Absorption Spectrum and Colorimetry of Co2+ in Water
Objectives: To obtain the absorption spectrum of Co2+ in water using a Spectronic 20
spectrophotometer, determine the wavelength and photon energy of maximum absorption, and
determine the concentration of an unknown Co2+ solution using colorimetric methods.
Introduction: Light in the visible and ultraviolet region of the spectrum contains photons of sufficient
energy to promote electrons to higher energy orbitals or to remove them from an atom, molecule , or
ion completely. Thus the study of light absorption or emission in the visible and ultraviolet region of
the spectrum is often referred to as electronic spectroscopy. If light in this region is emitted by an
atom, molecule, or ion it corresponds to a process where an electron falls from a higher energy orbital
(or state) to a lower energy orbital. These emission processes occur in phenomena such as
phosphorescence (exhibited by “glow in the dark” materials), fluorescence (exhibited by fluorescent
lights), and chemiluminescence (exhibited by fireflies when they glow). When light of a particular
wavelength is absorbed by an atom, molecule, or ion an electron is promoted to a higher energy orbital
and removes that color of light from a light beam but allowing the other colors to pass through or be
transmitted. Thus the color observed for a solution of a particular species is not the color of light
absorbed, but the color of light transmitted. The color of light absorbed is often the opposite (or
complementary) color on a color wheel. The basic color wheel shown below arranges the colors
clockwise from red to purple in order of decreasing wavelength and increasing frequency. Based on
the color wheel, a blue solution would be expected to absorb light in the orange region of the visible
spectrum.
In this experiment you will be working with the Co2+ ion in water, which has a characteristic
color. The absorption spectrum of the Co2+ ion will be measured using a Spectronic 20
spectrophotometer. This will enable you to dial through a series of wavelengths, measure the
absorbance at each wavelength, and find the wavelength of maximum absorbance. Unlike atoms,
which have very sharp line spectra, the line spectra of molecules and ions in solution contain very
broad lines as a result of atomic vibrations (although measurements close to absolute zero will have
minimal vibrational motion by atoms and much sharper line spectra as a result). So instead of a sharp
line at a single wavelength, the absorption spectrum of Co2+ will show a broad peak with a maximum at
a certain wavelength.
The wavelength of maximum absorbance can be used to analyze the concentration of solutions
2+
of Co once a calibration curve at known concentrations has been established. The intensity of the
color of the solution, and the absorbance of light, depends directly on the concentration of the solution.
Thus a quick and nondestructive measurement of concentration is to measure the light absorbance and
compare the absorbance value to a calibration curve of absorbance versus concentration.
Procedure
Part A: Absorption Spectrum of Co2+P
Starting at a wavelength of 700 nm, zero out the absorbance on a Spec 20 spectrophotometer.
Then place a sample of 0.2 M Co2+ solution in the instrument and measure the absorbance at that
wavelength. Repeat this process every 10 nm to the final wavelength of 400 nm. Identify the
wavelength of maximum absorbance. Plot absorbance versus wavelength on a graph to obtain the
visible absorption spectrum of Co2+.
Part B. Plotting an absorbance versus concentration calibration curve.
Dilute the Co2+ solution to 0.14 M, 0.10 M and 0.060 M by mixing each of 7.0 mL, 5.0 mL, and
3.0 mL of Co2+ solution with enough water to give a total volume of 10.0 mL in each case. Measure
the absorbance of these solutions at the wavelength of maximum absorbance you determined in Part A.
Plot an absorbance versus concentration curve with your four absorbance measurements with 0.20 M,
0.14 M, 0.10 M, and 0.060 M Co2+ solution. You can include a fifth point in your plot since at a
concentration of 0.0 M the absorbance should be 0.000. Draw the best straight line you can through the
five points.
Part C. Measuring the concentration of an unknown Co2+ solution
Obtain a Co2+ solution with an unknown concentration. Record the unknown number on your
data sheet. Measure the absorbance of this solution at your wavelength of maximum absorbance.
Compare this value with your calibration plot to identify the concentration of the unknown solution.
DATA SHEET for “Absorption Spectrum and Colorimetry of Co2+ in Water”
Name ________________________________________________________________
Partner Name(s) _______________________________________________________
Part A:
Wavelength (nm) Absorbance Wavelength (nm) Absorbance
700 550
690 540
680 530
670 520
660 510
650 500
640 490
630 480
620 470
610 460
600 450
590 440
580 430
570 420
560 410
400
Wavelength of maximum absorbance ________________________________________
Frequency of maximum absorbance __________________________________________
Photon energy of maximum absorbance ______________________________________
Absorption spectrum of Co2+(aq)
Part B:
Absorbance values at wavelength of maximum absorbance
Concentration (M) Absorbance
0.20
0.14
0.10
0.06
0.00
Plot of absorbance versus concentration
Part C:
Unknown number ___________________________________________
Absorbance of unknown ___________________________________
Concentration of unknown ____________________________________