the ground. It does not biodegrade, and it contaminates our groundwater. It should be recycled.
  4. Maybe; detergents and shampoos may contain phosphates or surfactants that may not be biodegradable. Read the label or other information provided with the product, or contact the manufacturer.
  5. Yes; Vanish cleaner is mostly sodium acid sulfate, a salt which may be washed down the drain.
  6. Yes; Woolite wash contains no phosphates, and the organic surfactants are biodegradable.
  7. Yes; bleach is a 5% solution of sodium hypochlorite, a salt which may be safely washed down the drain.
  8. Yes; ammonia decomposes to a salt and nitrogen.
  9. Yes; medicines are degradable.
  10. No; but latex paint is degradable and may be safely disposed of in a sanitary landfill.
  11. No; paint thinner is not biodegradable. If flushed down the drain or poured on the ground, it contaminates the groundwater. It should be taken to a hazardous waste collection center or recycled.
  12. Yes; hydrogen peroxide decomposes into oxygen and water.
  13. Yes; nail polish remover is degradable.
  14. Yes; Pepto-Bismol is degradable.
  15. Yes; fluoride treatment is a salt that is not harmful to the environment.
  16. Yes; rubbing alcohol is degradable.
  17. Yes; vegetable scraps may be safely disposed of in the garbage disposal; however, a more cost-effective method is to compost.
  18. Yes; although grease can clog drains and damage septic systems, it will degrade eventually and causes no environmental damage.

Additional Background

  1. Try the experiment yourself first. Some food colorings should be used undiluted, while some need to be diluted for the standard (1/1) solution so that there will be a difference between the first two readings. If that is true for the food coloring you are using, make a standard solution for the students to use for steps 3 and 4.
  2. This activity can be performed in a group setting by using different lab groups to prepare different portions of the serial dilution to analyze. In this way, maximum use may be made of limited equipment. Even with only one spectrometer or colorimeter, several groups can test and record data rather quickly.
  3. In using red food coloring for the experiment using the Spectronic 20, the optimum wavelength was determined to be about 500 nm. This was true for the two different brands of food colorings used. You may need to test the particular brand of food coloring you have available. There was very little difference in percent transmittance recorded from the two different colorings.
  4. Often the last two dilutions in the CBL experiment were measured as having the same absorbance and percent transmittance. This was just as good as using our eyes.
  5. Standard food colorings may be obtained from supply houses. One source is Warner-Jenkinson (1-800-325-8110). A source for bulk food coloring is Cuisenaire (1-800-237-0338). The use of pure food colorings may provide interesting results. Food colorings from grocery stores are often blends of different colors.
  6. The relationship between absorbance and percent transmittance is logarithmic and inverse.
  7. If a colorimeter or Spectronic 20 spectrometer is not available, this lesson may be done simply by observing the colors and discussing the limits of using our senses to detect impurities in water.
  8. If desired, relate the use of exponents to pH:
  • 1 ppt = 10-3. If the [H+] is 10-3, the pH of the solution is 3. Examples are soft drinks, vinegar, grapefruit juice.
  • 1 ppm = 10-6. If the [H+] is 10-6, the pH of the solution is 6. Rainwater has a pH of around 6.
  • 1 ppb = 10-9. If the [H+] is 10-9, the pH of the solution is 9. This includes many detergents.

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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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