2.5.2 Absorption  In the last laboratory you noticed that thermal or electrical excitation of atoms caused electrons to be promoted to higher levels of lying energy. The subsequent relaxation of the excited electrons to their ground energy levels was followed by photon emissions in the visible electromagnetic spectrum field. The distinctive colors emitted during this relaxation phase can in some cases be used to classify the individual responsible for the spectrum of emissions. You will be turning this phenomenon around for the next two test times by using the absorption spectrum of a transition metal ion in aqueous solution. The transition metal ions are known to show a wide variety of colours. Yes, many transition metal ions are responsible for the colored pigments in paints, the color of gemstones such as rubies or sapphires and stained glass colors. Absorption spectra of chemical species (atoms, molecules or ions) are produced when a beam of electromagnetic energy (i.e. light) passes through a sample and a portion of the photons of electromagnetic energy passing through the sample is absorbed by the chemical species. A perfect example of this phenomenon is our understanding of colour. Consider the case where a ray of white light (i.e., sunlight) passes through a chlorophyll-containing sample solution (the compound responsible for leaf colour). In the blue and red regions of the visible portion of the electromagnetic spectrum the chlorophyll molecules absorb only a few select photons. The energies of such absorbed photons cause electrons to be excited in the chlorophyll molecule and the energy of these excited electrons is used in the plant cell to drive the conversion of carbon dioxide and water into glucose. More important for our purposes is that when the red and blue photons which chlorophyll absorbs are subtracted from white light, the resulting light beam that leaves the solution appears green to our eyes and this is why leaves appear green to us. If we were able to calculate the total number of photons of all colors entering the sample and compare it with the total number of photons of all the colors leaving the sample, we would find fewer photons exiting the sample than entering it. This is consistent with the fact that some of the photons from the white light beam that entered the solution were absorbed by the chlorophyll molecules.A spectrophotometer is an instrument for doing this measurement. Using some well-understood electronics this tool effectively "counts" the number of photons entering a sample and compares it to the number of photons leaving a sample. Moreover, the device is capable of taking white light and splitting it into its constituent colors (i.e. much like Copyright © 2010; John Hardesty and Bassam Attili Page 2 a prism), enabling the user to analyze the light absorption of different wavelengths with a resolution of almost 1 nm. In optics, the portion of physics which deals with light properties The Intensity, I,.

1. 2.5.2 Absorption
In the last laboratory you noticed that thermal or electrical
excitation of atoms caused electrons to be promoted to higher
levels of lying energy. The subsequent relaxation of the excited
electrons to their ground energy levels was followed by photon
emissions in the visible electromagnetic spectrum field. The
distinctive colors emitted during this relaxation phase can in
some cases be used to classify the individual responsible for the
spectrum of emissions. You will be turning this phenomenon
around for the next two test times by using the absorption
spectrum of a transition metal ion in aqueous solution. The
transition metal ions are known to show a wide variety of
colours. Yes, many transition metal ions are responsible for the
colored pigments in paints, the color of gemstones such as
rubies or sapphires and stained glass colors. Absorption spectra
of chemical species (atoms, molecules or ions) are produced
when a beam of electromagnetic energy (i.e. light) passes
through a sample and a portion of the photons of
electromagnetic energy passing through the sample is absorbed
by the chemical species. A perfect example of this phenomenon
is our understanding of colour. Consider the case where a ray of
white light (i.e., sunlight) passes through a chlorophyll-
containing sample solution (the compound responsible for leaf
colour). In the blue and red regions of the visible portion of the
electromagnetic spectrum the chlorophyll molecules absorb only
a few select photons. The energies of such absorbed photons
cause electrons to be excited in the chlorophyll molecule and
the energy of these excited electrons is used in the plant cell to
drive the conversion of carbon dioxide and water into glucose.
More important for our purposes is that when the red and blue
photons which chlorophyll absorbs are subtracted from white
light, the resulting light beam that leaves the solution appears
green to our eyes and this is why leaves appear green to us. If
we were able to calculate the total number of photons of all

2. colors entering the sample and compare it with the total number
of photons of all the colors leaving the sample, we would find
fewer photons exiting the sample than entering it. This is
consistent with the fact that some of the photons from the white
light beam that entered the solution were absorbed by the
chlorophyll molecules.A spectrophotometer is an instrument for
doing this measurement. Using some well-understood
electronics this tool effectively "counts" the number of photons
entering a sample and compares it to the number of photons
leaving a sample. Moreover, the device is capable of taking
white light and splitting it into its constituent colors (i.e. much
like Copyright © 2010; John Hardesty and Bassam Attili Page 2
a prism), enabling the user to analyze the light absorption of
different wavelengths with a resolution of almost 1 nm. In
optics, the portion of physics which deals with light properties
The Intensity, I, is the calculation of the amount of photons
emitted at a point in a given time unit. (Higher intensity could
be regarded as' brighter' and lower intensity could be regarded
as' dimmer;' thus high intensity light would be bright and low
intensity light would be dim.) When we calculate the light beam
intensity entering our sample (Io) and equate it with the light
beam intensity leaving our sample (I) then we can use the I / Io
ratio to get an estimate of what proportion of the light entering
the sample was found leaving the sample. The ratio is called the
Transmission Transmittance: T I o We can translate this ratio to
a percentage by multiplying it by 100 to get Percent
Transmittance (percent T): percent Transmittance: percent T I o
100 Thus, if the light intensity leaving our sample is 76 and the
light intensity entering our sample is 100, the transmittance will
be 0.76 and the transmittance percentage will be 0.76 This will
be 76%, suggesting that 76% of the photons that enter our
sample find their way out of our sample. For our purposes the
interpretation of a new term, Absorbance (A) is mathematically
convenient: Absorbance: A log10 I o Absorbance is a direct
measure of how much light our sample absorbs. If you play with
the formula in your calculator you can note that absorbance will

3. take on values between 0 (at 100 percent transmittance) and
around 2.0 (at 1 percent transmittance); hence high absorbance
values are correlated with very little light passing through the
sample entirely and low absorbance values (i.e. those
approaching0) Are related to most light that passes through the
sample entirely. The Beer-Lambert Law: Consider a chemical
product solution that absorbs light of a given wavelength. We
might picture two fascinating situations. Next, if we pass
through a relatively dilute solution a ray of light of the correct
wavelength, We might picture photons meeting a limited
number of absorbing chemical compounds, so we could
anticipate a high percentage of transmission and low absorption.
Alternatively, if we move through a highly focused solution the
same beam of light, we may assume that the photons would
encounter a large number of copyrights © 2010; John Hardesty
and Bassam Attili Page 3 Chemical species absorbing, so we
should anticipate a low percent transmittance so strong
absorption. Accordingly, absorbance is proportional to sample
concentration. Third, We might assume that if we allow the
light beam to encounter a solution for a long period of time, we
might expect a low percentage of transmittance and high
absorption; while if the light beam was allowed to encounter the
solution for a short period of time, we could expect a high
percentage of transmittance and low absorption. As light travels
at a constant velocity, c= 3.0x 108 m / s, this means that the
absorbance should also be proportional to the beam's path
length across the sample. These two considerations allow us to
define the following proportionality: A k l c where, k= constant
proportionality, l= length of path, and c= concentration of the
absorbing chemical species If the length of the path is measured
in centimeters and the concentration of the absorbing species is
expressed in molarity, the proportionality constant is called
molar absorption, with units ofM-1 cm-1; And our
proportionality reduces to the Beer-Lambert Law: A l c This
strategy is used not only by chemists but by scientists in many
fields. The Beer-Lambert law helps you, the scientist, to

4. measure the absorption of a given sample and to deduce the
solution concentration from that calculation! Currently, The
concentration of a specific chemical species in a solution can be
determined as long as you know that the species absorbs light of
a specific wavelength. [24]