Determination of the Concentration of a Cobalt Solution
Introduction:
Every colored solution absorbs various amounts of light at different wavelengths of the
spectrum. Before doing a spectral analysis, the optimum wavelength must be determined.
This wavelength corresponds to the wavelength of light that is absorbed the most (or
transmitted the least) by the sample.
The first part of this experiment is to detail the procedure to be used to find the
wavelength of maximum absorbance for a cobalt solution. This process involves
recording the absorbance or percent transmittance (%T) over the range of 400 nm to 650
nm, usually in intervals of 10 to 15 nm. The data can be graphed either manually or by a
computer program to visualize the highest absorbance or lowest %T. The wavelength of
maximum absorbance is typically used to analyze the standards and unknowns in Part 2
because the instrument will be more sensitive to changes in absorbance at that
wavelength.
The second part of this experiment involves creating a standard curve, which will be used
to determine the concentration of the unknown solution. To generate the standard curve,
absorbances will be obtained for five standard solutions. A graph of this data should
yield a straight line. The absorbance for the unknown and the graph will be used to
determine the concentration of the unknown solution.
Purpose:
The purpose of this experiment is to determine the concentration of an aqueous cobalt
solution.
Safety:

Always wear goggles in the lab.
Materials:
standard cobalt solutions (0.1 – 0.5M)
unknown cobalt solutions
Kimwipes
Spectrophotometer 20
deionized water blanks
small test tube rack
Procedure – Part I: Determining the Wavelength of Maximum Absorbance.
1. Turn on the spec by twisting the front left knob clockwise. Allow the spec to warm
up for 15 minutes.
2.
Set the mode on the spec to transmittance. With the sample compartment empty, set
the transmittance to 0% by turning the left front knob. Note: Do not turn this knob
again during the experiment. If you do accidentally turn this knob again, notify
your instructor.
3.
Set the mode to absorbance. Set the spectrophotometer to 400 nm using the large
knob on top.
4.
Wipe the cuvette containing the water blank with a Kimwipe to remove dirt and
fingerprints.
5.
Insert the water blank into the sample compartment so that the white line on the
cuvette matches up with the notch in the sample compartment. Set the absorbance to
0.000 using the front right knob.
6.
Wipe and properly insert the cuvette containing the 0.1M cobalt solution into the
sample compartment. Record the absorbance of the cobalt solution on the data sheet
provided.
7.
Reset the wavelength to 410 nm. Zero the instrument with the blank as in step 5.
Next, record the absorbance of the cobalt solution at that wavelength as in step 6.
Note: If you refrain from fingering the cuvettes except at the very top, it is not
necessary to wipe the cuvettes each time. Be sure to use the 0.1M Cobalt solution
for all of Part I.
8.
Repeat step 7 at 10 nm intervals up to 650 nm. Be sure to zero the blank for each
new wavelength, and record the absorbance on the data sheet provided.
9.
After all the readings are recorded, graph the data with wavelength on the x-axis and
absorbance on the y-axis.
10. When plotted, the highest peak is the wavelength of maximum absorbance. This is
the wavelength that should be used in Part 2.
Data – Part I:
Wavelength
400 nm
410 nm
420 nm
430 nm
440 nm
450 nm
460 nm
470 nm
480 nm
490 nm
500 nm
510 nm
520 nm
Absorbance
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
Wavelength
530 nm
540 nm
550 nm
560 nm
570 nm
580 nm
590 nm
600 nm
610 nm
620 nm
630 nm
640 nm
650 nm
Maximum absorbance of the cobalt solution =
Absorbance
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________ nm
Procedure – Part II: Determination of the Concentration of a Cobalt Solution.
1. Set the wavelength to the wavelength of maximum absorbance.
2.
Insert the water blank and zero the absorbance.
3.
Insert the 0.5M cobalt solution and record the absorbance. Note: If the absorbance
is off scale, repeat steps 2 and 3 with a wavelength 10 nm less than the maximum.
Check several wavelengths at 10 nm intervals if necessary to bring the absorbance on
scale.
4.
Record the absorbance of the other standards and the unknowns at this same
wavelength. (It is not necessary to zero the absorbance if the wavelength is not
changed.)
5.
Construct a graph using concentration as the x-axis and absorbance as the y-axis.
6.
Use this graph to determine the concentration of the unknown solution.
Data – Part II:
Wavelength used: ________ nm
Concentration
0.5M
0.4M
0.3M
0.2M
0.1M
Unknown ____
Unknown ____
Absorbance
Calculations:
1. Determine the concentrations of the unknown solutions.
2. Determine the percent error of your results.
Questions:
1. What is the general relationship between absorbance and %T? Explain.
2.
Why is it necessary to wipe the cuvettes?
3.
Why is the wavelength of maximum absorbance used to analyze chemicals?
4.
When might the wavelength of maximum absorbance not be used to analyze a
chemical?