The Absorption Spectrum of
Cobalt ( II) Chloride
General Chemistry I (1411)
Professor Hitish Nathani
Manny Carenzo11/28/2011
Abstract
To determine the molar concentration of unknown sample, a known molar
concentration can be used to gives us a numerical value for absorbance so we can
then calculate for the unknown. A standard Cobalt (II) Chloride solution, 0.150M
was used to make successive molar concentrations of H2O dilutions which were
tested for their light absorbance spectrum. Light absorbance is used according to
Beer’s law that absorbance is directly linked to concentration.
A value of K constant was obtained, 2.927 M-1 from the Cobalt (II) Chloride
concentrations, equation (1). The unknown H2O dilution concentrations were also
tested for their light absorbance spectrum. The K constant was used, equation (2) to
calculate for the unknown concentration, Four cuvettes concentrations were
obtained, 0.0655, 0.0532, 0.0471, and 0.0386. These four concentration values were
then were used in formula which gave us the undiluted C2 value: 0.0935, 0.0886,
0.094, and 0.0965. The average of these four numbers is 0.0931. The unknown
test tube number three concentration was selected to be within the range of the
standard and closest to the average is 0.100M as calculated by equation (5).
Purpose
A series of solutions will be setup to show the effect of analyte concentration on
transmittance and absorbance. We will obtain absorbance values at different
concentration diluted solutions which we can then calculate the constant value of K
from the Beer’s law. The K constant will be used to calculate the value of the
unknown concentration. The proportional relationship of absorbance and
concentration allows us to safety determine concentration gradient of an unknown
dilution.
Introduction
The use of light absorption to measure transmittance is used in many fields of study:
Chemistry, Physics, Biochemistry, Molecular biology. In order to determine the
equilibrium constant of a solution, where the reactants move in a forward or reverse
direction to form products that eventually reach a balance of equilibrium, we can
determine the respective concentration of a solution. A spectrophotometer is used
to give us a numerical value of an equilibrium constant of a concentration indicating
the light absorbance corresponding to that concentration. An experiment will be
conducted to identify an unknown concentration of CoCl2 solution by using a
Spectrophotometer.
A known CoCl2 concentration will be use to obtain a constant concentration value of
K. The value of K is: sum of concentration of absorbance divided by the sum of
concentration squared.
k = ∑ cA /∑ c2 = M-1
(1)
The K Molarity of constant value of concentration will then be used in the
calculations using the formula A=kc to obtain the concentration of our unknown
concentration of CoCl2. ( A= absorbance, C = concentration)
The Spectrophotometer is an instrument that measures the fraction of visible light,
400nm to 700 nm. The instrument consists of two parts, a spectrometer for
producing light of any wavelength and photometer measuring light intensity (David
Caprete 8/96). Using the Beer’s law we can find the concentration of cobalt chloride
from the amount of light absorbed by the solution
Different compounds absorb wavelengths equally. Certain chemicals are able to
absorb particles of light as defined by the ratio of transmittance of intensity of light
entering and diminishing within the solution.
T= I/I0
(2)
The solutions will absorb light particles from the Spectrophotometer giving us a
numerical value of light absorbance.
A = - log10 of transmittance T = -log10 (I/Io)
(3)
According to Beer’s law, absorbance of light is directly related to concentration:
A =kc
(4)
where k = 1/M so that k=M-1 and where c = concentration in mole/liter
Concentration and absorbance are directly proportional according to Beer’s law.
Absorbance of light is directly related to concentration such that A =kc (where k =
1/M so that k=M-1 and where c = concentration of mole/liter). Both of these
proportion constants C=A will allow us to determine the concentration of our diluted
unknown. There is linear relationship between concentration and absorbance.
To get the concentration of the original undiluted unknown solution, the dilution
formula will be used:
C1V1 = C2V2
(5)
(I)
Procedure
Procedure for the Absorption Spectrum of Cobalt (II) Chloride
MATERIALS
Spectronic 20 D+ Thermo Spectronic , 0.150 M cobalt (II) chloride, CoCl2, solution,
unknown solution, distilled water , test tube rack, safety glasses
two Eppendorf 10 ml pipettes, tissues (preferably lint-free), 10 of 20 × 150 mm test
tubes
Procedures to calibrate transmittance and absorbance of the
Spectronic 20 + Thermo Spectronic.
(A picture diagram of the Spectronic 20 + has been provided in order facilitate
procedure)
1. Turn on power switch /zero control and warm up the Spectrophotometer
for 15 minutes.
2. Turn transmittance/absorbance control (control % T/0A all the way down.
3. Calibration of spec by turning zero control knobs to exactly zero.
4. Set filter switch to 340nm to 599 nm.
5. Set wavelength control to 400 nm.
6. Obtain test tube with water I/2 full to set the transmittance to 100%.
7. Place test tube into the sample compartment
8. Turn the 100% knob until it reads 100.00 transmittance. We know have
100 % transmittance and zero absorbance.
Procedure to obtain highest absorbance spectrum of 0.150 M CoCl
2.
1. Obtain a 5 ml test tube (# 1) of 0.150 M CoCl 2 undiluted by water.
2. The test tube will be used to calibrate absorbance at 25 nm
Intervals from 400 nm to 600 nm.
3. This test tube will placed in the sample compartment at each 25 nm
interval.
a. The Spectrophotomer will be recalibrated at zero and a reference blank is
used in the curvette for each 25 calibrations. The mode will set for
absorbance.
b. The test tube will be wiped clean at each 25 intervals before placement
into the compartment.
4. Nine 25 nm intervals will measure.
In order to achieve the highest wave wavelength absorbance of concentration
from our dilutions in this experiment, we need to obtain the constant, k, at
maximum absorbance wavelength and then use this constant for our
concentration dilutions. One test tube containing only CoCl2 0.15M will not be
diluted and this will be the test tube that we will use to obtain our maximum
wavelength absorbance constant. Measurement of absorbance at intervals of
25 nm between 400 and 600 nm will be used to obtain maximum absorbance
constant. The Spectrophotometer is zeroed out because you want only the
compound itself to be the absorbing species
Procedure to obtain absorbance as a function of concentration
A. Standard solution dilution set up
1. Eight test tubes are utilized.
2. 0.150 M CoCl2 solution of 5 ml is pipette into the first test tube ( # 1) and
the remainder of the 7 test tubes will receive respectively 4.0 ml, 3.5 ml , 3.0
ml, 2.5 ml, 2.0 ml, and 1.0 ml.
3. Distilled water will be Eppendorf pipette into these 7 test tubes.
a. The first tube (# 1) will not be diluted.
b. Test tube # 2 will receive 1.0 ml,
c. The other test tubes consecutively will be pipetted 1.5 ml, 2.0 ml, 2.5 ml, 3.0 ml
and 4.0 ml according to the following table
______________________________________________________________
Test Tube
0.150 M CoCl2
Distilled water
Solution ( ml)
(ml)
_____________________________________________________________
1.
5.0
0
2.
4.0
1.0
3.
3.5
1.5
4.
3.0
2.0
5.
2.5
2.5
6.
2.0
3.0
7.
1.0
4.0
Absorbance Spectrum of 0.150 M CoCl2 for the 7 test tubes
1. Calibrate the spectrophotometer at the highest spectrum
500 nm ( spectrophotometer should have been calibrated for 100%
transmittance and zero absorbance before testing for absorbance
can be done.
2. Set the mode for absorbance and field to 340 nm to 599 nm.
3. Each test tube is be cleaned before being placed into the sample
compartment.
Procedure for solution mixture of the concentration of the unknown.
A. 1. Four test tubes will be used .Mixture of unknown and dilution with H 2O
2. Test tube # 1 pipette 3.5 ml/1.5 H2O, # 2 , 3.0 ml/2.0 ml, 2.5 ml/2.5, 2.0
ml/3.0 ml.
B. 1. The four test are tested for absorbance spectrum
Test tube unknown number: three
_______________________________________________
ml of Unknown
ml of H20
_______________________________________________
3.5
1.5
3.0
2.0
2.5
2.5
2.0
3.0
(II) Experimental Data
A. Results of absorption of aqueous CoCl2 for maximum absorption.
(nm)
A
(nm)
A
(nm)
A
400
0.54
475
0.345
550
0.223
425
0.101
500
0.396
575
0.091
450
0.225
525
0.375
600
0.052
As the graph will indicates maximum absorbance will occur at
(Y) Absorbance 0.4 and ( X) Nano Meters 500. The concentration is fixed, the
change in absorbance indicates that K constant from Beer’s law is dependent
on wavelength. This is in agreement with Beer’s law that states absorbance
is directly related to the concentration of the substance.
B. Results of absorption for the 8 test tubes
Test tube No.
1.
2.
3.
4.
5.
6.
7.
A
0.396
0.345
0.313
0.289
0.243
0.213
0.101
c
0. 150
0.120
0.105
0.090
0.075
0.060
0.030
The known CoCl2 six test tube diluted concentrations will gives the numerical light
absorbance. These numerical values are then plotted and we then use linear
Regression to explain two variables, the dependent variable (Absorbance) and the
independent variable Molar concentration) ( Yale .ed 10/21/2011) .
The equation for y = mx +b (straight line) can be transposed to the Beer’s law
formula where y = A, x= c, m = slope = k, b= intersects with y axis = 0. Straight line
origins from (y= 0) and x =0 because the slope of the line, which is k, should pass
through the straight line of regression, k= A/c. The Beer’s law can only be used in
dilute solutions ( Columbia University Press 2007)
Trial solutions for the unknown with absorbance values.
ml of unknown
ml of H2O
A
____________________________________
1
3.5
1.5
0.192
2.
3.
4.
3.0
2.5
2.0
2.0
2.5
3.0
0.156
0.138
0.113
(III)
Order of Calculations to obtain the K constant
K=A/C
C=concentration
A=absorbance
1. To obtain molar concentration : Of known concentration by using
M1V1=M2V2
_________________________________________
Tube # 0.150 M CoCl2
H20
Concentration
Solution ( ml)
(ml)
M1V1/V2=M2
_________________________________________
1.
5.0
0.0
0.150
2.
4.0
1.0
0.120
3.
3.5
1.5
0.105
4.
3.0
2.0
0.090
5.
2.5
2.5
0.075
6.
2.0
3.0
0.060
7.
1.0
4.0
0.030
2. To obtain constant K we use the formula ∑cA/∑c2
Test tube
Absorbance
Concentration
CxA
C2
___________________________________________________________
1.
2.
3.
4.
5.
6.
7.
0.396
0.345
0.313
0.289
0.243
0.213
0.101
0.150
0.120
0.105
0.090
0.075
0.060
0.030
K= 0.1935/0.0661= 2.927 M-1
0.0594
0.0414
0.0328
0.0260
0.0182
0.0127
0.0030
_______
TOTAL 0.1935
0.0225
0.0144
0.0110
0.0081
0.0056
0.0036
0.0009
_____
0.0661
.
2. To obtain the concentration value of the unknown test tube use the flowing
formula:
C=A/K
Test tube
ml
ml of H2O
A
C=A/K
___________________________________________________
1
2
3
4.
3.5
3.0
2.5
2.0
1.5
2.0
2.5
3.0
0.192
0.156
0.138
0.113
0.0655
0.0532
0.0471
0.0386
To determine the concentration of C2 : C1V1/ V2= C2
3. To obtain the C2 molar concentration value from the formula :
C1V1/V2 =C2
a. Calculate average C2
1.
2.
3.
4.
(0.0655)(5)/ 3.5
(0.0532)(5)/3.0
(0.047)(5)/2.5
(0.0386)(5)/2.0
= 0.0935
= 0.0886
= 0.0940
= 0.0965
______
0.3726 ÷ 4 =0.0931
(III)Results and Discussions
The concentration determination of the unknown which was 0.100 was determined
from the average of the four ( C2 )diluted concentrations. Where our absorbance
values with agreement with the Beer’s law (equation 4) The decrease in numerical
absorbance order of the unknown concentration 0.192, 0.156, 0.138, and 0.113
was indeed agreement with Beer’s law, concentration is directly proportional to
absorbance. The absorbance values that we plotted on the graph that we obtained
from the 0.150 molarity Cobalt ( II) Chloride concentration was in agreement with
Beer’s law. The graph showed that absorbance values were plotted within range of
straight line therefore the K value reflected the Beer’s law and therefore our errors
dilution were not that major.
The precision of our data of (C2 ) is within an average curve (graph 2). I used a
standard deviation formula. I realize that have only four variables for the formula but
I wanted to know if my standard deviation was large or small. My calculation showed
that my ( C2) was small .003.
1.
2.
3.
4.
(.0935-.0931) = (.0004)2 =1.6 x10-5
(.0886-.0931) = (-.0045)2=2.05x10-5
(.094-.0931) = (.0009)2=8.1x10-7
(.0965-.0931)= (.0034)2=1.156x-5
4.862x10-5 ÷4 = 1.2155x10-5
=.003
I had hoped that a team approach could have been utilized on this project to
decrease systematic errors. My lab partners approached the project as lone
individuals and therefore we had unnecessary misunderstandings. They failed to
realize that the field of Chemistry is a team approach.
(IV)
Conclusion
The scope of experiment utilized the Beer’s law to determine
concentration. The many parts make up the whole in an experiment and
chemical light absorbance played a key role in determining concentration
gradient.
A value of K constant was obtained, 2.927 M-1 from the Cobalt (II) Chloride
concentrations. Four cuvettes concentrations were obtained, 0.0655,
0.0532, 0.0471, and 0.0386. These four concentration values were then
were used in formula which gave us the undiluted C2 value: 0.0935,
0.0886, 0.094, and 0.0965. The average of these four numbers is 0.0931.
The unknown test tube number three concentration was selected to be
within the range of the standard and closest to the average at 0.100M.
The experiment showed that use of Spectrophotometry was possible to
give us wavelength values of a chemical of known concentration which we
then used in our formula K=A/C. Two graphs were utilized to determine
maximum absorbance 500 nm and other to determine if our K value was
the within straight line slope.
References:
1. http://.harpercollege .edu/tmps/chm/100/dgodambe/thedisk/labtech/example.htm 10/11/2011. ( Beer’s
Law Example)
2. Houghton Miffin Company, The absorption Spectrum of Cobalt (II) Chloride, (
Experimental requirements were obtain from this source )
3. Columbia University Press, www.cc.columbia . edu/cu/cup ( 2007)
4. http://.stat.yale.edu/Course /1997-98/101/linreg,htm