A COMPLEX IRON SALT & BEER S LAW

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1 A OMPLEX IRO SALT & BEER S LAW LABORATORY OTEBOOK Objectives, hemical & Equipment Tables, and Procedures & Observations should all be entered into your EL. All spectra files should be attached in the Procedures & Observations section. A few pictures should also be attached that clearly show the solutions and equipment or instrumentation used. ITRODUTIO Transition metal cations, M +, react with charged or neutral ligands, L, to form complex ions. Many transition metal cations form octahedral complex ions with up to 6 ligands surrounding a central metal ion. The ligands act as Lewis bases, donating at least one pair of electrons to M + to form a coordinate covalent bond. (Ligands, therefore, coordinate or bind to M +.) Unidentate ligands (e.g., l -, O, H 3, - OH, -, etc.) create a bond with the transition metal cation by donating one electron pair, as shown to the right. Polydentate ligands donate 2 or more electron pairs creating two or more bonds with a transition metal center. These ligands are also known as chelates (from the Greek word for claw ) because a polydentate ligand clamps hold of the transition metal cation on at least two sides. Frequently, the binding of a chelating ligand to a transition metal cation creates a colored complex ion. helators such as oxalate (ox, 2 O 4 2- ), ethylenediamine (en, ), and ethylenediaminetetraacetate (EDTA) are used o 2+ The complex ion is coupled with counter ions to create a neutral ionic compound called a coordination compound. Often coordination extensively in environmental tests to detect trace amounts of metal cations because their reaction with many transition metals results in a dramatic color change. Typically a sample is treated with an excess of the ligand so that all binding sites on the transition metal are occupied, as shown on the right. K + K + K + 2 Fe 3+ K 3 Fe() 6 l - Fe 3+ o 2+ o(en) 2 l 2 l - 3-

2 compounds isolated in the solid form are found to contain waters of hydration (e.g., o(en) 2 l 2 i 4 O). In this experiment the complex ion that you will be working with is Fe(ox) 3-3. The oxalate ion (ox, 2 O 2 4 ), acts as a chelating bidentate ligand, binding to the iron (III) in a 3:1 ratio (3 oxalate ligands for each metal center). This complex ion creates a bright green solution when dissolved in water. The color observed is a result of the absorption of radiation from this visible part of the electromagnetic spectrum. (Typically, the color of light absorbed by a chemical complex is complementary to the color observed.) This absorption is a result of the excitation of valence electrons to higher energy electron orbitals of the Fe(ox) 3-3 complex. Therefore, the intensity of light at the absorbed wavelengths is reduced passing through solution; the amount of reduction is dependent on the concentration of the absorbing species and the distance the light travels through the solution (path length). This linear dependence is known as the Beer-Lambert Law (or Beer's Law): (1) A = ε l A = absorbance (no units) ε = molar absorptivity coefficient (units = L/mol-cm) = concentration of absorbing species (units = mol/l) l = path length (units = cm) Typically, the optical path length and molar absorptivity coefficient are held constant in an experiment, so the absorbance varies with concentration alone. A plot of absorbance vs. concentration is known as a Beer's Law Plot. In this experiment five standard solutions of the coordination compound, (H 4 ) 3 Fe(ox) 3 i3 O, will be created with volumetric glassware and their absorbance will be measured. A Beer s Law 3- Plot will be created with these absorbance, concentration data pairs. An unknown Fe(ox) 3 containing salt (a coordination compound with a different counterion) will then be massed and dissolved in a known volume of water. The absorbance of the unknown solution will be compared to the Beer s Law Plot to determine the concentration of Fe(ox) 3-3. The formula weight 3

3 can be determined from this data. Gravimetric analysis will also be performed to determine the mass percent of water and combined with the preceding data to determine the identity of the compound s counter ion. Before starting the experiment, the TA will asks you to do a quick demonstration or talk-through one of the following: 1) How to properly handle a cuvette (how to wipe it down, what to wipe it down with, how to hold it). 2) Before filling the cuvettes with a new solution, you need to prerinse it. What do you prerinse it with? SAFETY PREAUTIOS Safety goggles and aprons must be worn at all times. Fe(ox) 3-3 containing compounds should be handled with gloves. Avoid inhalation and skin or eye contact. Wash affected areas thoroughly with cold water. When using a pipet, always use a pipet bulb to provide suction, never pipet by mouth. PROEDURES Part A: Prep of Standard Fe(ox) 3-3 Solutions Work in pairs. Design a plan to create 5 standard solutions with concentrations between 1.0 x 10-3 M and 7.5 x 10-3 M by dissolving solid (H 4 ) 3 Fe(ox) 3 i3 O in DI O using the volumetric glassware available in lab. Describe your procedure & show the calculations. ote: DI O is deionized water. A few special faucets in the lab will provide this water. Part B: alibrate & Blank the Visible Spectrometer 1. Obtain a visible spectrometer from the stockroom. Use the USB cable to connect the visible spectrometer to the LabQuest2. 2. alibrate the spectrometer by clicking. The calibration dialog box will display the message: Waiting.seconds for lamp to warm up. (The minimum warm up time is one 4

4 minute.) ote: For best results, allow the spectrometer to warm up for at least three minutes. 3. reate a blank. What is a blank? What should be used as the blank? Wipe the outside of the blank with a kimwipe and insert the cuvette in the sample compartment. lick Finish alibration and then OK. 5

5 Part : Absorption Spectrum (Finding λ max ) 1. Measure the path length of the cuvette. What is this value? What does it represent? 2. Fill 5 labeled, prerinsed (with what?) cuvettes with the 5 different solutions created. 3. Wipe the outside of the cuvette (Why?) containing the solution of highest concentration, place in the cuvette holder, and click. lick once the data collection is complete. 4. Examine the graph and note the wavelength region of maximum absorbance. Remove the rainbow background spectrum by double clicking the rainbow background. lick and store latest run. What color of light is being absorbed by the sample solution? How is that color related to the color of the solution? Part D: Beer s Law Plot 1. Go to the screen, click on Sensors > Data ollection. hange Mode: Events w/ Entry, ame: oncentration, Units: molarity. lick OK. 2. Wipe the outside of the cuvette containing solution #1 with a kimwipe, place in the cuvette holder of the Spectrometer, click then click Keep. Type in your calculated value for the concentration of Fe(ox) 3-3. Repeat with all remaining solutions. 3. lick Stop once the absorbance values for all the standard solutions have been collected. Transfer the data to your EL, title the plot and label the axes. Part E: Absorbance Spectra of Unknown Work alone. 1. Weigh approximately ~0.125 g of the unknown and use it to create a 50 ml aqueous solution. Record the exact mass used. 2. Rinse a clean, empty cuvette with a few drops of the unknown solution and then fill the cuvette about 2/3 full with the solution. Record the absorbance for your unknown at λ max. When finished, pour all Fe 3+ containing solutions into the designated waste containers in the hood. Make sure to clear your address and password of the LabQuest2 so others can t access your account. Shutdown the LabQuest2 and not simply put it to sleep. To shutdown the LabQuest2: press the home key, select System! Shut Down! OK. 6

6 Part F. Gravimetric Analysis 1. Using a sharpie, put your name on a clean dry scintillation vial. Measure and record its mass. 2. Obtain ~0.100 g of unknown. Record the exact mass and appearance of the crystals. Place the container with crystals in the oven for at least one hour (make sure the oven is on i.e. it feels warm). arefully remove from the oven and place in a desiccator until cooled to room temperature and then record the mass. If the mass continues to decrease, place the container with crystals back in the desiccator. When finished throw the crystals in the designated waste container in the hood. Rinse out the vial with water and wipe off the sharpie marking with a small amount of acetone on a kimwipe. Post Lab Results & Discussion Answer each of the following questions on a Postlab page in your EL. reate a new heading for each question, so your TA can easily navigate through your answers. 1. reate a Beer s Law Plot with the data collected in Part D. 2. What is the value of the y-intercept on your Beer s Law plot? Provide possible explanations why the number is not zero. (Take into account sign.) 3. Using data from your Beer s Law Plot, what is the mean value of the molar extinction coefficient (also called molar absorptivity) for the Fe(ox) 3-3 ion? Find the mean value and standard deviation for ε. 4. Using the Beer s Law plot, find the concentration of in your unknown cuvette graphically (revise the plot created in #1) and mathematically (show all calculations). 5. Using the concentration calculated in #4 calculate the moles of Fe(ox) 3-3 ion present in the 50 ml of solution created in Part E and the mass % of Fe(ox) 3-3 ion in the unknown. 6. Using the data from the gravimetric analysis, determine the mass % of water in the unknown. 7. hallenge Question: The only other component to the unknown coordination compound is the counter cation that balances the 3- charge of the Fe(ox) 3-3 ion. With the information above, determine the counter cation s identity. (Hint: Find the atomic mass.) 8. Did any gross errors occur? Did you mess up? Did the equipment or instrumentation fail? If so, what was the effect on your results? 7

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