Experiment on the analysis of copper in brass by uv visible spectroscopy
KWAME NKRUMAH UNIVERSITY OF SCIENCE AND
DEPARTMENT OF CHEMISTRY
TITLE: ANALYSIS OF COPPER IN BRASS BY UV-VISIBLE SPECTROSCOPY
NAME: OPOKU ERNEST
DATE: 21ST JANUARY, 2014
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TITLE: ANALYSIS OF COPPER IN BRASS BY UV-VISIBLE
AIMS AND OBJECTIVES:
By the end of this experiment, the student should be able to demonstrate the following
1. To determine the percentage of copper in brass by UV-visible spectroscopy.
2. Properly calibrate and use a spectrophotometer.
3. Convert percent transmittance to absorbance, and vice versa.
4. Construct a calibration curve relating absorbance and concentration for solutions of
5. Use a calibration curve to determine the concentration of an unknown solution.
6. Convert a molar concentration to a mass percent value.
Electromagnetic radiation, of which ultraviolet and visible light are but two examples, has
properties of both waves and particles. When light acts as a particle, called a photon, each
light particle possesses a discrete amount of energy called a quantum. When a molecule is
exposed to electromagnetic energy it can absorb a photon, increasing its energy by an amount
equal to the energy of the photon. The energy of the absorbed photon can be calculated if the
frequency, ν, of the light is known according to Equation 1.
E = hν
Where h is a constant known as Planck’s constant after Max Planck, the German scientist
who first proposed it. Planck’s constant has a value of 6.63 X 10-34 J.s. Frequency is
measured in units of 1/s or Hertz (Hz). The frequency of a light wave is inversely
proportional to its wavelength, λ, which is typically measured in meters. The product of ν and
λ is the speed of light, c as shown in Equation 2, c has a value of 2.998 X 108 m/s.
c = λν
Molecules are highly selective in the wavelengths of light they can absorb. The photons
absorbed depend on the on molecular structure and can be measured by instruments called
spectrometers. The data obtained from a spectrometer are very sensitive indicators of
molecular structure and concentration.
UV-visible spectrophotometry (UV-VIS) uses only the ultraviolet and visible regions of the
electromagnetic spectrum. UV light ranges from approximately 10 nm to about 400 nm. The
visible region of the spectrum ranges from 400 nm to 700 nm. Red light lies at the low
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energy end of the visible spectrum and violet lies at the high energy end. UV-VIS
spectroscopy depends on transitions of electrons in a molecule from one electronic energy
level to another. It is used mostly in studying transition metal complexes and conjugated π
systems in organic molecules. One of the principal uses of UV-VIS spectrometry is in
determining the concentration of an absorbing (coloured) molecule. The amount of light
absorbed, and thus the intensity of the colour of the solution, depends on the concentration of
the absorbing species in the solution. UV-VIS absorption peaks are typically quite broad and
are often spoken of as bands rather than as peaks. The wavelength at maximum absorption is
referred to as λmax and is the optimal wavelength for a pure solution. For an impure solution,
a “blank” solution can be prepared which contains all components of the solution except the
one being analyzed. This solution is used to subtract out absorbance due to interfering
When a beam of light with intensity, Io, passes through a solution, a coloured species, or
analyte, will absorb some of the light energy. The beam of light that passes through (or is
transmitted through) the solution will have a lower intensity, I, than the incident light, Io
because some of the light will be absorbed. Spectrometers typically measure either
transmittance, T, which is the amount of transmitted light, or absorbance, A, which is a
measure of the light absorbed. Both transmittance and absorbance are measures of the amount
of light that is absorbed by the analyte. Transmittance is calculated by dividing the
transmitted light by the incident light (Equation 3). Experimentally, however, transmittance is
usually measured as the percent of the incident light or %T and is defined as shown in
T = I/Io
%T = (I/Io) X100%
Absorbance is defined by Equation 5.
A = -log(T)
But since %T rather than T is actually measured, Equation 5 is changed to Equation 6.
A = -log(%T/100) = log(100/%T) = log 100 – log(%T) = 2-log(%T)
Each chemical species absorbs a different quantity of light at any given wavelength. The
amount of light absorbed by a compound depends on the structure of the compound and the
solvent. However, for any chemical species in a given solvent, the amount of light absorbed
at a given wavelength will be a constant called the molar absorptivity, ε (also referred to as
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the molar extinction coefficient). The absorbance, A, is a function of the concentration, c, of
the absorbing species and the distance the light travels through the solution, that is the path
length, l. Absorbance can be calculated for any solution for which the molar absorptivity is
known (Equation 7).
A = ε.c . l
Equation 7 is often referred to the Beers-Lambert Law or Beers’ Law for short.
Since molar absorptivity and path length are both constants for a given instrument, one may
safely assume that the absorbance is directly proportional only to the concentration of the
analyte. The concentration of a solution can be determined from a Beers’ Law calibration
plot prepared by measuring the %T (or A) for several solutions of known concentration
(standard solutions). The Beers’ Law plot uses concentration as the independent variable
(x-axis) and absorbance as the dependent variable (y-axis). When the absorbance of an
unknown sample is determined, the concentration can then be determined by graphical
interpolation from the prepared calibration graph.
Brass is an alloy composed of zinc and copper. In order to determine the copper content in a
sample of brass we must first dissolve the sample. To achieve this we will use nitric acid, a
strong oxidizing acid. The reaction can be described by the following equation.
3Cu(s) + 8H+(aq) + 2NO3-(aq) J 2NO(g) + 4H2O(l) + 3Cu+2(aq) Equation 8
The nitric oxide, NO, gas evolved in this reaction reacts immediately with oxygen to form
brown NO2 and N2O4 gases. NO, NO2, and N2O4 are all toxic. The dissolution process
therefore must be done in a working fume hood.
8M nitric acid
5 conical flasks
Electronic balance; model: adventurer Pro AV 812; S/N: 1126492837
100ml measuring cylinder
100ml volumetric flasks
Shimadzu UV-160 spectrophotometer
CHEMICALS AND EQUIPMENT
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An amount of brass and
copper were measured into
two separate clean beakers
20ml of 8M nitric acid was
added to the brass in
fumehood and warmed.
20ml of 8M nitric acid was
added to the copper also in
fumehood and warmed.
The contents of each beaker
was transferred into a 100ml
volumetric flask and topped
to the mark.
10ml, 20ml and 30ml of the
prepared copper solutions
were measured into three
conical flasks and diluted
with 30ml, 20ml and 10ml of
distilled water respectively.
The masses obtained were
1.5000g and 2.0000g for
brass and copper
There was an evolution of
a brown smoke and a
formation of a green
solution after warming.
There was an evolution of The copper present was copper
a brown smoke and a
formation of blue solution.
The absorbances of the samples were determined with the help of a UV-160
spectrophotometer at a wavelength of 760nm. The results are as follows:
Distilled water was used as the blank solution.
10 ml of copper standard solution and 30 ml
of distilled water
20 ml of copper standard solution and 20 ml
of distilled water
30 ml of copper standard solution and 10 ml
of distilled water
40 ml of copper standard solution only
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Further details of the results can respectivefully be found on the endorsed sheet at the
extreme end of this document.
CALCULATIONS AND EVALUATION OF DATA
6. Mass of copper = 2.000g, volume of copper solution = 100ml = 0.1dm3,
Molar mass (Cu(NO3)2) = 63.55 + (14 + 16 x 3) x2) = 187.55g/mol
Concentration = mole
but mole = mass
Concentration = 2.000
187.55 x 0.1
i). C1 = 0.1066M, V1 = 10ml, C2 = ?, V2 = 40ml
C1V1 = C2V2
C2 = 0.1066 x 10
ii). C1 = 0.1066M, V1 = 20ml, C2 = ?, V2 = 40ml
C2 = 0.1066 x 20
iii). C1 = 0.1066M, V1 = 30ml, C2 = ?, V2 = 40ml
C2 = 0.1066 x 30
= 0.07995M = 0.0800M
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y = 29.862x - 0.6536
From the graph above,
Concentration of copper in brass is 0.0723M
9. Concentration of copper = 0.0723, molar mass of copper = 63.55g/mol, volume =
Mass of copper = conc. x molar mass x volume
= 0.0723 x 63.55 x 0.1 = 0.4595g
Percentage of copper in brass = 0.4595 x 100%
From the result above, the experiment tries to shows the quantity of copper in brass, since
brass is an alloy made of cooper and zinc. By so doing nitric acid which can dissolve most
metals is being used. Foils of brass and copper are dissolved in nitric acid, brass react with
the acid by liberating a brown poisonous gas as the foil dissolves. After dissolution pale blue
solution was obtained the pale blue colour was as a result of copper component in the brass,
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the solution was diluted by adding 20ml of distilled water to it before transferred to 100ml
volumetric flask. Zinc nitrate is colourless.
Brass is an alloy of copper in brass and zinc which contains a small quantity of lead. Alloys
are actually uniform mixtures of two or more solid substances. For example, brass is a solid
solution of primarily copper and zinc. Alloys often possess properties which are unique and
distinct from those of the individual components. It is also true, however, that the individual
solids still retain more or less the same individual properties. Thus, copper atoms in a brass
sample have more or less the same atomic properties, such as ionization energy, as in a copper
metal sample. Alloys can usually possess any ratio of the component solids, not just
stoichiometric ratios as would be typically found in chemical compounds. Some brasses also
have small amounts of other types of metals, such as tin. Brass’ properties make it easy to
form and bend.
Copper foils reacted in the nitric acid and liberated a brownish gas (NO2) gas as the foil
completely dissolves. After the dissolution a sea blue solution was obtained, the solution was
diluted by adding 20ml of distilled water in the fumehood before sent to the laboratory bench.
It was transferred into 100ml volumetric flask and was labelled as the standard copper
solution. 10ml, 20ml and 30ml of the standard copper solution was measured into three
different beakers, 30ml, 20ml and 10ml volume of distilled water was added respectively to
The various beakers and its content were sent to the spectrophotometric room for
measurement. The instrument had being switched on for about 60 minutes before
measurement, the wavelength was set at 760nm. Distilled water was used as the blank
solution to zero the machine. The absorbance of the various solutions were then recorded.
A graph of absorbance versus concentration of copper is plotted. This graph is used to find
the concentration of copper in the brass solution by tracing the absorbance to the
concentration axis, since zinc nitrate is colourless.
Mass of copper in the sample was calculated and was used to calculate the percentage of it in
brass using spectroscopy.
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The acid might not completely react with the metal foil before water was added.
1. The gases produced during the dissolution of the brass sample are toxic. Therefore all
reactions that involved the evolution of a gas were done in fumehood.
2. Goggles were worn to prevent any of the solution from getting into contact with the eye
3. Gloves were worn to prevent any of the solution from getting into contact with the body.
From the experiment above, we can conclude that the aim of the experiment has being
achieved. That is the UV-visible spectroscopy method was successfully used to determine the
mass and percentage composition of copper in brass.
1. Modern Inorganic Chemistry, Second Edition by William L. Jolly, 2010, pages 46 to 49.
2. Journal of Solid State Chemistry, 2012, pages 4 to 7, retrieved from
3. Concise Inorganic Chemistry, Fifth Edition by J.D Lee, 2007, pages 202-204 and 951.
4. KNUST Chemistry laboratory manual for second year, CHEM 269 & 270, page 32- 34.
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