Spectrophotometric Determination
of Copper in Brass: Beer’s Law
Purpose:
In this experiment, you will determine the percent copper in a sample of brass using spectrophotometry.
Background:
When light is absorbed by a substance, the energy of the light is transferred to the substance, and
induces a transition from the ground state to an excited
state. A particular substance is not able to absorb at all
wavelengths, but only certain wavelengths. A substance
will absorb light at a wavelengths corresponding to
specific transitions to excited states. Since different
substances require different amounts of energy for
transition to an excited state, they will absorb different wavelengths of light.
A spectrophotometer is an instrument that measures the intensity of the light entering and exiting a
sample and compares the two. The basic design of optical system of a spectrophotometer is illustrated
below.
The light source is focused on a prism or diffraction grating which breaks up the beam of light into its
spectrum. The spectrum is passed through a slit which is adjusted to exclude all but a selected
wavelength. The beam emerging from the slit is focused on a sample contained in a cuvette. The
cuvette is made of optical glass and is designed so that it does not absorb or reflect an appreciable
amount of the light beam. After passing through the sample, the light beam falls on a photoelectric cell
that is used to measure the intensity of the beam.
A spectrophotometer can be used in two ways. It
used to measure the light absorption as a function of
wavelength to obtain what is called the absorption
The absorption spectrum can be used to determine
identity or purity of a substance. Or, it can also be
determine the concentration of a substance by
comparing the absorption of a solution of unknown
concentration with the absorption of one or more
can be
spectrum.
the
used to
standards.
The ratio of emergent intensity, I, to incident intensity, Io, is called transmittance, T.
Therefore, %T = 100% x (I/Io).
The amount of light absorbed by the sample is expressed in terms of the absorbance, A, defined as
A = log (Io/I)
or
A = -log (%T/100%)
where Io is the incident light intensity and I is the emergent light intensity. The smaller I is relative to Io,
the greater the fraction of light absorbed by the sample and the greater the absorbance. The absorbance
depends upon: the wavelength of the incident light, the concentration of the absorbing molecules, and on
the sample thickness. The wavelength of maximum absorption by a material is one of the characteristic
properties of that material.
The relationship between concentration and absorbance is known as Beer's law. It states that the
absorbance, A, is directly proportional to the concentration of a solution:
A=εcl
where c is the concentration in moles per liter (M or mol•L-1), l is the path length (typically 1 cm), and ε
(the Greek letter epsilon) is the molar absorbtivity (also called the extinction coefficient with units of
L•mol-1•cm-1)
If you plot absorbance versus concentration, the resulting graph yields a straight line. The equation for
the line can be used to determine the concentration of an unknown solution once the absorbance of the
unknown solution has been measured.
Absorbance vs. Concentration for [Co(SCN)4]2-
Materials:
Spectronic 20 spectrophotometer
25mL, 100 mL volumetric flasks
10 mL volumetric flasks or test tubes
1 mL and 5 mL pipets
concentrated nitric acid
copper (II) nitrate
brass bullet shells
test tube
cuvette
Procedure:
1. Determine the mass of two brass .22 caliber rifle shells to + 0.0001 g. Place the shells in a
small beaker. Under the fume hood, add about 10 mL of 15.8 M HNO3(aq) and cover with a
watch glass. After the metal dissolves completely, add 50 mL of distilled water to the beaker.
Transfer the solution to a 100 mL volumetric flask and dilute to 100.0 mL. The nitric acid will
dissolve the zinc and copper metals in the brass. Zinc nitrate solution is colorless, but the
copper (II) nitrate has a deep blue color. The unbalanced net ionic equation for the copper
reaction is:
Cu(s) + NO3-(aq) → Cu2+(aq) + NO2(g) in an acidic solution
2. Prepare 25 mL of an aqueous 1.00 M Cu(NO3)2 solution using copper nitrate and a volumetric
flask.
3.
Prepare more standard solutions. Use a volumetric pipet to transfer 5.00 mL of this Cu(NO3)2
solution into a clean test tube and add 5.00 mL of distilled water. Thoroughly mix the solution.
This makes 10.00 mL of 0.500 M Cu(NO3)2(aq). Continue the dilution process to make 10.0
mL each of three more dilute solutions that are 0.100 M, 0.0500 M, and 0.0100 M.
4.
Determine the wavelength of maximum absorbance (max) for Cu+2. First, observe the color of
your solutions. Based on the color that you see, predict the wavelength maximum for Cu+2.
Now determine the wavelength for maximum absorbance by determining the absorbance of one
of your solutions at these wavelengths (in nanometers): 400, 420, 440, 460, 480, 500, 520,
540, 560, 580, 600, 620, 640, 660, 680, 700. Plot the absorbance versus wavelength.
5.
Construct the calibration curve. Once you have determined the max, measure the absorbance
of each standard solution at this single wavelength. Make a graph of absorbance versus
concentration (molarity) of Cu+2. There will be a linear relationship between absorbance and
molarity. Find the best-fit line through your data using linear regression. You will use this line
to determine the molarity of the Cu+2 in your dissolved solution.
6.
Place some of the brass solution in the cuvette, and position it in the spectrophotometer. The
absorbance of the solution will be displayed. Record the absorbance value. Use the
absorbance value and the calibration curve (it’s your best fit line) to determine the
concentration of Cu2+ in the solution.
7.
From the concentration of Cu2+ determined above, calculate the percent copper in the brass
shells.
Directions for measuring the absorbance of a solution with the Spectronic 20 spectrophotometer.
1. Be sure that the instrument is turned on. These instruments need 15 minutes to warm up!
2. Set the wavelength knob to the first wavelength on the list.
3. Using the zero adjust knob on the left side, set the needle to read 0% transmittance (% T) on the top
scale of the meter. Nothing should be in the sample compartment.
4. Fill one cuvette with deionized water, wipe it with a tissue, and insert it in the sample compartment
with the line on the cuvette aligned with the line on the sample holder. Close the cover.
5. Use the 100% adjust knob on the right hand side to set the needle to read 100% T with the watercontaining cuvette in the sample holder. Remove the cuvette and set it aside. Be certain the instrument
can be scaled to 100% transmittance for water (blank) at both 400 nm and 700 nm before making all
measurements from 400700 nm.
6. Rinse the other cuvette with the standard solution, and fill it with the solution. Wipe it with a tissue,
and insert it in the holder. Make sure that the cuvette is properly aligned as before. Read the absorbance
to three significant figures on the bottom scale of the meter.
7. For each new wavelength, you will need to repeat this procedure.
8. Record the wavelength and the absorbance in your notebook. Make a plot of your data to determine
the wavelength for maximum absorbance. To find the exact value, you may want to collect more data
for wavelengths that are close to the maximum on your graph. For example, if 400 and 420nm gave the
same large amount, you may want to take a measurement at 410 nm.
The Spectronic 20 spectrophotometer