Determination of the Amount of Copper in Brass
Spectrophotometry is an extremely important tool used in forensic science to determine the detailed
chemicals composition of evidence obtained from a crime scene. It can be used to determine the
concentration of either single chemicals species on solution or even the concentration of a species within a
mixture of species in solution. For example it can be used to determine the mass percent of copper in brass
shell casings collected by the crime scene investigator (CSI), and then match the brass composition to a
particular manufacturer.
The primary objective of the first part of this experiment is to determine the concentration of an unknown
copper(II) nitrate solution. You will use a Colorimeter (a side view is shown in Figure 1) or a
spectrophotometer to measure the concentration of each solution containing copper(II) ions which have a
distinctive color. As with most colored solutions, there is a relationship between the concentration of the
solution and the amount of light that the solution absorbs.
You will first make a standard solution containing the copper(II) ion then dilute that standard solution so that
you make four additional solutions containing the copper(II) ion that are less concentrated. Additionally,
you will be able to calculate the concentrations of all these diluted solutions. Each standard solution is
transferred to a small, cuvette that is placed into the colorimeter or spectrophotometer. The amount of light
that penetrates the solution and strikes the photocell is used to compute the absorbance of each solution.
Figure 1
When you graph absorbance vs. concentration for the standard solutions, a direct relationship should result.
The direct relationship between absorbance and concentration for a solution is known as Beer’s law, and the
graph that you create is called a calibration plot or standard curve.
The primary objective of the second part of this laboratory exercise is to determine the amount of copper in a
sample of brass. You will dissolve the brass and use the calibration curve from Part I to determine the
concentration of Cu2+ in your sample which will allow you to calculate the mass of copper in your sample
and ultimately the percent of copper in your brass sample.
OBJECTIVES
In Part I of this experiment, you will
•
•
•
Prepare and test the absorbance of five standard copper(II) nitrate trihydrate solutions.
Calculate a standard curve from the test results of the standard solutions.
Dissolve a brass sample in nitric acid to extract the Cu2+ ion and determine its concentration using your
standard curve.
• Determine the percent of copper(II) in the original brass sample.
Adapted from Advanced Chemistry with Vernier & The AP Chemistry Lab Manual published by the College Board.
Colorimetric Analysis and Determination of Percent Copper in Brass
MATERIALS
Data collection device
colorimeter or spectrophotometer
solid Cu(NO3)2
balance
Buret & clamp
100.0 mL volumetric flask
10.0 mL volumetric flask
5 test tubes
test tube rack (or large beaker for holding test tubes)
5 cuvettes
Beral pipette
tissues (preferably lint-free)
Stir rod
Brass sample
≈ 6M HNO3, nitric acid
1. Obtain and wear goggles.
2. Mass the amount of Cu(NO3)2 • 5 Η2Ο needed to make exactly the volume of ≈ 0.400 M Cu(NO3)2
solution dictated by the size of the volumetric flask you have been given (50.0 mL or 100.0 mL). This
will be your stock solution. Transfer the solid into the volumetric flask, add water to just below
the mark on the flask and mix well. Once the solid has completely dissolved, use a pipette to fill the flask
to the mark on the volumetric flask and mix a final time. Record the exact concentration of your solution
in the blank provided in question #3 below. Transfer the solution to the buret.
3. Determine the volume of each diluted solution you wish to make based upon the size of the 5 test tubes
you have available. Your first test tube will simply contain some of your stock solution. Use the dilution
formula M1V1 = M2V2 to make your five dilutions.
Design a data table that includes the concentration and absorbance of each of the 5 standards you chose to
make by diluting your __________ M Cu(NO3)2 “stock” solution.
4. Set up either the spectrophotometer or the data collection system. If using a colorimeter, connect it to the
interface.
• Start the data collection program. Be sure the program shows correct readings for the colorimeter.
• Set up data collection for Events with Entry.
• Follow the appropriate steps to calibrate the colorimeter or zero the spectrophotometer.
5. Collect and record the absorbance values for each of the six standard solutions taking care to fill the
cuvette about ¾ full and wipe off any fingerprints prior to placing it in the colorimeter or
spectrophotometer.
6.
Use graph paper, graphing calculator or graphing software to construct a calibration plot which is a
graph of absorbance vs. concentration. Print the graph if using computer software. You will use this
calibration curve to determine the amount of copper in your brass sample in Part II.
7. Discard the solutions as directed by your instructor.
Adapted from Flinn Scientific Chem Fax and Vernier Advanced Experiments in Chemistry.
Colorimetric Analysis and Determination of Percent Copper in Brass
PART I: PRE-LAB QUESTIONS
1. What are the name, formula and molar mass of the colored species being measured in this laboratory
exercise?
2. How do you propose to determine the volume of your test tubes?
3. Why is it important to fill the cuvette to about ¾ full before inserting it into the colorimeter or
spectrophotometer?
4. The wavelength setting you are told to use in this
lab is not chosen arbitrarily. A previous
experiment was conducted to determine the best
wavelength setting. The experiment used a
spectrophotometer and collected absorbance data
for a given sample of Cu2+ at 10 nm wavelength
increments starting at 450 nm and ending at
850 nm. The graph of the data is shown at right.
Explain how this graph is used to determine the
wavelength setting for this experiment.
6. A student is given the task of determining the
molar concentration for a sample of copper(II)
nitrate solution of “unknown” concentration.
In order to establish a standard or calibration
curve, the student prepares a set of five standard
solutions of copper(II) nitrate. The student
collects the data shown in the following table.
Standard
1
2
3
4
5
Concentration
(mol/L)
0.0800
0.160
0.240
0.320
0.400
Absorbance
0.205
0.404
0.599
0.789
0.982
(a) Use the set of axes provided above to graph the data. What is the equation for the calibration plot
(regression equation or “line of best fit”)?
(b) The student then measures the absorbance for the unknown sample and determines it to be 0.669.
Determine the concentration of the unknown Cu(NO3)2 solution and explain your method.
Adapted from Flinn Scientific Chem Fax and Vernier Advanced Experiments in Chemistry.
Colorimetric Analysis and Determination of Percent Copper in Brass
PART I: POST-LAB QUESTIONS AND DATA ANALYSIS
1. Use computer graphing software to plot your data and run a linear regression analysis to evaluate your
standard curve. Print your graph and be sure you include the equation for your linear-regression and the
value of the linear regression correlation coefficient.
2. What statistical indicator convinces you that your data is trustworthy? Justify your answer.
3. Beer’s Law appears in the “Gases, Liquids and Solutions” section of the AP Chemistry Formula Sheet
as A = abc but is more commonly seen in reference material as A = ε c . Either way A = absorbance,
a or ε = molar absorptivity (a constant specific to each solution), b or  = path length of the cuvette in
which the sample is contained, and c = concentration in mol/L or molarity. The molar absorption
coefficient, molar extinction coefficient, or molar absorptivity (ε), is a measurement of how strongly a
chemical species absorbs light at a given wavelength. It is an intrinsic property of the species. Calculate
the concentration of a solution having an absorbance equal to 0.600 and a molar absorptivity constant of
1.5 cm−1 M −1 with a path length of 1.0 cm.
4. You arrive in the lab to discover that the lab group working at your station did a fine job of cleaning up
after themselves. They even rinsed out the cuvettes with deionized water as evidenced by droplets of
water clinging to the bottom and sides of the cuvette. What, if anything should you do to prior to
measuring the absorbance of your samples?
5. A student fails to wipe the cuvette prior to inserting the cuvette into the colorimeter or spectrophotometer
correctly. What effect would a drop of solution left on the outside of the cuvette have on the measured
absorbance?
6. A different student fails to wipe the cuvette prior to inserting the cuvette into the colorimeter or
spectrophotometer correctly. What effect would a fingerprint left on the outside of the cuvette have on
the measured absorbance?
Adapted from Flinn Scientific Chem Fax and Vernier Advanced Experiments in Chemistry.
Colorimetric Analysis and Determination of Percent Copper in Brass
Part II: Determining the Percent Copper in Brass
Brass is an alloy made of copper and zinc. The proportions of zinc and
copper can be varied to create a range of brasses having different
properties. It is a substitutional alloy meaning atoms of the two
constituents may replace each other within the same crystal structure.
By comparison, bronze is principally an alloy of copper and tin.
Brass is used for decoration due to its bright gold-like appearance.
It is also used in applications to reduce friction in locks, gears, bearings,
doorknobs, ammunition casings and valves for plumbing. And you are
already aware brass is used extensively for its acoustic properties in
musical instruments such as horns and bells. Most importantly, it is also used in zippers!
In this part of the experiment, you will dissolve a sample of brass in nitric acid (the oxidizing acid)
and use your calibration plot from Part I to determine the amount of copper in your alloy.
OBJECTIVES
In this experiment, you will
•
Determine the amount of nitric acid necessary to dissolve your brass sample.
Use extreme caution and perform this step under a fume hood!
•
Determine the amount of copper present in your brass sample.
PART II: DETERMINATION OF PERCENT COPPER IN BRASS
1. Determine the mass of your brass sample to the nearest 0.001 g and place it in a small volumetric flask.
Construct a data table and record any pertinent data.
2. Assuming your sample is made entirely of copper; calculate the amount of 6 M nitric acid required to
react completely with your brass sample.
3. Select appropriate glassware and measure the calculated amount of 6 M HNO3 using EXTREME
CAUTION while wearing goggles, an apron and rubber gloves. THIS REACTION MUST BE
CARRIED OUT IN A FUME HOOD AS TOXIC NO2(g) IS RELEASED! Gently swirl the mixture
in the volumetric flask occasionally until the metal has completely dissolved (about an hour).
Disregard small insoluble flecks, if present. However, do allow the flecks to settle.
4. When the sample has completely dissolved, carefully add distilled water to the mark on the volumetric
flask, cap it and invert it several times to mix. Let any solids settle, then draw out the solution with a
clean pipette and place it in a small beaker. Stir with a glass stir rod to release any NO2 gas remaining
in the solution.
5.
Use spectrophotometric techniques to determine the molarity of the Cu(NO)2 (aq) contained in
exactly 100.0 mL of the brass solution.
Adapted from Flinn Scientific Chem Fax and Vernier Advanced Experiments in Chemistry.
Colorimetric Analysis and Determination of Percent Copper in Brass
PART II: PRE-LAB QUESTIONS
1. Write the balanced molecular equation for the reaction taking place with regard to the copper in brass
in this laboratory exercise. (This will be much easier if you read through the procedure. Go, figure!)
2. Write the balanced net ionic for the reaction taking place in this laboratory exercise.
3. A student is tasked with dissolving a brass shell casing with a mass of 3.579 g in 8.0 M nitric acid.
Assuming the shell casing is 100% copper, calculate the minimum volume of 8.0 M nitric acid the
student needs to accomplish this task.
4. A student determines the absorbance of their sample to be 0.669. Calculate the percent copper present
in their brass shell casing.
PART II: POST-LAB QUESTIONS AND DATA ANALYSIS
1. A student accidentally spills some of their copper solution prior to determining its absorbance.
What effect does this error have on the calculated percent of copper present in the shell casing?
2. A student wishes to repeat this experiment in order to determine the percent zinc present in a brass
shell casing. Is this experimental procedure suitable? Justify your answer.
Adapted from Flinn Scientific Chem Fax and Vernier Advanced Experiments in Chemistry.