C.S.Rye: Mascot Murder Mystery

Transcription

1 DEPARTMENT OF CHEMISTRY AND BIOLOGY FACULTY OF SCIENCE C.S.Rye: Mascot Murder Mystery Examine the evidence to see who had it in for Eggy the Ram. Eggy the Ram was found dumped in an alley behind the Mattamy Centre. Eggy s hair and clothing were soaking wet, and the coroner estimated that he had died within two hours of when the body was found. An autopsy revealed large quantities of water in the lungs, and the ultimate cause of death was determined to be drowning. The police have labeled this death a homicide. Primary suspicion falls on the mascots from the surrounding universities (as it is a well-known fact to authorities that there is a fierce rivalry between mascots). The three suspects are Trevor true blue (the U of T beaver), Hunter (the UOIT Ridgeback), and the anonymous York lion. Knowing where the drowning took place will help the police to possibly narrow their list of suspects. The whereabouts of each of the suspects has been confirmed at the time of death and there are limited bodies of water in each area that could be used to drown Eggy. Trevor was at a waterpolo game at the University of Toronto Mississauga pool. Hunter was at a UOIT rowing heat at Lake Scugog, and the York Lion was at a swim meet at York University. A chemical analysis of the water from Eggy s lungs showed an abnormally high concentration of phosphates. It is hoped that analysis of the water at the location of each of the suspects will determine where the murder took place and who the murderer is. In this experiment you are working for the centre of forensic science and are tasked with determining the amount of phosphate first in the sample from Eggy s lungs and subsequently each of the water samples. You will use a colorimetric method to find the concentration of phosphate ions in the parts per million (ppm) range. The principle behind this method is to find a chemical agent or agents that react specifically with phosphates, so that a coloured solution results. The intensity of the colour depends quantitatively on the amount of phosphate ions in the sample. One such agent is ammonium molybdate and subsequently a reducing agent is used to create molybdenum blue. The intensity of the blue colour increases as the phosphate concentration increases and is sensitive to concentrations less than 0.1 ppm phosphorous per litre. The colorimetric procedure involves making up a series of known phosphate solutions and treating them with ammonium molybdate and stannous chloride, a reducing agent. At the same time, we also treat the four water samples (one from Eggy and one from each of the potential crime scenes). After the colour develops in the solutions, we compare the colour of the water samples with the colours of the known phosphate solutions. 350 Victoria Street, Toronto, Ontario, Canada M5B 2K3 Tel: Fax:

2 A person can, of course, compare colours by visual observation, but this method is not always reliable, especially if the colours are very similar in intensity. A much better method is to use a device called a colorimeter, an instrument that measures the intensity of the colour electronically. The colorimeter does this by measuring the absorbance of a specific wavelength of light as it passes through the coloured sample. Each of the known phosphate solutions absorbs a different amount of light. The more phosphate in the solution, the more light it absorbs. The observer reads the absorbance of each sample on the colorimeter meter and then draws a graph, plotting the concentration of each known phosphate solution against its absorbance. A sample graph is shown below. The procedure should give a straight line, called a calibration curve. The calibration curve enables one to determine the phosphate content of the water samples by finding the absorbance of the unknown solutions on the calibration curve, and reading off the corresponding concentration of phosphate in ppm. Figure 5-1. Graph of absorbance versus phosphate concentration. The calibration curve is plotted from absorbance of solutions of known phosphate concentration Absorbance y = x R² = Concentration Phosphate (ppm)

3 SAFETY ALERTS! Ammonium Molybdate reagent (corrosive) contains 5 M H 2 SO 4, causes severe burns to eyes and skin, avoid contact with clothing. Eye contact: flush eyes with water for 15 minutes, seek medical advice. Skin contact: wash affected areas with warm running water for 15 minutes. Stannous Chloride - causes irritation to eyes and skin, avoid contact with clothing. Eye contact: flush eyes with water for 15 minutes. Skin contact: wash affected areas with soap and warm water. Hazardous waste All waste should be disposed of in the aqueous inorganic waste containers. Please check your data prior to disposing of any of your solutions. 1. Complete the following table for preparing five solutions of known phosphate content, and for preparing one blank to serve as a reference solution. The stock solution contains mg of phosphate/ml of solution. Sample calculations follow after the table. Test Tube # Phosphate Content (mg) Concentration of Phosphate (ppm) Volume Required of Stock Phosphate Solution (ml) Volume Required of Water (ml) TOTAL Volume (ml) 1 BLANK Eggy s lungs unknown U of T pool unknown York pool unknown Lake Scugog unknown

4 Sample Calculations Using Test Tube #2. (i) Concentration of Phosphate (ppm) Recall from Experiment #1, that the unit of ppm can be defined as mg solute per L solution. Test tube #2 contains 0.01 mg phosphate in 10.0 ml solution. What is the phosphate concentration in ppm? First, convert volume to L. Then calculate the concentration in ppm (ii) Volume Required of Stock Phosphate Solution (ml) The stock solution contains mg of phosphate /ml of solution. If 0.01 mg of phophate is needed for your standard solution, let X be the volume of stock solution required (iii) Volume Required of Water (ml) The total volume for each test tube is to be 10.0 ml, and a volume of 0.4 ml of stock phosphate solution has already been added to test tube #2. Therefore the amount of water required is: 10.0 ml ml = 9.6 ml WATCH THE NUMBER OF SIGNIFICANT FIGURES! 2. Plug the colorimeter into the electric socket at your bench and allow the machine to warm up. 3. Label ten screw cap tubes #1 through to #10. Put these tubes in a test tube rack. There is already 1 ml of an ammonium molybdate solution in each of these tubes. Obtain 10 ml of stock phosphate solution in a clean dry beaker. 4. Add to tubes #1 through #6, the required volume of distilled water as calculated. Use a 10 ml graduated pipette for your additions. Touch the tip of the pipette to the side of the test tube while draining the water.

5 5. Using a graduated 2 ml pipette, add to tubes #2 through #6, the required amount of stock phosphate solution. Take care that there is no contamination of tube #1 with phosphate. This is the blank solution that you need to standardize your colorimeter. 6. Obtain a little more than 10 ml of each of the four water samples. Pipette 10 ml of each of the samples into the respective tubes (#7, #8, #9, and #10). 7. Add 2 drops of the stannous chloride solution to each of the 10 tubes. 8. Cap each tube and invert 5 times to mix the solutions. 9. Wait 5 minutes to allow the colour to develop. If no colour develops please contact your teaching assistant after the 5 minutes. 10. Your instructor will demonstrate how to operate the colorimeter and instructions are also on the next page. Please ensure that the wavelength is set to 650 nm. 11. Obtain the readings for all your solutions, going from the lowest phosphate concentration to the highest concentration. The darker the solution, the higher the content of phosphate. Note: Before each reading, rinse the cuvettes with a little of the solution to be tested before filling it for the reading. This will prevent contamination by the previous solution. Dry the outside of the cuvette with Kimwipe before inserting it into the colorimeter. Pour the contents of each solution back into the test tubes after you are finished. This will allow you to recheck any measurement. 12. Take a look at your tubes. Do you see the range of intensity of blue colour that should develop? How good is your ability to determine concentration by eye? Record all your results in the attached table. Plot a graph of absorbance (see calculation below) versus phosphate concentration (ppm) and draw the line-of-best-fit. This is your calibration curve. Plot the absorbances of your four samples on it, and determine the phosphate concentration (ppm) of the four water samples. State the concentration of the samples on the graph. Transfer your results to your report sheet, answer the questions below, and include the graph in your report. Absorbance = log (1 / T), For example: where % T = 88.1 % (88.1 / 100), Absorbance = log (1 / 0.881)

6 Spectronic 20 Instructions Key 1. Sample Compartment 2. Pilot Lamp 3. Wavelength control 4. Transmittance / Absorbance control (100%T / 0 A) 5. Power switch / Zero control 6. Filter lever (Note: Only the 20+ models have a filter lever) Operation 1. Turn on the instrument by turning the Power switch (#5) clockwise. Allow the spectrophotometer to warm up for at least 15 minutes to stabilize. 2. After the warm-up period, set the desired wavelength (650 nm) with the Wavelength control knob (#3). 3. Set the filter lever (#6, if equipped) to the right 4. Remove the cuvette from the sample compartment (#1), close the cover, and adjust the Zero control (#5) so that the meter reads 0% T. 5. Fill the cuvette ¾ full with the blank solution (from test tube #1) and wipe the cuvette with a Kimwipe to remove liquid droplets, dust, and fingerprints. 6. Place the cuvette into the sample compartment, close the cover, and adjust the 100%T using the Transmittance / Absorbance control (#4) so that the meter reads 100%T. 7. Remove the cuvette and empty the contents into waste. 8. Rinse the cuvette twice with small volumes of the solution to be measured and fill it ¾ full with the solution to be measured. 9. Wipe the cuvette with a Kimwipe, insert the cuvette into the sample compartment (#1) and close the lid. 10. Read the %T value from the meter 11. Remove the cuvette from the sample compartment and repeat steps 7-10 for the remaining solutions 12. When all measurements are complete, turn off the spectrophotometer by turning the Power Switch / Zero Control counter clockwise until it clicks.

7 C.S.Rye: Mascot Mystery REPORT SHEET Name: Date: Teaching Assistant: 1. Review pages on graphing in appendix. Draw a line-of-best-fit through all of your data points. Include the graph with your report sheet. 2. Results. Tube # Phosphate Content (mg/10 ml) Phosphate Concentration (ppm) % Transmittance reading Absorbance (calculated) Eggy s lungs 8 U of T Pool 9 York Pool 10 Lake Scugog Based on the above data, which suspect was closest to the crime scene.