A COMPLEX IRON SALT & BEER’S LAW

Iron Salt
Revised 10/21/14
A COMPLEX IRON SALT & BEER’S LAW
LABORATORY NOTEBOOK
Objectives, Chemical & Equipment Tables, and Procedures & Observations should all be entered
into your ELN. 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.
INTRODUCTION
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
N
central metal ion. The ligands act as Lewis bases, donating at
least one pair of electrons to M+ to form a coordinate covalent
N
bond. (Ligands, therefore, coordinate or bind to M+.) Unidentate
ligands (e.g., Cl-, H2O, NH3, -OH, -C≡N, etc.) create a bond with
C
C
N
Fe3+
C
C
C
N
C
the transition metal cation by donating one electron pair, as shown
3-
N
N
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. Chelators such as oxalate (ox, C2O42-),
ethylenediamine (en, H2NCH2CH2NH2), and ethylenediaminetetraacetate (EDTA) are used
extensively in environmental tests to detect trace amounts of metal cations
NH 2
H 2N
C o 2+
H 2N
NH 2
H2
N
because their reaction with many transition metals results in a dramatic color
N
H2
binding sites on the transition metal are occupied, as shown on the right.
change. Typically a sample is treated with an excess of the ligand so that all
The complex ion is coupled with
N
counter ions to create a neutral ionic
compound called a coordination
compound. Often coordination
N
K+
C
C
Fe3+
K+
C
C
C
N
K+
C
N
K 3Fe(CN)6
2
NH 2
N
H 2N
N
Co 2+
H 2N
Cl-
NH 2
Co(en) 2Cl2
H2
ClN
N
H2
Iron Salt
Revised 10/21/14
compounds isolated in the solid form are found to contain waters of hydration (e.g., Co(en)2Cl2 i 4
H2O).
In this experiment the complex ion that you will be working with is Fe(ox)33-. The oxalate ion
(ox, C2O42–), 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)33- 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 =εCl
A = absorbance (no units)
ε = molar absorptivity coefficient (units = L/mol-cm)
C = 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, (NH4)3Fe(ox)3i3H2O,
will be created with volumetric glassware and their absorbance will be measured. A Beer’s Law
Plot will be created with these absorbance, concentration data pairs. An unknown Fe(ox)33containing 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)33-. The formula weight
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Iron Salt
Revised 10/21/14
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.
SAFETY PRECAUTIONS
Safety goggles and aprons must be worn at all times. Fe(ox)33- 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.
PROCEDURES
Part A: Prep of Standard Fe(ox)33- 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 (NH4)3Fe(ox)3i3H2O in DI H2O using the volumetric
glassware available in lab. Describe your procedure & show the calculations. Note: DI H2O is
deionized water. A few special faucets in the lab will provide this water.
Part B: Calibrate & 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. Calibrate 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
minute.) Note: For best results, allow the spectrometer to warm up for at least three
minutes.
3. Create 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. Click Finish
Calibration and then OK.
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Iron Salt
Revised 10/21/14
Part C: Absorption Spectrum (Finding λmax)
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
. Click
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. Click
store latest run. What color of light is being absorbed by the sample solution?
and
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 Collection. Change Mode: Events w/ Entry,
Name: Concentration, Units: molarity. Click 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)33-. Repeat with all remaining solutions.
3. Click Stop once the absorbance values for all the standard solutions have been collected.
Transfer the data to your ELN, 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 λmax.
When finished, pour all Fe3+ containing solutions into the designated waste containers in the
hood.
Make sure to clear your email address and password of the LabQuest2 so others can’t access your
email account. Shutdown the LabQuest2 and not simply put it to sleep. To shutdown the
LabQuest2: press the home key, select System à Shut Down à OK.
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Iron Salt
Revised 10/21/14
Part F. Gravimetric Analysis
1. Using a grease pen or labeling tape, put your name on a 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). Carefully 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, return the crystals to your TA.
Post Lab Results & Discussion
Answer each of the following questions on a Postlab page in your ELN. Create a new heading for
each question, so your TA can easily navigate through your answers.
1. Create 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)33- 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)33- ion present in the 50
mL of solution created in Part E and the mass % of Fe(ox)33- ion in the unknown.
6. Using the data from the gravimetric analysis, determine the mass % of water in the unknown.
7. Challenge Question: The only other component to the unknown coordination compound is the
counter cation that balances the 3- charge of the Fe(ox)33- 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?
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