Download Spectrophotometry Copper Sulfate

NAME _________________________________
PURPOSE:
SPECTROPHOTOMETRY
a) To generate a calibration (standard) curve of absorbance as a function of molar concentration
b) To determine the molar concentration of an unknown
Per the pre-lab readings we note that UV-VIS spectroscopy is an effective tool for solutions containing
transition metal complexes, due to the color of the transition metal ion, in water. From where does this color
originate?
We know now, that most transition metal ions do not exist as monatomic ions, when dissolved in water. Rather,
they exist in water as some sort of complex ion (the metal ion, surrounded by some sort of ligand … in the case
of being dissolved, only in water, the ligand is water). We also know that there is a coordination number associated
with the number of ligands. We ALSO know that many compounds possessing transition metal ions absorb
visible light when dissolved in water. For example, copper(II) sulfate appears as a blue solution in water.
Nickel(II) chloride produces a green solution in water. The reddish tint of the cobalt(II) chloride solution results
from the absorbance of light that is blue-green. Once the blue-green light has been removed from the white light,
the perception of color results from the light that was not absorbed. If we were to shine white light on a sample
of cobalt(II) chloride in water we would find that light is absorbed over a range of about 450-600 nm. This broad
range of wavelengths actually corresponds to several colors. But most of the absorbance occurs near 520 nm, in
the blue-green part of the spectrum.
Because different molecules absorb light in different wavelength ranges we can make determinations of what
type of molecule is in solution based on the wavelengths of light absorbed.
The absorption of light by transition metals can be described through d-orbitals. When compounds containing
metal ions like copper(II) or cobalt(II) are dissolved in water the water molecules hydrate the metal ions. The
energies of the d-orbitals in the metal ion are affected by the closest water waters around the metal ion. Normally,
all 3d orbitals possess the same energy (degeneracy). But in water this is not the case:
Cu atom
   

3dxy 3dxz 3dyz 3dx2-y2 3dz2
Cu2+(aq)
 
3dx2-y2 3dz2
Where  = 1 orbital
  
3dxy 3dxz 3dyz
In the copper(II) ion there is an energy difference between the 3d orbitals. If an electron occupies one of the
lower energy levels and a photon possessing energy that matches the difference in energy between the lower
and upper levels then absorption can take place. (citation) Department of Physical Sciences Kingsborough Community College The City University
of New York Winter 2010
Today’s work is pretty straight forward. You will be provided stock solutions and λmax for Cu2+(aq). This is just
an introductory exercise. However, introductory should not put you off from bringing your best game. Lab
hygiene, pipetting skills, and measurement all matter….
We need to work in larger-than-usual teams, due to a availability of Spec 20 machines.
Consider how you might divide up the workload/tasks
PART 1
1. Be sure your goggles are on throughout the time period.
2. Obtain 1 test tube and approximately, 15.0 mL of 0.50 M CuSO4(aq). This is known as your stock
solution.
3. Obtain 4 more clean test tubes
4. Obtain a small beaker of distilled water.
5. Using a pipet, measure and dispense 5.00 mL of distilled water, into each of the 4 test tubes.
6. Now, from your stock solution, and using a pipet … begin your serial dilution process: Extract a
5.00 mL aliquot of 0.50 M CuSO4 (aq) from your stock solution and dispense the aliquot into the first of
your 4 test tubes.
7. Mix the contents of the first test tube very well. This is known as your first dilution.
8. Rinse the pipet very well with distilled water.
9. From your first dilution, extract a 5.00 mL aliquot and dispense the aliquot into the second of your 4 test
tubes.
10. Mix the contents of the second test tube very well. This is known as your second dilution. With a wax
pen, label the test tube.
11. Rinse the pipet very well with distilled water.
12. Using your second dilution, extract a 5.00 mL aliquot and dispense the aliquot into your third test tube.
13. Mix the contents of the third test tube very well. This is known as your third dilution. With a wax pen,
label the test tube.
14. Rinse the pipet very well with distilled water.
15. Using the solution of this third dilution extract a 5.00 mL aliquot and dispense it into your fourth test
tube.
16. Mix the contents well. With a wax pen, label the test tube.
17. Rinse the pipet very well with distilled water.
18. At this point you should now have 5 test tubes … The first is the remaining 10.0 mL of your stock
solution of 0.50 M CuSO4(aq). You have 4 other test tubes, each holding a serial dilution of the stock
solution. Each of these test tubes has 10.0 mL of solution.
Take a minute out and using the dilution formula: M1V1 = M2V2, complete the following table of the serial
dilution:
Table of Serial Dilutions of Copper(II) Sulfate
Test Tube
Volume of Distilled
Water (mL)
Volume of CuSO4 (mL)
Total Volume (mL)
Final Molar
Concentration (M)
Stock
0
10.00
10.00
0.50
1
5.00
2
3
5
PART 2: When working with cuvettes, do not touch the cuvette with your fingertips, save for the upper
lip. Just prior to inserting a cuvette into the cradle or sample holder of a spectrophotometer, wipe the
sides of the cuvette with a dry wipe.
1. Assuming the Spec 20 has been turned on and warmed for at least 20 minutes … you can set the
absorbance to 620 nm. This is the lambda max (λmax) … that wavelength at which Cu2+ will best absorb
light.
2. Before measuring a sample, the spectrophotometer must first measure the absorbance of a blank
solution. A blank solution is one that contains all of the components that the sample possesses, with the
exception of the analyte. For this experiment, the analyte is the metal hydrate complex. So distilled
water will serve as the blank. Using a transfer pipet, place distilled or deionized water into a cuvette,
dropwise until it is about 2/3 full. Place the cuvette with the frosted circle facing front, into the sample
holder of the spectrophotometer.
3. Close the lid of the sample holder and ensure that the wavelength is set to 620 nm.
4. Set the absorbance of the spectrophotometer to zero. Remove the cuvette of distilled water.
5. Transfer a sufficient volume of your stock solution to a clean cuvette, so that it is about 2/3 full. Use a
NEW transfer pipet.
6. Open the lid of the sample holder and place the cuvette with the stock solution into the sample holder.
Ensure the frosted circle is facing you. Close the lid and record the absorbance value of the stock
solution at λmax.
7. Remove the cuvette of stock solution, and repeat steps 5-6, using each of the dilutions you made. Be
sure to handle the cuvette appropriately and be sure the lid of the sample holder is always closed, except
when removing a cuvette. Close it after removal….
8. Obtain a sample of copper(II) sulfate, of unknown molarity and use this sample, repeating steps 5 & 6.
Absorbance of Blank, Stock, & Dilution Solutions and Unknown
Test Tube
Molar Concentration
(M)
Blank
0.00
Stock
0.50
Absorbance
1
2
3
4
Unknown
PART 3:
1. Using an appropriate program, create a graph of Absorbance as a function of Molarity for the stock
solution and first four test tubes. Insert a best fit line. This is your calibration (standard) curve)
2. Using the curve, determine the molar concentration of your unknown. Record that value on the curve.
3. Seek out He-Who-Is-Wisest-And-Most-Handsome-Among-All-Men a.k.a. LGMU and ask what the
molarity of the unknown was …Calculate your percent error. Record both values on your graph.