Chem27.1 Spectroscopy

8/4/2019 Chem27.1 Spectroscopy

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CHIU, Ina Cathrina R.

SALACUP, Esther Eunice U.


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SPECTROSCOPYthe science in which the

interactions of radiation andmatter are used for qualitative

and quantitative analysis of

matter


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SPECTROPHOTOMETRY

a subset of spectroscopy which involves a

procedure that uses light interaction with

matter in order to measure the concentration

of a substance

SPECTROPHOTOMETERApparatus that measure the amount of

light absorbed at a given wavelength


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TRANSMITTANCE

Attenuation

Absorbing

solution ofconcentration c

Po P

Because of the interaction

between photons and the

particles of the solution, the

beam decreases from Poto

P.

Particles of the solutionabsorb electromagnetic

radiation.

Energy istransferred into

the moleculesof the solution.

Intensity ofradiation

transmitted outof the solutionis decreased.


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TRANSMITTANCE

the fraction of incident radiation transmitted by

the solution

Often expressed as percent transmittance

T = Transmittance

Po = transmitted intensity of light

P = incident transmission


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ABSORBANCE

measure of how much radiant power is absorbed by the

solution

Inversely related to transmittance

absorbance = transmittance

Directly related to concentration of absorbing species

concentration = absorbance


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BEERS LAW(absorption law)

A = absorbance = molar absorptivity (M-1 cm-1)

b = path length

c = concentration of absorbing species in M

Beers Law shows that there is a linear

relationship between absorbance and

concentration of absorbing species.


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Beers Law

For mixtures, the total absorbance is the sum of the

individual absorbance

Atotal = 1bc1 + 2bc2 + + nbcn


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Methodology

Prepare 100 mL of 0.001M KMnO4 and 0.001MK2Cr2O7 solutions.

In 20 mL test tubes, prepare (10) 10 mL solutions by diluting0.001M KMnO4 solution with distilled water in the following

ratio: 0:10, 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, 9:1

A. Determination of Concentration Range for Analysis of KMnO4

in solution


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Warm up the spectrophotometer for 20 minutes. Adjust thewavelength to 545 nm.

Insert the cuvette with the reference blank (distilled water) andset the absorbance (A) and transmittance (T) to 100%.

Read the absorbance of the prepared solutions, starting from theleast concentrated.

Measure the absorbance of the unknown solution.


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A 2:8 dilution ofKMnO4 soln at 440

nm

Prepare a 2:8dilution of K2Cr2O7(Procedure A) andread the A of the

soln at 545 nm and440 nm.

Measure the A of theunknown soln at 545

nm and 440 nm.

Calculate the

concentration ofMnO4

-1 and Cr2O7-2

in the mixture asppm Mn and ppm

Cr, respectively.

B. Spectrophotometric Determination of Mn and Cr in a Mixture


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Results and DiscussionDetermination of Concentration Range for Maximum Precision

Dilution Concentration Absorbance %Transmittance

[MnO42-] ppm Mn 545 nm 440 nm 545 nm 440 nm

0:10 0 0 0 0 100% 100%

1:9 1 x 10-4 M 5.494 0.165 0.010 68.4% 97.7%2:8 2 x 10-4 M 10.99 0.442 0.028 36.1% 93.8%

3:7 3 x 10-4 M 16.48 0.668 0.048 21.5% 89.5%

4:6 4 x 10-4 M 21.98 0.914 0.061 12.2% 86.9%

5:5 5 x 10-4

M 27.47 1.175 0.073 6.68% 84.5%6:4 6 x 10-4 M 32.96 1.425 0.089 3.76% 81.5%

7:3 7 x 10-4 M 38.46 1.651 0.094 2.23% 80.5%

2:8 8 x 10-4 M 43.95 1.877 0.126 1.33% 74.8%

1:9 9 x 10-4 M 49.95 2.130 0.140 0.741% 72.4%


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Sample computations

for 1:9 dilution of KMnO4


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Concentration range for max precision

Correlation Coefficient, r 0.9996

Absorbance of unknown 0.537

Concentration of unknown (ppm Mn) 13.14 ppm

Linear regression

- plot A (545 nm) vs. ppm Mn

m = b = 0.04382b = -0.038875

X = concentration of Mn in ppm

= 13.14 ppm


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Absorbance (545 nm) vs. ppm Mn

-0.5

0

0.5

1

1.5

2

2.5

0 10 20 30 40 50 60

A

bsorbance

ppm Mn


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%Transmittance (545 nm) vs. ppm Mn

0%

20%

40%

60%

80%

100%

120%

0 10 20 30 40 50 60

%T

ransmittance

ppm Mn


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Results and DiscussionDilution Concentration Absorbance %Transmittance

[Cr2O72-] ppm Cr 545 nm 440 nm 545 nm 440 nm

0:10 0 0 0 0 100% 100%

1:9 1 x 10-4 M 10.349 0.048 89.5%

2:8 2 x 10-4 M 20.798 0.002 0.095 99.5% 80.4%

3:7 3 x 10-4 M 31.198 0.003 0.142 99.3% 72.1%

4:6 4 x 10-4 M 41.597 0.007 0.185 98.4% 65.3%

5:5 5 x 10-4 M 51.996 0.230 58.9%

6:4 6 x 10-4

M 62.395 0.268 54.0%7:3 7 x 10-4 M 72.794 0.311 48.9%

2:8 8 x 10-4 M 83.194 0.354 44.3%

1:9 9 x 10-4 M 93.593 0.013 0.392 97.1% 40.6%


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Concentration range for max precision

Correlation Coefficient, r 0.9995

Absorbance of unknown 0.193

Concentration of unknown (ppm Mn) 44.52 ppm

Linear regression

-plot A (440 nm) vs. ppm Cr

m = b = 4.1804 x 10-3b = 6.8731 x 10-3

X = concentration of Mn in ppm

= 44.52 ppm


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Absorbance (440 nm) vs. ppm Cr

0

0.050.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0 20 40 60 80 100

A

bsorbance

ppm Cr


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%Transmittance (440 nm) vs. ppm Cr

0%

20%

40%

60%

80%

100%

120%

0 20 40 60 80 100

%T

ransmittance

ppm Mn


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KMnO4 = Max = 545 nm

K2Cr2O7 = Max = 440 nm

Beers Law can beapplied to solutions

containing more than

one kind of absorbing

substance provided thatthere is no interaction

among the various

species.

K2Cr2O7 KMnO4

A

Spectrophotometric Determination ofMn and Cr in a Mixture


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A1= 1 x bcx + 1y bcy (at 545 nm)A2= 2 x bcx + 2y bcy (at 440 nm)

Pagkatapos ng

nakakaiyak na pag-

iisolate...

LEGEND:

1 = A at 545 nm

2 = A at 440 nm

x = Mn

y = Cr


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Unknown Known

Mn (x) Cr (y) Mn + Cr

A at 545 nm (1) 0.537 1.4335

A at 440 nm (2) 0.193 0.146

Concentration of Mn (ppm) : 32.6630 ppm

Concentration of Cr (ppm) : 12.3949 ppm


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Calculating for (b)Mn,545 , (b)Cr,545,(b)Mn,440, and (b)Cr,440 :

Linear regression (plot A at 545 vs ppm Mn)

(b)Mn,545 = slope = 0.043827 ppm-1 cm-1

Linear regression (plot A at 440 vs ppm Mn)

(b)Mn,440 = slope = 2.7986 x 10-3 ppm-1 cm-1

Linear regression (plot A at 545 vs ppm Cr)

(b)Cr,545 = slope = 1.4466 x 10-4 ppm-1 cm-1

Linear regression (plot A at 440 vs ppm Cr)

(b)Cr,440 = slope = 4.1803 x 10-3 ppm-1 cm-1


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Concentration of Mn (ppm)

CMn = 32.663 ppm


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Concentration of Cr (ppm)

CCr = 12.3949 ppm


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Answers to Guide Questions

How does absorbance vary with concentration?Absorbance is directly proportional to concentration.

If the plot of A against C is extrapolated, will it intersect thepoint of origin?

Yes, it will only intersect the point of origin when there is noabsorbing species present in the sample. This was 0:10 solutionwhere in there was 0 ml of KMnO4 and 10 ml distilled water,which may be considered as a reference blank.

How does % Transmittance vary with concentration?

Transmission refers to how much light remains after it passesthrough. As the absorber concentration goes up, theabsorbance goes up, but the transmission and the percenttransmittance go down. In short, they have an indirectrelationship.


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How does curve of A vs. C compare with that obtainedby plotting %T vs. C? Explain the difference.

0

20

40

60

80

100

120

0 10 20 30 40 50 60

C

%T

%T

0

0.5

1

1.5

2

2.5

0 10 20 30 40 50 60

C

A A(Y)



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There is a linear relationship between absorbance

and concentration in dilute solutions. A plot of

absorbance vs. concentration gives a straight line at

a particular wavelength and temperature. On theother hand, percent transmittance is indirectly

proportional to the concentration, resulting in a

highly non-linear plot, an exponential plot in fact.



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A 20.0 ppm solution of a colored compound gives a 70.0%

transmittance in a 1.00cm cell. Find the absorptivity of the

solution.

A = abc *a for absorptivity, for molar absorptivity*

A = -log T

A = -log 0.7 = 0.1549

0.1549 = a (1.0 cm) (20.0 ppm)

a = 7.745 x 10-3

Solution:



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A solution of a colored species gives 25% transmittance. Whatwill be the % transmittance of the solution whose concentration

is three times that of the given? Assume linearity of response

over the given concentration range.

A = bc

A = -log T

%T = (T/To) x 100

Solution:%T = 25%

T = 0.25

A = -log 0.25

A = 0.602

A =b (3c) -> 3A [bec. of linearity]3A = 3(0.602) = 1.806 (A at 3x concentration)

10-1.806 = 0.0156 = T

25 % to 1.56% WOW! Anlaki ng pagbaba!

%T = T x 100 = 1.56%Final Answer!



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RecommendationsIt is important to remember not to have fingerprints on

the cuvette because fingerprints absorb and scatter light

slightly, even though they might not be readily visible.

The samples containing the standard solutions could be

photodecomposed, therefore, the vessels that contain the

standard solution must be amber bottles.


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RecommendationsWhen the solutions are placed in test tubes, as much as

possible, the sample must be exposed to minimal or

totally no light in order for the sample not to absorb the

light which could contribute to the reflection and

scattering losses, which are major error contributors to

the experiment proper.

Chem27.1 Spectroscopy

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