? % White KNO3
? % Blue CuSO4.5H2O
Cu2+ + 4NH3 → Cu(NH3)42+ (deep blue)
COLORED SOLUTIONS
A solution will appear a certain color if it absorbs its complementary color
from the color wheel
If a solution appears red, this means it is
primarily absorbing its complimentary color,
green
Sample absorbs green, but
transmits all other colors
White light
containing all
colors shines on
the sample
Eye sees the remaining
combination of colors as red
SPECTROPHOTOMETER – A device that measures the amount of light
absorbed by a sample
A light bulb
emits white
light
A diffraction grating
separates the colors
of light
Light passes
through a slit to
form a narrow
beam
Light passes
through the
sample
Another slit
allows just one
color to pass
A detector
measures the final
amount of light
I0
It
The less light that gets through,
the less the transmittance,
the greater the absorbance
TRANSMITTANCE (T) – the fraction of the incident light that passes
through the sample
T = It /I0
ABSORBANCE (A) – negative logarithm of the transmittance
A = -log (T)
ABSORBANCE SPECTRUM – A graph of the absorbance of a solution at
different wavelengths
Notice that there is a
peak at 600 nm
Since the solution is
absorbing orange, it
must appear blue,
which is across from
orange in the color
wheel
LAMBDA MAX (λmax) – The wavelength of maximum absorbance
For best accuracy, when
measuring the
absorbance of several
solutions, it is best to
measure as close to λmax
as possible
CONCENTRATION AND ABSORBANCE
Which of these 2 solutions contains a higher concentration of red
component?
Solution A – a higher concentration leads to a darker color
CONCENTRATION AND ABSORBANCE
Which of these 2 solutions will have a higher absorbance at λmax ?
Solution A – a higher concentration leads to a higher absorbance
 concentration and absorbance are directly proportional
BEER’S LAW – The mathematical relationship between concentration and
absorbance
A = ɛbc
A = absorbance
ɛ = extinction coefficient
(constant for a given solute at a given wavelength)
b = width of the tube holding the sample
(1.00 cm in our lab)
b = 1.00 cm
c = concentration
(in our lab today, it’s in “percent CuSO4.5H2O”)
In the above equation, ɛ and b are constants, so A and c are the variables
BEER’S LAW – The mathematical relationship between concentration and
absorbance
A = ɛbc
Imagine that you tested the absorbance of the 4 solutions shown below:
0.25%
A: 0.241
0.50%
0.75%
1.00%
0.478
0.722
0.961
What trend do you predict for their relative absorbance readings at λmax?
There is a linear relationship between A and c
A = ɛbc
c+ 0
y = mx + b
C: 0.25%
A: 0.241
0.50% 0.75%
0.478
0.722
1.00%
0.961
Beer's Law Graph for Red Dye
1.2
Absorbance
1
y = 0.9612x - 0.0002
C: 0.25%
A: 0.241
0.8
0.50% 0.75%
0.478
0.722
1.00%
0.961
0.6
0.4
A = ɛbc
A = (0.9612%-1)c
0.2
0
0
0.2
0.4
0.6
0.8
1
1.2
% Red Dye
This is called a CALIBRATION LINE because it is made using known
values and can be used to determine an unknown
m = Δy
= Δ Absorbance
= no units
____
_____________________
____________
Δx
Δ Concentration
%
= %-1
Beer's Law Graph for Red Dye
1.2
Absorbance
1
y = 0.9612x - 0.0002
C: 0.25%
A: 0.241
0.8
0.50% 0.75%
0.478
0.722
1.00%
0.961
0.6
0.4
A = ɛbc
A = (0.9612%-1)c
0.2
0
0
0.2
0.4
0.6
0.8
1
1.2
% Red Dye
If an unknown solution has an absorbance of 0.351, find its concentration
0.351
= (0.9612%-1)c
0.351
= c
____________
0.9612%-1
0.365% = c
Beer's Law Graph for Red Dye
1.2
Absorbance
1
y = 0.9612x - 0.0002
C: 0.25%
A: 0.241
0.8
0.50% 0.75%
0.478
0.722
0.6
0.4
A = ɛbc
A = (0.9612%-1)c
0.2
0
0
0.2
0.4
0.6
0.8
1
1.2
% Red Dye
Calculate the extinction coefficient of this substance, with units
m = ɛb
m = ɛ
___
b
1.00%
0.961
=
0.9612%-1
_____________
1.00 cm
=
0.9612%-1cm-1