Advanced Chemistry
The Determination of Copper in Brass
Purpose: To calculate the % of copper in Brass.
Background: Brass is a combination of copper and >>>>. When a brass sample is dissolved in nitric acid the
result is the following reaction.
8 H+ (aq) + 2 NO3- (aq) + 3 Cu (s)  2 NO (g) + 4 H2O (l) + 3 Cu2+ (aq)
The concentration of copper (II) ion, and hence the concentration of copper in the brass sample can be
measured by evaluating the intensity of the blue color. The color intensity will be measured visually by a
comparison of the sample’s color with that of a series of copper (II) ion solutions of known concentrations.
Alternatively, since the absorbance of the colored solutions is directly proportional to its concentration (A=abc),
a Beer’s Law plot can be constructed using solutions of known concentration. The copper (II) ion
concentration of the brass sample may then be read directly from this plot.
Materials:
1.00 M Cu(NO3) 2
5 mL volumetric flask
24 well plates
toothpicks
15.8 M HNO3
10 mL volumetric pipettes
96 well plates
¼ x #2 brass screw
spectrophotometer
Beral thin-stem pipets
Safety Precautions:
15.8 M nitric acid is very corrosive to all body tissues and toxic by ingestion and inhalation. Nitric acid is also a
strong oxidizing agent. Cupric nitrate is an oxidizing agent and moderately toxic by ingestion. NO 2 is a colorful
gas therefore POISONOUS. Lab should be performed in the fume hood. Wear goggles, gloves and aprons at all
time during lab.
Procedure:
First Things First
1. Measure the mass of a brass screw to ± 0.001 g using an analytical balance.
2. Assuming your brass sample is 100 percent copper by mass (even though it isn’t…), calculate the
minimum volume, in mL, of concentrated 15.8 M HNO3 (aq) that needs to be added to react completely
with the brass. SHOW YOUR TEACHER THIS CALCULATION, then have them measure it out
carefully in the fume hood and add it to a 10.000 – mL volumetric flask.
3. Allow the screw to react completely.
Pre-Experiment Procedure
You will be collecting data between wavelength, concentration, and absorbance. The interaction of light with
solutions to determine the concentration of an absorbing species is an important aspect of this experiment.
1. You will be given 0.100 M solutions of salts:
a. copper (II) sulfate
b. copper (II) nitrate
c. zinc nitrate
d. zinc sulfate
e. iron (III) nitrate
f. iron (III) sulfate
 Choose either salts a and d, b and e, or c and f.
2. Using a spectrophotometer (you will be shown how to do this), take measurements every 20
nanometers from 400-700 nm in order to generate a spectrum and to determine the “best” wavelength
at which to measure the absorbance of the two solutions.
3. Print your data and bring it to the front of the room so that we can have a class discussion about
absorbance, Beer’s Law, and the spectra you created.
Method A
1. Prepare comparison samples of copper (II) nitrate solutions by serial dilution. Using the 24 well plate
and a Beral pipet, begin with 20 drops of the copper (II) nitrate solution provided in one well, and where
each subsequent well has been diluted by one drop while maintaining an overall volume of 20 drops.
2. Calculate the concentration of Cu+2 ions in each cell.
3. When the screw has completely dissolved, carefully add distilled water to the mark on the volumetric
flask and invert several times to mix. Let any solids settle, then draw out the solution with a clean pipet
and place it in a well of a 24-well plate. Visually match the color intensity of this well with the closest
standard dilution and record the appropriate Cu2+ ion concentration.
4. Use solution stoichiometry to calculate the amount of grams of Cu and then the % of Cu in the brass
screw.
Method B
A standard curve of absorbance versus concentration must be prepared by determining the absorbance of a
set of standard solutions. A Beer’s Law graph then is plotted to determine the unknown solution.
Prepare the stock solutions in the following way:
1. Find the wavelength of maximum absorbance of the Cu2+ ion using the standard 1.00 M Cu(NO3) 2
solution (this was done in the pre-experiment procedure).
2. Obtain 10.00 mL of 1.00 M Cu(NO3)2 stock solution using a volumetric pipette. Place the solution in a
test tube.
3. Determine what volume is required to make 10.00 mL of 0.8000 M Cu(NO3)2. Use a volumetric pipette
to transfer this volume of the stock solution into a clean test tube, and pipette the appropriate amount of
water as well. Thoroughly mix the solution.
4. Repeat the dilution process to make 10.00 mL each of three additional dilute solutions (0.600 M, 0.400
M, 0.200 M).
5. Place each of these solutions into cuvettes and determine their respective absorbance. Create a graph
of absorbance vs. concentration (a standard curve).
Determining % copper in brass of your sample:
6. Using your copper ion solution from your lab, Find its absorbance simply by looking and reading the
number off of the computer and writing it down in your written data table. Using the graph constructed
determine the concentration of of the copper ions in the solution.
7. Place your sample in a clean cuvette and determine its absorbance value.
8. Using the equation of the line on your standard curve, determine the concentration of your sample.
9. Use solution stoichiometry to calculate the amount of grams of Cu and then the % of Cu in the brass
screw.
Data Table:
1) Method A
2) Method B
3) All results data from calculations
Calculations:
1) Beer’s Law = A=abc
Find the molar absorptivity of Cu+2 ions using the pre-experiment data,
choosing the maximum wavelength and appropriate absorbance
(the path length is 1.00 cm).
2) Method A 
concentration of copper ions in each well
reasoning used to determine concentration of your sample
moles of copper in screw
grams of copper in screw
% copper in brass screw
3) Method B 
molarity of aliquots
y = mx + b to find concentration of copper ions in unknown
moles of copper in screw
grams of copper in screw
% copper in brass screw
Questions:
1) Give the advantages and disadvantages of both methods.
2) Error Analysis
a) Effect of not preparing your solutions accurately in method B on % brass?
b) Effect of not using 20 uniform drops in method A on % brass?
Vocabulary:
Brass, Alloy, Beer’s Law, Aliquot, & Molar absorptivity
Lab modified from Flinn Scientific Workshop-St. Xavier Univ.