Advanced Chemistry
The Determination of Copper in Brass
Purpose: To calculate the % of copper in Brass.
Background: Brass is a combination of copper and other materials. 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. You’ll come back to it tomorrow.
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. Determine what volume of 0.500 M Cu2+ and what volume of water is required to make 10.00 mL of
0.4000 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.
3. Repeat the dilution process to make 10.00 mL each of four additional dilute solutions (0.300 M, 0.200
M, 0.100 M).
4. Place each of these solutions (0.100 M, 0.200 M, 0.300 M, 0.400 M, and 0.500 M) into cuvettes and
determine their respective absorbance. Create a graph of absorbance vs. concentration (a standard
curve).
Determining % copper in brass of your sample:
5. 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.
6. Using the equation of the line on your standard curve (the computer should be able to generate a
regression line for you), determine the concentration of your sample.
7. Use solution stoichiometry to calculate the amount of grams of Cu and then the % of Cu in the brass
screw.
Data Tables must include qualitative and quantitative observations from:
1) Method A
2) Method B
3) All results data from calculations
Calculations:
1) 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
2) Method B 
dilution equation to find volume of 0.500 M Cu(NO3)2 needed
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
2) Beer’s Law = A=abc
Find the molar absorptivity of Cu+2 ions using the 0.500 M concentration
and its absorbance. The path length is 1.00 cm.
Questions:
1) Give the advantages and disadvantages of both methods (FOUR total parts)
2) Error Analysis
a) Effect of having drops of water in the cuvette on % brass?
b) Effect of not using 20 uniform drops (some drops too big) in method A on % brass?
Vocabulary to be Incorporated in Conclusion:
Brass, Alloy, Beer’s Law, Aliquot, & Molar absorptivity
Lab modified from Flinn Scientific Workshop-St. Xavier Univ.