Stage 1 Desired Results

Transcription

1 Lesson Title: Phosphate Testing Contract Discipline Focus: Chemistry Grade level: Length of lesson: 1.5 class periods Content Standard(s): Stage 1 Desired Results Understanding (s)/goals Students will understand: The process of planning and preparing an experimental procedure. Essential Question(s): What elements are prepared before starting and investigation? How will you prepare yourself for the exercise? What are some things you can do before class starts? Student objectives (outcomes): Students will be able to: Conduct an investigation of the content of the phosphate ion and report their findings in a format that non-scientists are familiar with (letter). Stage 2 Assessment Evidence Performance Task(s): Measuring concentrations by absorbance, calculating molarities, following protocols. Learning Activities: Other Evidence: Writing a letter communicating protocols, theory, and results. Stage 3 Learning Plan The students were given a letter outlining a task from a mock environmental testing company asking them to win a contract. The company provided a method to test for phosphates and would reward the contract to the most accurate student run company. The students reported back to the company in a written letter outlining the theory behind their method and their results. The students have learned about molarities, protocols, and measuring concentration by absorption (Beer s Law) before starting the activity. After the students read the letter, a similar problem is solved with the class (0.5 period). Students were given about two days outside of class to come up with their protocol. On lab day, the students carried out their protocol and recorded results (1 period). Outside of class students analyzed data, came up with conclusions, and wrote the letter.

2 ACME Analyzers Inc. Head Chemist Duluth Area Chemical Companies Duluth, MN Date 12/4/08 To whom it may concern: ACME Analyzers Inc. 156 Floor, Grindy Tower St. Paul, MN Due to the recent oil boom in North Dakota, our company, ACME Analyzers Inc., has been unable to meet some of our obligations to The State of Minnesota. The oil fields of ND are keeping our chemists busy and the environmental testing in MN is being left undone. As you know, the lake and stream health of MN is important to its citizens. We were hired by The State of MN to test its lakes and rivers for the phosphate ion. The quality of the water can be greatly affected by unnatural levels of phosphate. Laws were passed to limit the use of phosphates as cleaners, which were used extensively in detergents. Despite this, commercial cleaners and dishwashing detergents still contain phosphates. Phosphate is arguably the only nutrient keeping algae from covering an entire lake, killing the inhabitants, and filling it with organic matter. This process, called eutrophication, which normally takes tens of thousands of years, is accelerated by phosphates- one pound produces 700 pounds of algae. Eutrophication is not limited to small lakes. Lake Erie was said to have aged by 15,000 years in the last 50 from 20,000 pounds of phosphates being produced every day in the 1970 s. The fish were decimated- all species were gone near the shore where thick blankets of algae were consuming all of the oxygen. MN does not want this to happen to Lake Superior or any of its 10,000 lakes. We would like to offer your company a chance to complete these important measurements for us. We are willing to compensate you more than your normal fee. However, we have an reputation to uphold and do not want unqualified companies doing a poor job in our place. We are sending some phosphate samples to your lab. Please keep detailed records while analyzing them, and report back. In your letter, include 1. the method you used to analyze the samples, 2. the theory behind your chosen method, and 3. your results (report as many significant figures as you see fit). We look forward in hearing from you by Friday, December 12, Sincerely yours, E. Fudd & Y. Sam CEOs and CRHs of ACME Analyzers Inc.

3 ACME Analyzers Inc. Specifics for samples: The sample will be between M of PO4 3- and M of PO4 3-. To get the normally clear solution of phosphate to turn color, use the following reagents: 1. Add 1mL of the aqueous Ammonium Molybdate (NH4)6Mo7)24 solution to 25mL of your solutions of PO4 3- to be tested. 2. Add one drop of the Stannous Chloride in glycerin solution to the solution after the ammonium molybdate was added. 3. Wait around 5 minutes to for all of the phosphate to react before measuring the absorbance. The small volume change because of the addition of the above reagents will not affect the concentrations you measure by absorbance as long as you add the exact same amount to each solution you test. The colorful complex formed, molybdenum blue, is actually a quite complex one. If you are curious to learn more about it, inquire about it with someone from a chemistry department at a local university.

4 Phosphate Ion Testing Protocol- The concentration mentioned in the letter was not the actual concentration. The simple concentrations were used to focus the lesson on the process, not the molecules. The numbers work the exact same way. I have included the original instructions with permission from Dr. Martin Ondrus of the Univeristy of Wisconsin- Stout. His lesson includes testing local water supplies. The local waters I tested near the school did not have phosphate ion concentrations that were measurable (a good thing). Treated effluent from water treatment plants can be a source of community phosphate ions to test.

5 Analysis of Phosphate in Water General Discussion In the present experiment, we will analyze a series of natural 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 (builders). 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. 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 mg phosphorus per liter. Apparatus The Spectronic 20 spectrophotometer will be employed in the measurement of color intensity of the blue solutions. A wavelength of 650 nanometers will be used in these analyses. Reagents a. Ammonium molybdate reagent: (prepared by instructor) The solution is prepared by dissolving 25 g of (NH4)6Mo7O24 4H2O in 175 ml purified water. Cautiously, 280 ml of concentrated H2SO4 is added to 400 ml of purified water. The acid solution is cooled, the molybdate solution is added, and the entire mixture is diluted to 1 liter. b. Stannous chloride reagent: (prepared by instructor) 2.5 g of SnCl2 2H2O is dissolved in 100 ml of glycerine. The mixture is gently heated to hasten dissolution. a. Stock 20.0 mg/l phosphate solution: (prepared by instructor) To prepare this solution, g of KH2PO4 is dissolved in 1.0 liter of water. This is a 200 mg/l stock solution. Then dilute 100 ml of this solution to 1.0 liter.

6 Procedure Note: Glassware should be washed thoroughly with hot water followed by rinsing with purified water. Do not use phosphatecontaining detergents to clean equipment for this experiment. Prepare the following standard phosphate solutions: a. 1.0 mg/l standard: Place 2.00 ml of 20.0 mg/l phosphate solution in a 100-mL graduated cylinder and dilute to 40 ml with purified water. (Save exactly 25 ml in an erlenmeyer flask for analysis with the spectrophotometer.) b. 2.0 mg/l standard: Place 4.00 ml of 20.0 mg/l phosphate solution in a 100-mL graduated cylinder and dilute to 40 ml with purified water. (Save exactly 25 ml in an erlenmeyer flask for analysis with the spectrophotometer.) c. 3.0 mg/l standard: Repeat the directions for the 1.0 mg/l standard using 6.00 ml of 20.0 mg/l phosphate. (Save 25 ml) d. 4.0 mg/l standard: Repeat the directions for the 1.0 mg/l standard using 8.00 ml of 20.0 mg/l phosphate. (Save 25 ml) e. 5.0 mg/l standard: Repeat the directions for the 1.0 mg/l standard using ml of 20.0 mg/l phosphate. (Save 25 ml) f. Blank: Set aside 25 ml of purified 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 versus concentration as described by your instructor.

7 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. Place in an erlenmeyer flask 25 ml of the water sample to be analyzed. Put 1.00 ml (using a pipet) of ammonium molybdate solution into the flask and swirl to mix. To the flask add 2 drops of stannous chloride solution and mix by swirling. If phosphate is present, a blue color will develop to a maximum 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 nanometers wavelength, measure the absorbance (after 5-10 minutes color development) of the blue sample.* Waste Disposal All solutions may be rinsed down the drain with water. Calculations From the concentration and the absorbance of the five standards and the blank, make a plot of absorbance as a function of concentration (a "standard curve" or "Beer's Law plot"). Use the plot and the absorbance of each river, lake, sewage, or unknown solution to determine the concentration in milligrams phosphate ion per liter (mg/l PO4 3- ) in that sample. We sometimes use concentration units of parts per million (ppm) synonymously with mg/l because 1.0 liter of water at room temperature weighs 1000 grams (to two significant figures). Thus 1.0 mg is one millionth of the weight of one liter. For the sake of consistency in this manual we use units of mg/l and related units such as μg/ml and μg/l. *Should one of your samples produce a very dark blue color which can not 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 purified water to bring the volume up to 100 ml. Now 25 ml of this diluted sample may be analyzed according to directions for color development in sample, previously given. Remember that the concentration which you ultimately obtain from this sample will have to be multiplied by 100 because of the 100 fold dilution.

8 Report the following data: Sources of Water Samples Absorbance of Standards: Blank 1.0 mg/l PO mg/l PO mg/l PO mg/l PO mg/l PO4 3- Note: Prepare a standard curve and turn it in with the report sheet. Absorbance of Water Samples Dilution Factor (if any) Concentration of Phosphate in Water Samples Discussion Identify those aspects of the procedure which may have introduced some error into the final results. In each case indicate whether the error would cause the measured value to be too high, too low, or either.