Spectrophotometric Analysis of Iron Experiment Lab Report

Spectrophotometric Analysis of Iron(II) Ion:Instructions
Introduction
No great discovery was ever made without a bold guess. —Isaac Newton
Iron exists in the environment largely in the form of Fe2+ and Fe3+ ions. Fe(OH)3 is only soluble
in very acidic water (pH < 3); normal dissociation of water into H+ and OH– results in the precipitation of any Fe3+ ions as Fe(OH)3. Fe(OH)2 is much more soluble, so it does not precipitate from water at the pH normally found in the environment or in water supplies. Excessive iron(II) ion in water gives the water and anything cooked in it strange tastes. In addition, such water leaves stains on things washed in it. In today’s experiment you will analyze water samples for traces of iron(II) ion. Because the iron level in the local water supply is, fortunately, quite low, you will analyze an artificial sample. Figure 1: 1,10phenanthroline Iron(II) ion has no discernible color. To develop a color to measure, you will introduce an excess of 1,10-phenanthroline (“o-phen” for orthophenanthroline). Three 1,10-phenanthroline molecules react with the iron(II) ion to give a highly colored red complex ion. The iron(II) ion is bound to the six nitrogens on the three o-phen molecules by covalent bonds in which both electrons are provided by the nitrogen. Because the reaction goes to completion and the iron(II) is limiting, the concentration of the colored complex is a measure of the amount of iron(II) originally present. You will add a solution of hydroxylamine as an antioxidant to prevent the air from oxidizing the iron(II) ion to iron(III). Apparatus and Supplies top loading balance MicroLAB measurement system side-loading balance 10 mL syringes (3) 6 spectrometer vials vial block graduated cylinder 25 mL spatula Beral pipets volumetric flasks, loo and 250 mL small funnel 0.1% 1,10 -phenanthroline (o-phen) solution 0.2% hydroxylamine solution beaker 50 mL (2) beaker 150 mL Parafilm® ferrous ammonium sulfate [official name: ammonium iron(II) sulfate hexahydrate] Procedure Wear Eye Protection Wear Gloves Work in pairs, but each student will analyze his or her own unknown. Do not transfer any chemicals in the weighing room, especially not in the side-loading balances. A. Standard Solutions Stock Solution Weigh a clean, dry 50 mL beaker on a side-loading balance in the weighing room. Record all digits of the mass. Put the beaker on a top-loading balance and add between 0.18 and 0.20 g of your standard, ferrous ammonium sulfate: Fe(NH4)2(SO4)2⋅6H2O. Weigh the beaker again on the same side-loader and record all digits of the exact mass. Quantitatively transfer the standard to a well-rinsed, but not necessarily dry 250 mL volumetric flask according to the following procedure. Add about 15 mL of water to the beaker, swirl carefully and pour the solution into the volumetric flask without losing any. Add more water to the beaker and pour into the volumetric flask. Repeat until there is no visible trace of solid in the beaker, and then twice more. Swirl the flask to dissolve the standard. Add 10 mL of 0.2% hydroxylamine (NH2OH) to prevent oxidation. Add deionized water to the flask until the level is exactly at the mark. You can use a dropper for the last bit; if you overshoot you will have to repeat the weighing and transfer. Cover the flask with a piece of Parafilm, and mix it thoroughly by inverting it 30 times, allowing the bubble to travel from top to bottom and back each time. Secondary Solution Rinse a 10 mL syringe three times with the stock solution of ferrous ammonium sulfate, discarding the rinsings. Accurately shoot 5.00 mL of stock solution into a well-rinsed (not necessarily dry) 100 mL volumetric flask. Add about 10 mL of 0.2% hydroxylamine solution. Fill exactly to the line with deionized water, cover and mix by inverting 30 times. Calculate the concentration of this solution from C1V1 = C2V2. Spectrophotometric Standards Line up five clean, dry spectrometer vials free of marks, tape, or labels in a vial block. Rinse one 10 mL syringe three times with deionized water and another three times with secondary solution. Don't mix them up. Use syringes to shoot carefully measured amounts of secondary solution and water, according to the table below. Figure 2 Figure 3: vial block Calculate the concentrations of these solutions from C1V1 = C2V2. Get about 15 mL of 0.1% ophen solution in a small beaker. Use a syringe to add 1.00 mL of 0.1% o-phen solution to each of the spectrometer vials; cover each and shake well. Unknown solution Get one or more unknown solutions, as assigned, and record their identifications. Each student will do his or her own unknown. The instructor may ask you to use tap water as one of the unknowns. Thoroughly rinse a syringewith water, then rinse it 3 times with an unknown solution. Shoot 10 mL of the unknown solution into a clean dry spectrometer vial. Shoot in 1.00 mL of ophen solution with syringe; cover and shake well. Concentration Calculations Stock solution Calculate the number of moles of iron(II) ion in the stock solution from the mass of ferrous ammonium sulfate used. Each mole of ferrous ammonium sulfate provides one mole of iron(II) ion. Calculate the concentration from c = n/V (V must be in liters). Secondary solution Calculate the concentration of iron(II) in the secondary solution from C1V1 = C2V2. C1 is the concentration of iron(II) in the stock solution, V1 is the volume of stock solution that you used to prepare the secondary solution, V2 is the volume of the secondary solution, and C2 is the concentration of iron(II) in the secondary solution. Spectrophotometric standards Calculate the concentration of iron(II) in each of the spectrophotometric standards based on the volumes in the table and the concentration of iron(II) in the secondary solution. Note that V2 is the sum of the volumes of the secondary solution and of water used to prepare the standard. Spectrophotometer Measurements Make sure the power strip is on. Turn on your MicroLAB unit. Run the MicroLAB program and select Beer's Concentration. Fill a clean spectrometer vial with deionized water, wipe the outside, and put it into the well. Cover the well with a cap and click the 1. Read Blank button. Select Names to Print and type your names. Select Absorbance from the options at the top of the screen. Go to tab 2. Read Knowns. Put one of the vials of spectrophotometric standard in the well, cover and click Add. Enter the sample identity and concentration (number only, no units). Use 'E' notation for small numbers (1.54 × 10-5 = 1.54e−5). Do the same for each of the five spectrophotometric standards (not the unknown). Be sure to place the vial in the well and cover it before you click the Add button. After you run all five standards, go to tab 3, Curve Fit. Select First Order (Linear). Select the color bar with the highest absorbance on the spectrum window. If the Beer's law (absorbance vs. concentration) line is not a straight with minimal scatter, try another wavelength. The data points should form a good straight line. If they do, go to tab 4. Read Unknowns. Put the vial of unknown in the well, cover and click Add. Enter the identification. The program will put a point for the unknown on the graph according to its absorbance. It will also record the concentration in the spreadsheet view. If you have a second sample to run, do so before ending the experiment. You can move the column boundaries in the spreadsheet to get the concentrations on single lines. Save the experiment file to your section's folder on the T drive. Print out the items indicated under "Report" below. Clean up All solutions, including standards and unknowns must be put into the marked waste container. Rinse all glassware with deionized water and return it. Rinse the yringes with deionized water, especially the tips. Clean up your lab station and the balance area. Report Calculate the iron(II) concentration in mg/L: multiply the molar concentration by the molar mass of iron. This gives the concentration in grams per liter. Multiply the grams per liter by 1000 to get mg/L. Print out the spectrum for one of the samples, the Beer’s law plot, and the spreadsheet from MicroLAB. Attach the print-out to your report. Be sure to put your name and lab station on your report. Do not fail to report your unknown identification code, otherwise you will have an unknown unknown. Each student must turn in her/his own report. Figure 4 Images All figures courtesy of authors. Title: Spectrophotometric Analysis of Iron(II) Ion 1) What is the scientific purpose of this experiment? 2) What is the underlying scientific theory that will allow you to achieve the purpose of this experiment? 3) Show the procedure that you intend to follow to complete this experiment. 4) Write the chemical profile for 1,10-phenanthroline. Include hazard information. 5) Write the chemical profile for hydroxylamine. Include hazard information. 6) Write the chemical profile for Iron(II) ammonium sulfate. Include hazard information.