S P E C T R O S C O P Y

Marie Giecel Dela CruzAlma Lipnica Pabilane

KEYWORDS

Spectroscopy Absorbance Transmittance Beer-Lambert’s Law Electromagnetic Spectrum Wavelength, Frequency

INTRODUCTION

Spectroscopy is a general term for the science that deals with the interactions of various types of radiation with matter.

The interactions of interest were between electromagnetic radiation and matter

INTRODUCTION

INTRODUCTION

Visible light, X rays, microwaves, radio waves and so forth are all different kinds of electromagnetic radiation

The spectrum is divided into regions, with the visible region accounting for only a small portion, from 380 nm to 750 nm

This visible region is flanked by the infrared and ultraviolet regions.

INTRODUCTION Electromagnetic radiation is said to have a

dual behavior: particle and wave Therefore, it is characterized by

wavelength, frequency, and amplitude The wavelength, λ (Greek lambda), is the

distance from one wave maximum to the next

The frequency, ν (Greek nu), is the number of waves that pass by a fixed point per unit time, usually given in reciprocal seconds (s-

1) or hertz, Hz (1 Hz = 1 s-1)

INTRODUCTION The amplitude is the height of a wave,

measured from the midpoint to the peak Multiplying the wavelength of a wave in

meters (m) by its frequency in hertz (s-1) gives the speed of the wave in meters per second (m/s) : v λ = speed

The rate of travel of all electromagnetic radiation in a vacuum is a constant value, “speed of light”, denoted by c, and has an exact value of 2.99792458 x 108 m/s, often rounded to 3.00 x 108 m/s

INTRODUCTION

In this experiment, absorption spectroscopy is used.

At certain wavelengths, a normally transmitting medium will absorb light of some wavelength and transmit the others.

The photon’s energy is converted into an atomic or molecular process: excitation of electrons, vibration, rotation etc.

INTRODUCTION

The molecule’s energy is increased with exactly the same amount of energy of the photon it absorbed.

M + hv M*M* is the molecule’s excited state A photon whose energy is not equal to

the difference between 2 energy states will not be absorbed

OBJECTIVES

To prepare the absorbance spectrum of colored species;

To determine the concentration range for maximum precision; and,

To determine by spectroscopy the concentrations of colored species in a mixture.

METHODOLOGY

One hundred milliliters each of 0.001 M KMnO4 and 0.001 M K2Cr2O7 were prepared.

10-mL solutions were prepared from the 0.001 M KMnO4 using the following dilution ratio of H2O:KMnO4: 10:0, 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, 1:9

Prior to usage, the spectrophotometer was warmed for 20 mins and then, the wavelength was adjusted to 545 nm.

METHODOLOGYThe cuvette containing reference liquid was inserted and the absorbance was zeroed and the transmittance was set to 100%.

The absorbance of the solutions containing different H2O:KMnO4 ratio were recorded. The concentration in molarity and in ppm were computed.

The concentrations were plotted against absorbance and %transmittance. The concentration range for max precision was determined. The absorbance of the unknown at 545 nm was also measured..

METHODOLOGY

A 2:8 K2Cr2O7:H2O solution was prepared. The absorbance of this was measured at 440 nm and at 545 nm.

The absorption of the unknown and the 2:8 KMnO4:H2O solution at 440 nm were also measured.

The concentrations of Mn and Cr in the unknown were computed from the readings.

RESULTSDilution MnO4 ppm Mn A T %T

1:9 1 x 10-4 5.494 0.208 0.6194 61.94

2:8 2 x 10-4 10.988 0.489 0.3243 32.43

3:7 3 x 10-4 16.402 0.749 0.1782 17.82

4:6 4 x 10-4 21.976 1.019 0.09572 9.572

5:5 5 x 10-4 27.470 1.285 0.05188 5.188

6:4 6 x 10-4 32.968 1.541 0.02877 2.877

7:3 7 x 10-4 38.458 1.772 0.01690 1.690

8:2 8 x 10-4 43.952 2.050 0.08913 0.8913

9:1 9 x 10-4 49.446 2.229 0.05902 0.5902

RESULTS

Concentration Range for Maximum Precision (ppm)

10 – 22 ppm

Correlation Coefficient, r 0.9989

Absorbance of Unknown 0.449

Concentration of Unknown (ppm Mn) 9.698

RESULTS

0 10 20 30 40 50 600

0.5

1

1.5

2

2.5

Concentration vs Absorption

Concentration Mn (ppm)

Ab

so

rpti

on

Figure 1. Plot of concentration of Mn (in ppm) versus the absorbance

RESULTS

0 10 20 30 40 50 600

20

40

60

80

100

120

Concentration vs Transmittance

Concentration Mn (ppm)

Tra

nsm

itta

nce

(%)

Figure 2. Plot of the concentration of Mn (in ppm) versus the transmittance (in %)

RESULTS

Sample Computations:A = εbc where slope=εbBased from the linear regression of the data, εb = 0.0463 Concentration of the unknown:

RESULTS Spectrophotometric Determination of

Mn and Cr in a Mixture  Known Unknown

Mn Cr Mn + Cr

A at 545 nm 0.489 0.016 0.449

A at 440 nm 0.031 0.089 0.180

 

Concentration of Mn (ppm)

9.471

Concentration of Cr (ppm) 35.82

RESULTS

Sample ComputationsA440 = A440,Mn + A440, Cr A440 = εbcCr + εbcMn

A540 = A540,Mn + A540,Cr A540 = εbcCr + εbcMn

RESULTS

Plug in the known values to the equations (b=1cm, and absorbance A) and solve for the concentration of Mn and Cr.

The experimental results were:CMn = 9.471 ppm

CCr = 35.82 ppm

a and ε are both absorptivity. ε is molar absorptivity, used when c is in M and b is in cm.

DISCUSSION Absorption Spectroscopy Transmittance (T)

Attenuation of a beam of radiation by an absorbing solutionDue to interactions of photons and absorbing particles, the power of the beam is attenuated from Po to P.

DISCUSSION Transmittance of the solution is the fraction of

the incident radiation transmitted by the solution.

It is given by,T = P/Po %T = (P/Po)x100

Absorbance is given by,A = -log T = log(1/T) = log(Po/P)

Absorbance is directly proportional to path length b and concentration c, and is a log function of T;*as seen in in the plot of %T vs. conc, the graph is logarithmic: as concentration increases, %T decreases*as seen in the plot of A vs. conc., the graph is linear: as concentration increases, absorbance increases

DISCUSSION

Other equation for Absorbance (A)A = abc

where a is a proportionality constant called absorptivity

When b is in cm and c in molarity (mol/L), absorptivity is in L mol-1cm-1 and denoted by

The equationsA = abc and A= are the statements of

Beer-Lambert-Bouger Law or simply Beer’s Law

DISCUSSION

For mixtures, Beer’s Law can still be used.

A = 1bc1 + 2bc2 + 3bc3 + … + nbcn

where 1, 2,…,n corresponds to the different composition of the mixture.

In the experiment, an unknown mixture of KMnO4 and K2Cr2O7 was subjected to absorption spectroscopy to measure its absorbance at 545nm and 440 nm.

DISCUSSION

The absorbance measured was compared to the absorbance of the standard 0.001 M K2Cr2O7 and 0.001 M KMnO4

The absorptivity (a) of the 2 components were calculated a = A/concentration

(a = varies on different wavelengths)

DISCUSSION

This was substituted to the two equations, one for 440 nm and another for 545 nm

A440 = εbcCr + εbcMn

A540 = εbcCr + εbcMn

System of equations was used to calculate for the concentrations of KMnO4 and K2Cr2O7 in the mixture

Also, Cramer’s Rule can also be used

DISCUSSION

The violet color of the Mn is observed because it absorbs the color green ( = 500-560 nm). This would explain why Mn has higher absorbance at = 545 nm.

Chromium absorbs the color blue (accounting for its yellow color) which occurs at = 435-480 nm; therefore, Cr should register a higher absorbance reading at = 440 nm than at = 545 nm.

DISCUSSION

A substance exhibits a color which has a wavelength opposite to the wavelength of the color it absorbs

CONCLU./RECOMM.

Spectroscopy is an efficient method that can be used in both quantitative and qualitative analyses

It is more efficient when used in organic compounds because all organic compounds absorb EM radiation

Many inorganic compounds do not absorb EM radiation, and they first need to be transformed into absorbing species

CONCLU./RECOMM.

Beer-Lambert’s Law relates Absorbance to concentration of the analyte, path length and absorptivity.

Absorbance is dependent on concentration (directly proportional)

Absorptivity is constant for a certain wavelength and different in another wavelength

The concentration range for max precision is estimated by getting the most linear portion of the graph

CONCLU./RECOMM.

Keep the KMnO4 in a colored container to avoid its photodecomposition

Make sure that the measurements to be done would be as accurate as possible (i.e. weighing etc.)

Answers to Questions How does absorbance vary with

concentration?As concentration increases, absorbance

increases linearly. If plot A against C is extrapolated, will it

intersect the point of origin?Yes because the absorbance was zeroed at

the initial concentration (blank). How does % transmittance vary with

concentration?As concentration increases, % transmittance

decreases, logarithmically.

Answers to Questions

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

In Figure 1, it was established that as concentration increases, absorbance also increases linearly.

From the equation, A = -logT. This would explain the logarithmic decrease of the transmittance as the concentration of the substance is increased.

Answers to Questions

A 20.0ppm solution of a colored compound gives a 70.0% transmittance in a 1.00cm cell. Find the absorptivity of the solution.

c = 20.0 ppmT = 0.700b = 1.00 cm

Answers to Questions A solution of a colored species gives 25%

transmittance. What will be the % transmittance of the solution whose concentration is 3 times that of the given? Assume linearity of response over the given concentration range.

REFERENCES

Bhondwe, A. "Molar Absorptivity." Buzzle Web Portal: Intelligent Life on the Web. 3 Jan. 2011. Web. 03 Mar. 2011. <http://www.buzzle.com/articles/molar-absorptivity.html>.

Hargis, L. (1988). “Analaytical Chemistry”. Prentice-Hall. New Jersey.

Skoog, D. et al. “Introduction to Spectrochemical Methods”. Fundamentals of Analytical Chemistry, 8th ed. 2004. Brooks/Cole – Thomson Learning: California. pp 710, 715-716, 718, 720, 730.

THE ENDThanks for listening!:)

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