Quinn Nutter
CHM 144.M/H
Dr. Danielson
Fan
Nick Neuenschuander
November 14, 2013
Spectrophotometric Analysis of a Copper Penny
Abstract:
In this experiment, the main objective was to see if the claim by the US government was
proven through a spectrophotometric analysis of a copper penny. The US Government claims
that after 1982, that each copper penny has 2.5% copper composition. The penny was dissolved
and diluted in a solution of HNO3, concentrated ammonia and water and then undergone a
spectrophotometric test using a red LED light. The results were then used to find the
interpolation of each penny and then the present of copper per penny. The results had a mean of
1.13% copper per penny, much less then the claim that the United States Government had
previously stated that they would be. Due to the age of the pennies and the weathering conditions
that were observed before hand, the copper coating on the pennies must have lost it’s copper due
to years of weathering and rubbing on its copper coating, proving that at one point, the pennies
did have 2.5% copper.
Introduction:
In this experiment, quantitative analysis requires that the penny to be in a solution. To do
this though, it takes a high oxidizing agent to dissolve a less active metal such as Cu. The one
used for this experiment is nitric acid. When the penny is immersed into nitric acid, nitrogen
dioxide is a product along side of two complex ions of copper (Cu(H2O)42+) and Zinc
(Zn(H2O)42+). The copper complex ion has Cu2+ as the central metal ion and the H2O is the
ligands attached to I, giving the ion a 2+ charge to it. The ligands can be replaced by ammonia to
give the ion a new chemical composure of Cu(NH3)42+ and Zn(NH3)42+. Due to the copper ion
being blue, the absorbance readings will not include zinc due to zinc being colorless.
The main objectives of this experiment are to take a penny, and learn how to preform
spectrophotometric test upon it. Before that, another objective is to learn about oxidation of less
active metals and the complex ions that they form. Once the absorbance have been recorded,
being able to take the data given from the test and to make a calibration curve. With that curve is
to use it to find the percent of copper in a penny. With that data in mind, compare the results to
the hypothesis and determine an conclusion.
The United States government has stated that after 1982, the penny has been reduced from 95%
copper, to 2.5%. This statement means that any penny, made from 1982 to today has 2.5%
copper in them. Atlanta Christian College Prep School states that each penny has a distinctive
composition “which is composed of zinc metal inside a thin shell of copper metal.” Using their
claim, taking a penny, that is not recently new, and weathered down, will have less than 2.5%
copper due to weather, and how old the penny is.
Methods:
The methods used for this experiment were separated into three parts. First part was the
preparation of the standard solutions, second, the preparation of the penny sample, and part three
is spectrophotometric analysis using the MeasureNet colorimeter.
Solutions Preparations:
The preparations for part one started with making six standard solutions of Cu2+ into
separate 50 mL volumetric flasks starting at 0.00mL for the first standard solution and going by
increments of 2 mL to 10.00mL. 3.00mL of concentrated ammonia solution is added to each
standard solution and is swirled until no solid precipitation is visible. The concentrated ammonia
solution is at 15 molar concentrations. The solutions are then diluted with water and then a
stopper is put on the end. Next is preparation of the penny sample solution. The penny is
observed for its weight, mint date, and the condition. The penny is then placed inside a 150mL
beaker and 25ml of HNO3 is added using a graduated cylinder. The concentration of the HNO3 is
6 moles. Once the penny has been fully dissolved in the solution, the solution is then transferred
over to a 100mL volumetric flask. 14ml of concentrated ammonia (same concentration as the
solution used in the preparation of the standard solutions) is added by graduated cylinder to the
penny solution. The penny solution is swirled until there is no more solid blue precipitation is
visible. Add water to dilute the solution, and put a stopper on the end of the volumetric flask.
Absorbance Measurements:
The wavelength of maximum absorbance is determined from an absorbance versus
wavelength spectrum of the colored Cu(NH3)42+ solution using a MeasureNet PDA spectrometer.
Based on that data, an appropriate LED light color to be used upon the solutions is made. The
MeasureNet system is then calibrated to the right variables and then a reference is made using
both cuvettes filled a third of the way with the colorless 0.00mL standard solution. The tests are
preformed starting with the least diluted solution to the most dilute. When the standard solutions
have been tested upon, the penny solution is then tested in the spectrometer.
Calculations:
The first calculation was done to calculate the concentration of Cu2+ in each of the six
solutions. The formula used to find such data was “M1V1=M2V2”, whereas the concentration of
the stock solution is 5.00g/L. During the setup of the penny solution, 24.00mL of concentrated
ammonia solution was added into it to fully dissolve the precipitation, and make the solution
transparent. When picking a LED color to use for the spectrophotometric analysis, the color red
was chosen due to the red LED had a higher peak around 627-642nm.
Results:
When the experiment began, the pennies were observed for 3 main categories. The
categories were the date the penny was made, the condition of it and finally, the weight of the
penny. Once this data was recorded and the penny was made into its solution, the absorbance of
the solution was recorded. This data is shown in Figure 1.
Penny Data:
Quinn
Nick
Joanna
Ryan
Mint of penny:
1990
1991
1988
1993
Condition:
Very Weathered
Slightly
Some weather
Slightly
Weathered
rust onto it.
Weathered
Weight:
2.4583g
2.4940g
2.4850g
2.4882g
Absorbance:
.616au
.544au
.501au
.650au
Figure 1. Shows the information on the pennies before the experiment trials.
Before the penny solution is put into the spectrophotometer, six stock Cu2+ solutions were
tested. Figure 2 shows the calculated concentration of the stock Cu2+ in the solution. This was
achieved by solving for M2 in the equation ‘M1V2=M2V2’ where the variables ‘M1’ and ‘V2’ are
constants throughout each equation. Once the concentration was found, each standard solution
was put into the spectrophotometer. 0.00mL solution does not have an absorbance unit number
due to it was used to be the reference solution for the other stock solutions (Figure 2).
Standard Solution Data:
0.00ml
4.00ml
6.00ml
8.00ml
10.00ml
Solution Solution
Solution
Solution
Solution
Solution
Concentration:
M2=0.0
M2=0.20g/
M2=0.40g/
M2=0.60g/
M2=0.80g/
M2=1.00g/
(M1V1=M2V2)
g/mL
mL
mL
mL
mL
mL
Absorbance
-
2.00ml
.162au
.324au
.486au
.647au
.81au
Figure 2. Shows the calculation and data of the concentration and the result of the
spectrophotometer tests.
Figure 2 is visually represented in Graph 1. Graph one also shows the equation for the
tread line between the data points.
Concentration verses Absorbance
0.9
0.8
y = 0.809x + 0.0006
R² = 1
Absorbance
0.7
0.6
0.5
Concentration
0.4
Linear (Concentration)
0.3
0.2
0.1
0
0
0.2
0.4
0.6
0.8
1
1.2
Concentration
Graph 1. Visual representation of the absorbance in relationship to the concentration of the
solution applied.
Using the equation of the line in Graph 2, the interpolate can be determined by replacing
the ‘y’ variable with the absorbance unites for each penny. The interpolate for each penny goes
as follows:

Quinn’s penny interpolate: 0.761

Nick’s penny interpolate: 0.672

Joanna’s penny interpolate: 0.619

Ryan’s penny interpolate: 0.803
Using the data up above, the interpolate can be put into a equation to find the grams of
copper in the original penny by taking the interpolate and multiplying it by the liters of measured
solution that was in the volumetric flask. This unit was constant at .039L due to there were 25mL
of HNO3 used to dissolve the penny and 14ml of concentrated ammonia used to dissolve any
precipitation in the penny solution.
Calculations For Original Penny
Grams of ‘Cu’
Quinn’s Penny
Nick’s Penny
Joanna’s Penny
Ryan’s Penny
.0297g Cu
.0262g Cu
.0241g Cu
.0313g Cu
1.21%
1.05%
.97%
1.26%
(interpolate)*
(L of measured
solution)
Percent of ‘Cu’
In Penny
Figure 3. Shows the data of the calculations for copper in each penny sample.
Mean and Standard Deviation
Mean
Standard Deviation
(Σ𝑑) ÷ 4
𝑆𝑥 = ((Σ𝑑12 ) ÷ (𝑛 − 1))1/2
1.13%
Sx=.1358
Figure 4. Shows the Mean and Standard deviation of the percent of copper in the penny.
Discussion:
This experiment done is to show the amount of copper in a penny. The penny was
dissolved into a solution and then reduced even more to get the copper element by itself, and
then putting it into a spectrophotometer and it seeing how much of the selected LED light is
absorbed by the copper solution. In 1982, due to the rise of copper prices, the amount of money
it took to make the penny with 90% copper was much more than the face value of the penny.
With that, the United States mint then “changed to 97.5 percent zinc and 2.5 percent copper
(copper-plated zinc).” (US Mint). The pennies composition changed from being majority core of
copper, to then only having a thin copper coating for a zinc core. Using the data from the
experiment on pennies done after 1982, it shows that the pennies have a lower amount of copper
then stated and assumed by the United States mint. Using four different mints, from four
different years, the present of copper in them are on average 1.37% lower then the stated 2.5%
copper per penny. The pennies used in the experiment are between the years 1988 and 1993, a
five-year span of which each penny had ether some weathering to them, or most of the penny
was weathered down. Using the standard deviation of the percent’s of the pennies, it shows that
the difference between the pennies is in a good range of each other, making other pennies
statistically around the same copper composition as the pennies that were conducted in this
experiment. But, looking at the penny’s age and how much copper is in the penny compared to
how the US mint states that there is, it can be seen that the penny has lost copper over the years
due to its outer shell being rubbed off, weathered down, and just plain ageing of the penny. The
pennies used in this experiment are on average around 1990.5 years old and have each lost
around 1.37% of their copper composition shell. The zinc in the penny might have not been lost
as much as the copper due to the zinc making up the core of the penny and the copper is the outer
shell, exposing it to the weathering extreme. Some potential error variables that were in the
experiment were the conditions of the pennies. Having pennies with different weathering
conditions can affect of how much of the copper was lost on the shell coating. The pennies
should have all been though the same cleaning process to then become the same weathering
conditions. Another variable that potentially could cause errors is the closeness of each penny.
Partners and whose data they shared with should have been the same mint date increments, so
that not all pennies are at random and could yield near same percent copper lost and would have
made an error in the standard deviation. Pennies dating back older than the ones used in the
experiment have more likely lost more copper the older they get till 1982. If this experiment had
the same objectives of finding out how much copper is in a penny prior 1982, there would need
to be some changes into the experiment. For one, the amount of HNO3 would have to be
increased to dissolve a penny with a high copper composition. The Cu2+ stock solutions would
also needed to have a greater copper input due to the spectrophotometer readings would have to
increase to match the copper penny solution’s absorbance readings.
Conclusion:
I learned a lot in this lab experiment, especially about spectrophotometry and how to find
concentration of an object by using its absorbance of a certain LED light source. I now about
using different oxidizing agents to dissolve a certain compound and separate and isolate a certain
element that I wanted to analyze. I also thought of the variables in an experiment that you might
not have control over, such as weathering of the pennies, or errors in calculations in determining
certain valuables needs for results of the experiment.
Something else I learned a little bit more about complex ions. I learned about how that
they have a metal ion covalently bonded with anions called ligands. I also learned a little bit
more about chemical reactions that make fumes and toxic gas and that they need to be
preferment with certain safety precautions.
References:
Lab Experiment 10, Percent Zinc in a Penny, Greater Atlanta Christian College Prep School,
Norcross, GA, PDF file available on the internet.
"The United States Mint · About The Mint." The United States Mint · About The Mint. United
States Treasury, n.d. Web. 14 Nov. 2013.
<https://www.usmint.gov/about_the_mint/fun_facts/?action=fun_facts2>.