Introduction:
In this experiment, the Beer-Lambert Law, A = abc , where A is the absorbance, a is the molar absorptivity, b is the path length, and c is the concentration, will be applied to a series of aqueous solutions of cobalt (II) chloride (CoCl
2
). The pink color of the solutions is due to the presence of Co
2+
ions, not Cl
-
ions. The wavelength corresponding to the maximum absorbance in the absorption spectrum of Co 2+ will be identified. Then, in order to achieve maximum sensitivity in measurements, that wavelength will be utilized in determining the absorbances of a series of solutions of know concentration (i.e. standard solutions). When the measured absorbances are plotted against concentration, a linear relationship will be evident. The slope of the line will correspond to ab . Since the path length b , can be measured, the value of a can then be calculated. As long as the wavelength is not changed, a will be a constant for any solution of
CoCl
2
. Thus, the concentration of Co
2+ in an unknown solution can be calculated from the measured absorbance of the unknown solution.
Purpose: To determine the wavelength of maximum absorbance of a cobalt (II) chloride solution, to validate the Beer-Lambert Law, and to use the Beer-Lambert Law to determine the concentration of a solution with unknown concentration.
Equipment/Materials:
0.10 M solution of CoCl
2
6 cuvettes stirring rod
Spectrophotometer
CoCl
2
solution of unknown concentration pipet and bulb ruler deionized water
Safety:

Goggles should be worn at all times in the laboratory.

Cobalt (II) chloride is toxic by ingestion.

Gloves should be worn if open wounds are present on the experimenter’s hands.

Use a safety bulb for pipetting. Never pipet by mouth.
Procedure: Part I – Determination of the Wavelength of Maximum Absorbance
1.
Obtain approximately 30 mL of 0.100 M CoCl
2.
2.
Using a pipet and safety bulb, transfer 5.0 mL of the solution to a clean cuvette.
3.
Prepare a “blank” by placing 5.0 mL of deionized water in another cuvette.
4.
Using the up/down arrow keys on the spectrophotometer, adjust the wavelength to
400 nm .
5.
Make sure the instrument is in Absorbance mode by using the left/right arrow keys.
6.
Place the “blank” cuvette in the sample compartment and close the lid.
7.
Set the absorbance to zero by pressing the Calibrate key.

8.
Remove the blank and replace it with the cuvette containing the CoCl
2.
Close the lid.
Record the absorbance on the Data Table.
9.
Change the wavelength to 425 nm.
10.
Repeat Steps 6-8.
11.
Continue to change the wavelength by 25 nm increments and repeat steps 6-8 until you reach 600 nm. Remember that each time you change the wavelength, you must calibrate the absorbance to zero using the “blank” (press the CAL key).
12.
Graph the absorbance (on the “y” axis) vs. wavelength (on the “x” axis). Draw a smooth curve to fit the experimental points. Identify the wavelength with the maximum absorbance to the nearest multiple of 25 nm and record in the Data Table.
Part II: Validating the Beer-Lambert Law
13.
Prepare a series of CoCl
2
standard solutions according to the following table. Using a pipet, add the indicated volume of CoCl
2
and the indicated volume of deionized water.
Mix the contents of each cuvette using a clean, dry stirring rod. NOTE: Tube 1 was already prepared in Step 3, Part I.
Tube
1
Tube
2
4.0
Tube
3
3.0
Tube
4
2.0
Tube
5
1.0 Volume of 0.100 M CoCl
2
sol. (mL) 5.0
Volume of water (mL) 0 1.0 2.0 3.0 4.0
14.
Measure and record the absorbance of each of the series of solutions at the wavelength of maximum absorbance (determined in Part I).
15.
Calculate the concentration of each of the solutions based on the dilution factor.
16.
Graph absorbance vs. [Co
2+
]. Use a ruler to draw the best straight line through the origin and as close as possible to all of the experimental points.
17.
Calculate the slope of this line.
Part III: Determining the concentration of the Unknown
18.
Obtain about 5 mL of a CoCl
2
solution of unknown concentration. (If there is more than one available, be sure to note which one you got.) Place the solution in a cuvette and place it in the sample compartment of the spectrophotometer. Record its absorbance and the unknown number in the Data Table.
19.
Calculate the concentration using Beer’s Law and the data you gathered. (An alternative method for finding the unknown’s concentration is to read it from the graph that you constructed. Find the absorbance value for the unknown on the “y” axis of your graph.
Follow this point across until you intersect the line (calibration curve), then go straight down to the “x” axis to find the concentration.)

Date: Name:
Part I:
Period/Group:
3
4
5
Wavelength
400 nm
425 nm
Absorbance
450 nm
475 nm
500 nm
525 nm
550 nm
575 nm
600 nm
Use the data above to construct a graph of Wavelength vs. Absorbance.
The maximum absorbance (to the nearest multiple of 25 nm) occurs at
Part II: Data Table – Validating the Beer-Lambert Law
Absorbance as a function of [Co
2+
] measured at
Tube No. Absorbance
nm.
[Co 2+ ]
1
2
Data Table – Determination of Wavelength of Maximum Absorbance
nm.
Use the data above to construct a graph of Absorbance vs. Concentration of CoCl
2.
Draw the best straight line that will go through the origin and as close as possible to all other data points.

Part III: Determining the Concentration of an Unknown
Unknown # Absorbance
Calculations:
1.
Attach the graph from Part I.
2.
Attach the graph from Part II.
3.
Using Beer’s Law, calculate the concentration of the unknown in the following way.
Since b (pathlength) is a constant when using the spectrophotometer, and since a (molar absorptivity) is a constant for a given compound (such as CoCl
2
), we can set up a ratio of
A (absorbance) to c (concentration) for the standard (with known concentration) and the unknown in order to find the unknown concentration.
A std
= A unk
c std c unk or c unk
= A unk c std
A std
4.
Find the concentration of the unknown(s) by reading from the calibration curve as explained in number 21 of the Procedure.
Questions:
1.
Calculate the concentration of chloride ions (Cl
-
) in the unknown sample(s).
2.
In the graph for Part 2, why must the straight line pass through the origin?
3.
What experimental problems would confront you if you attempted to determine the concentration of the metal in solution in each of the following reagents? a.
Cr
3+
has two maximum absorbance peaks, one at 407 nm and one at 574 nm b.
Mn
2+
has a maximum absorbance at 530 nm, but the molar absorptivity value is very low