(CMC's online logo)

Forensic Chemistry Lab Exercise

The Case of the Drowned Businessman: Analysis of Phosphate in Water


Students will learn about:

  • quantitative analysis using a spectrophotometer
  • the use of redox reactions to convert a colorless compound to a colored compound
  • preparation of an absorbance versus concentration plot
  • dilution of samples

    This lab is best done late in the first semester or early in the second semester of a first-year college chemistry course (or possibly in a high school AP chemistry course)

    The instructions for this lab exercise consists of two sections:

  • Student Handout
  • Instructor Notes

    STUDENT HANDOUT

    The case of the drowned businessman: Analysis of phosphate in water
    Peter Jeschofnig, Colorado Mountain College

     

    Purpose: It is the purpose of this crime scenario-based lab to introduce students to the quantitative analysis of phosphate in water using a spectrophotometer.

     

    Scenario: THE CASE OF THE DROWNED BUSINESSMAN

    The body of a wealthy local businessman was found dumped alongside a country road a few miles outside of Small-town, U.S.A. The man's hair and clothing were completely wet, and the coroner estimated that the man had died within an hour of when the body was found. An autopsy revealed large quantities of water in the man's lungs, and the cause of death was listed as drowning.

    In view of the circumstances, the police have labeled this death a homicide. Primary suspicion falls on the businessman's three adult children; a socialite daughter, a rancher son, and another son who is a mining engineer. All of the adult children would inherit substantial fortunes from their father, and all were known to have recently argued with him.

    Knowing where the drowning took place will help the police to possibly narrow their list of suspects. There are only three bodies of water within a one-hour radius of where the businessman's corpse was found. They are the swimming pool at the local country club; a large stock pond on one son's ranch, and a mine-waste settling pond.

    A chemical analysis of the water from the businessman's lungs reflected abnormally high concentrations of phosphates. It is hoped that analysis of water from the three possible drowning sites will pinpoint exactly where the businessman was drowned and thus provides a clue regarding his murderer.

    Materials Needed:

    Spectronic 20 or similar spectrophotometer

    Cuvettes; graduated cylinders; Erlenmeyer flasks; beakers

    ammonium molybdate reagent; stannous chloride reagent

    stock 20 ppm phosphate solution; distilled or deionized water

    5 phosphate standards (1-5 pmm) to be prepared by students

    Safety, handling, and Disposal: It is recommended that students read the M.S.D.S.s of all chemicals used in this lab experiment. As always, the use of safety goggles is required, as is the compliance with standard laboratory safety rules. Dispose of used reagents according to local ordinances.

    Procedure:

    In this experiment, we will analyze a series of water samples for their phosphate content. Detergents are among the greatest contributors to phosphate content in rivers and lakes because phosphate-containing compounds are used in detergent formulation as water softeners. Phosphate is not toxic to animals or plants. In fact, it is a plant nutrient, which stimulates the growth of aquatic weeds and algae. This may cause lakes and rivers to become clogged and overrun with plants (eutrophication).

    The principle of this method involves the formation of molybdophosphoric acid, which is reduced to the intensely colored complex, molybdenum blue. This analytical method is extremely sensitive and is reliable down to concentrations below 0.1 ppm (mg) phosphorus per liter.

    Apparatus:

    The Spectronic 20 spectrophotometer will be employed in the measurement of color intensity of the blue solution. A wavelength of 650 nm will be used in these analyses.

    Reagents used:

  • Ammonium molybdate reagent
  • Stannous chloride reagent
  • Stock 20.0 ppm phosphate solution

    Procedure:

    Note: Glassware should be washed thoroughly with hot water followed by a rinsing with distilled water. DO NOT use phosphate-containing detergents to clean equipment for this experiment.

    Prepare the following standard phosphate solutions:

    a. 1.0 ppm standard: Place 2.00 ml of 20.0 ppm phosphate solution in a 100 ml graduated cylinder and dilute to 40 ml with distilled water.

    b. 2.0 ppm standard: Place 4.00 ml of 20.0 ppm phosphate solution in a 100 ml graduated cylinder and dilute to 40 ml with distilled water.

    c. 3.0 ppm standard: Place 6.00 ml of 20.0 ppm phosphate solution in a 100 ml graduated cylinder and dilute to 40 ml with distilled water.

    d. 4.0 ppm standard: Place 8.00 ml of 20.0 ppm phosphate solution in a 100 ml graduated cylinder and dilute to 40 ml with distilled water.

    e. 5.0 ppm standard: Place 10.00 ml of 20.0 ppm phosphate solution in a 100 ml graduated cylinder and dilute to 40 ml with distilled water.

    f. Blank: Set aside 25 ml of distilled water, which will be treated with the color-developing reagent to serve as a blank.

    These five standard solutions and the blank should now be treated according to the following "color development" procedure. After measuring the absorbance of these solutions, make a plot of absorbance (Y-axis) versus concentration (x-axis).

    Color development in sample:

    This procedure is used for the five standard solutions and for any river, lake, or sewage water samples, which are to be analyzed for phosphate. In this case, the samples represent the water from the victimâs lungs and the three samples from the water sources, which are the potential drowning sites.

    Place 25 ml of the water sample to be analyzed in an Erlenmeyer flask.

    Put 1.00 ml (using a pipet) of ammonium molybdate solution into the flask and swirl to mix.

    Add 2 drops of stannous chloride solution to the flask and mix by swirling.

    If phosphate is present, a blue color will develop to a maximum intensity in 5 minutes. NOTE: The time period is somewhat critical. Measurements should be taken anywhere from 5 to 15 minutes after addition of stannous chloride.

    While you are waiting for the blue color to develop, set the wavelength to 650 nm on the spectrophotometer. Use the blank solution to set it to read zero absorbance. Using 650 nm wavelength measure the absorbance (after 5-15 minutes development) of the blue sample*.

    Preparation of standards plot:

    Use the absorbance values of the five phosphate standards and make a plot of absorbance (y-axis) versus the concentration of the standard (x-axis). Draw a best-fit line through the data points and use this plot to interpret the results of the crime samples.

    * Should one of your samples produce a very dark blue color which cannot be read with the spectrophotometer, dilute the original water sample 100 fold. This is accomplished by placing 1.0 ml of the water sample in a 100 ml graduated cylinder and then adding enough distilled water to bring the volume up to 100 ml. Now, this diluted sample may be analyzed according to the directions for color development above.

    Remember that the concentration, which you ultimately obtain from this sample, will have to be multiplied by 100 because of the 100-fold dilution.

    Question: After analyzing the 4 crime-related water samples, determine which drowning location is the best match to the sample from the victimâs lungs.

    Suggested reading:

    Standard methods for the examination of Water and Wastewater. American Public Health Association. 1985 or current edition.

    INSTRUCTOR NOTES

    The case of the drowned businessman: Analysis of phosphate in water
    Peter Jeschofnig, Colorado Mountain College

    Time Required: 2 hours

    Safety, handling, and Disposal: It is recommended that students read the M.S.D.S.s of all chemicals used in this lab experiment. As always, the use of safety goggles is required, as is the compliance with standard laboratory safety rules. Dispose of used reagents according to local ordinances.

    Points to Cover in Pre-Lab: Pre-lab should include a short discussion on the use of redox reactions to produce a colored complex compared to the colorless starting compound and the use of the spectrophotometer to analyze such compounds. A review of how to determine the appropriate wavelength for analysis might be included as well.

    Procedural Tips and Suggestions:

    It is suggested that the instructor prepare a total of 4 unknown samples of two should be identical and the rest different. One of the unknowns should represent the sample from the autopsy, the identical sample is from the drowning location, while the other two (different) samples are from the other water sources.

    It is useful to make at least one sample of high enough concentration to require the students to use appropriate dilution techniques.

    Glassware should be washed thoroughly with hot water followed by rinsing with distilled water. Do not use phosphate-containing detergents to clean equipment for this experiment.

    Preparation of reagents:

    Ammonium molybdate:

    1) Cautiously, add 280 ml of concentrated H2SO4 to 400 ml of distilled water

    2) Dissolve 25 g of (NH4)6Mo7O24:4H20 in 175 ml distilled water.

    3) When the acid solution has cooled, the molybdate solution is added and the entire mixture is diluted to 1 liter.

    Stannous chloride:

    Dissolve 2.5 g of SnCl2:2H20 in 100 ml of glycerine.

    Gently heat to speed up dissolution.

    Stock 20.0 ppm phosphate solution:

    Dissolve 0.286 g of KH2PO4 in 1 liter of water. This is a 200 ppm stock solution.

    Dilute 100 ml of this solution to 1 liter.

    This lab could be modified or extended by testing for additional chemicals commonly found in water such as nitrates, various heavy metals, etc. If a heavy metals analysis is desired and an AA (Atomic Absorption Spectrophotometer) is available, an Atomic Absorption lab could be substituted or added.




    Return to Inquiry-Based Labs Page

    Return to Peter Jeschofnig's Home Page


    Click on address below to send Peter Jeschofnig an e-mail message:
    Pjeschofnig@coloradomtn.edu

    This page was created by Peter Jeschofnig and was last updated: March 7, 2003