Absorbance and Transmittance

8/2/2019 Absorbance and Transmittance

1/38

Absorption Spectroscopy in the visible region is considered to be one of the

oldest physical methods used for quantitative analysis and structural elucidation.

Spectrometer is mainly used for quantitative analysis and serves as a useful auxiliary

tool for structural elucidation. Analytical application of absorption of radiation by matter

can be either qualitative or quantitative. The qualitative application of absorption

spectrometry depends on the fact that a molecular species absorbs radiation only in

specific regions of the spectrum where the radiation has the energy required to raise

the molecule to some excited state. A display of absorption versus wavelength or

frequency is called an absorption spectrum of that molecular species and serves as a

Fingerprint for identification. The wavelength of visible radiation starts at 8000 and

ends at 4000 . The main types of instruments are use for measuring the emission or

absorption of radiant energy from substance is called by various names such as

Photometers, Colorimeters and spectrophotometers.

Photometer:-

It is an instrument which measures the ratio of some function of two

electromagnetic beams. This is an inexpensive instrument employing a filter to isolate

a narrow wavelength region and photocell or photometer to measure the intensity of

radiation.

Spectrophotometer:-

The instrument measures the ratio of a function of the two of the radiant power

of two electromagnetic beams over large region. In this instrument a monochromatic

radiation is used instead of a filter. A monochromator allows a large wavelength region

1


2/38

to scan. In addition a spectrophotometer employs most secretive, detectors like

photometer or photomultipliers.

Colorimeters:-

Any instrument used for measuring absorption in the visible region is gradually

called as Colorimeter. In fact some commercial filter photometers are called

Colorimeters.

I.1: Theory of Spectrophotometer:-

When monochromatic or heterogeneous light is incident upon homogeneous

medium a part of incident light is reflected, a part is absorbed by the medium and the

reminder is allowed to transmit. If Io denotes the incident light, Ir the reflected light, I

the absorbed light and It the transmitted light, then we can write,

I0= I + It + Ir -----------------------(1)

If a comparison cell is used, the value of Ir which is very small can be

eliminated for air-glass interfaces, under this condition equation (1) becomes as

I0= I + It ----------------------- (2)

Bouger actually investigated the range of absorption of light with the thickness

of medium. But this credit was enjoyed by Lambert who simply explains the concepts

developed by Bouger. Beer later applied Lamberts concept to solution of different

concentrations and reported his results just power to those of Bernard. However two

laws governing absorption are generally known as Lamberts and Beers law. We will

discus this one by one.

2


3/38

Lamberts Law:-

This law1,2 can be studied as When a beam of light is allowed to pass through a

transparent medium, the ratio of decrease of intensity with the thickness of the

medium is directly proportional to the intensity of the light.

Mathematically the Lamberts law may be studied as follows

-di / dt I

OR

-di / dt = KI ------------------ (3)

Where I denotes intensity of the incident light of wave length, It denotes the thickness

of medium and K denotes the proportionality factor, on integration equation (3) and

putting I = Io when t = 0 , we get

ln Io / It = Kt

It = Io e-kt --------------------(4)

Where Io denotes the intensity of incident light, Iv denotes the intensity of

transmitted light and K is a constant which depends on the wavelength and absorbing

medium used. On changing equation (4) from natural to common logarithms, we get

It = Io . 10-0.4343kt

= Io . 10-kt

3


4/38

Where K = k / 2.3026 --------------

(5)

In equation (5) K is absorption coefficient which is defined as The reciprocal of

thickness the light to 1 /10 0f its intensity.

The above definition follows from equation

It / Io = 0.1 = 10-kt

or Kt = 1

or K 1/t ------------- (6)

The ratio It / Io is termed as the transmittance T and log 1/T is termed as

absorbance A of the medium. The ratio Io / It is termed as Optical density.

So that

A = log Io / It -------------- (7)

Beers law:-

Lamberts law shows that there exists a logarithmic relationship between the

transmittance and the length of the optical path through the sample.

4


5/38

Beer1,2 observed that a similar relationship holds between transmittance and

concentration of the solution, i.e the intensity of a beam of monochromatic light

decreases exponentially with the increase concentration of absorbing substance

arithmetically.

Thus equation (4) becomes as

It = Io e-kc

= It .10-0.4343kt ---------------- (8)

= Io. 10-Kc ----------------(9)

Where k and K are constants and is concentration of the absorbing substance of the

absorbing substance combining equations (5) and (9) we get

It = Io .10-act

Or

log Io / It = act ------------------ (10)

Equation (5) is termed as mathematical statement of Beer- Lamberts law. This is also

a fundamental equation of spectrophotometer.

In equation (10) the value of a depends on the unit of concentration. If C is

expressed in mole dm-3 and in centimeters, then a is replaced by symbol and is

termed as the molar absorption coefficient or molar absorptivity.

5


6/38

It is important to remark here that exist a relationship between the absorbance

A, the transmittance T, and the molar absorption coefficient ,

i.e. A = ct

= log Io / It

= log 1 / T

= - log T --------------------(11)

In spectrophotometers, the scales are calibrated and the absorbance is read directly.

I.2: Deviations from Beers Law:-

From Beers law it follows that if we plot absorbance against concentrations a

straight line passing through the origin should be obtained in figure 1.1. But there is

usually a linear relationship between concentration and absorbance and an apparent

failure of Beers law may ensure. Deviation from the law is reported positive or

negative according to whether the resultant curve upward or concave downwards.

I.3: Deviation from Beers law can arise due to following factors:-

1. Beers law will hold over a wide range of concentrations provided the structure

of coloured ion or of coloured non-electrolyte is the dissolved state does not

change with concentration. If a coloured solution is having foreign substance

whose ions do not react chemically with that coloured components, its small

6


7/38

concentration (foreign substance) does not affect the light absorption may affect

extinction coefficient.

2. Deviations may also occur if the coloured solute varies, dissociates or

associates in solution.

3. Deviations may also occur due to the presence of impurities that fluoresce or

absorb at absorption wavelength. The interference introduce an error in the

measurement of absorption of radiation penetrating the sample.

4. Deviations may occur if monochromatic light is not used.

5. Deviations may occur if the width of slit is not proper and therefore, it allows

undesirable radiations might be absorbed by impurities present in the sample.

The magnitudes of two deviations becomes appreciable at higher

concentrations.

6. Deviations may occur if the solutions species undergoes polymerization.

7. Beers law can not be applied to suspensions but the lather can be estimated

calorimetrically apply preparing a reference curve with known concentrations.

I.4: The review of work done in absorption measurements:-

7


8/38

Measurements have been made of visible region absorption spectrum of a

dichloric dye in the chiral nematic host by H.S.Cole, Jr. and S. Aftergut3 . Data are

replaced for a range of conditions of boundary state birefringence and pitch.

Intrinsic polarized ultraviolet absorption of crystalline tetragonal Geo2 are

studied by M.Stapllroek and B.D.Evans4 for in the range 5-103 /cm at room

temperature and below sharpline structure strongly polarized. The optical absorption

spectra from 5000 to 30,000 /cm of single crystals of chromium chloride have be

studied from 300 to 6k by D.R.Rosseinsly and I.R.Dorrily5 . The ultraviolet absorption

spectra of MBBA is the multi stable solids, the nematic and isotropic liquid states havebeen measured by M.Mizuno and T. Shinoda6 . Further more the spectra of dilute

solutions and linear dichroisum spectra of nematic single liquid crystal in

homogeneous orientation have also been observed. The temperature dependence of

absorption of diacetylene chains dispersed in partially polymerized monomer matrices

has been measured by D.Bloor and C.L. Hubble7).Over the range from 2 to 380 k for

number of different monomers. The results are interpreted in terms of the effects of

the lattice environments on the polymer chains. It is shown that in general the polymer

chain length does not have a major effect on the absorption spectrum.

The optical number of organic compounds have been examined by

W.c.mcColgin and etal8, In low temperature glassy solutions. According to the

experimental conditions of excitation a given sample can yield either the usual broad

bands complete with stokes shift or a set of very narrow fluorescence lines,

comparisons of these two distinct type of spectra from the same sample make it

possible to explain such a features of the convention all spectra as their broad band

8


9/38

width peak positions and stokes shifts. The absorptivity of carefully purified water have

been measured by T.I.Quicken and J.A. Irvin(9) at 1 nm intervals in wavelength range

196 to 320 nm.

2.1: Absorbance and Transmittance Measurements:-

Figure 2.1 shows diagrammatically the measurement of absorbance in

Cuvette(10)

lying vertically. The cuvette is longitudinal to the laser beam. A very

simple apparatus is developed using cuvette detector and laser. The detector

used here is light detecting resistor. Fig 2.2 shows the circuit diagram for

detecting light in terms of current. Through empty cuvette, laser beam is

passed and intensity of laser beam is noted in terms of current Io. Cuvette is

filled with acqueous solution and transmitted intensity of laser beam is noted

in terms of current as It. Noting Io & It for various length of solution in

cuvette ; Absorbance & transmittance have been estimated. Length of liquid

is varied. Concentration dependence of absorbance & transmittance also

found out for various concentration of methyl orange in distilled water. The

absorbance and transmittance have been studied in 0.1%, 0.2%, 0.5%, 0.7%,

1%, 1.2%, 1.4%, 1.6%, 1.8%, and 2% acqueous solution of methyl orange.

2.2: materials & preparation of solution:-

In problem methyl present orange is selected, for the preparation

of acqueous solution methyl orange is of A R grade. Supplied by Fishery

9


10/38

Industries. Acqueous solution of methyl orange is prepared for0.1% , 0.2% ,

0.5% , 0.7% , 1% , 1.2%, 1.4% , 1.6% , 1.8% , and 2% acqueous solution of

methyl orange in distilled water. The absorption and transmittance of all

concentration have been studied at room temperature 30.2 C0 using the

procedure described in article 2.1.

10


11/38

2.3: density measurement:-

Density measurement are carried out using specific gravity bottle,

electronic balanced with accuracy of 0.01 gm has been used for weighing

empty and filled specific gravity bottle with various concentration of

acqueous solutions of methyl orange. Density measurements are carried out

at room temperature 30.2 0C

2.4: precautions taken during measurements:-

Following precautions must be observed using the absorption and

transmittance measurement.

i. Instrument should be placed in a clean and dust free environment.

ii. Instrument must be installed at a place free from vibration and light.

iii. It should be always covered with dust cover when not in use.

iv. Only the matched cuvette should be use

v. The sample holder should be cleaned before use to obtained best

results.

11


12/38

vi. The instrument should not be used in presence of inflammable gasses.

12


13/38

The absorption measurement technique has been already discussed in chapter-

II and all the results obtained during this study are presented in table 3.1 to 3.12 and

figure 3.11 to 3.12. table shows the experimental values obtained during the

absorption and transmission study in aqueous solutions of methyl orange. Figure

shows variation of absorbance and transmission of laser beam through the aqueous

solutions of methyl orange.

13


14/38

Table - 3.1:

Variation of Transmittance & Absorbance with length for

0.01 % methyl orange aqueous solution

The intensity of incident light Io = 500 A

Density = 1.7088 gm/ml

ObsNo

Length (cm) Current (A) Transmittance Absorbance

1 01 490 0.98 0.0087

2 02 470 0.94 0.026

3 03 460 0.92 0.036

4 04 440 0.88 0.055

5 05 430 0.86 0.065

6 06 420 0.84 0.075

7 07 410 0.82 0.086

8 08 400 0.80 0.96

9 09 390 0.78 0.107

10 10 380 0.76 0.119

14


15/38

Table - 3.2:





ObsNo


1 01 480 0.96 0.0177

2 02 470 0.94 0.026

3 03 450 0.90 .045

4 04 440 0.88 .055

5 05 420 0.84 0.075

6 06 400 0.80 0.096

7 07 380 0.76 0.119

8 08 370 0.74 0.130

9 09 360 0.72 0.142

10 10 350 0.70 0.154

15


16/38

Table - 3.3:





ObsNo


1 01 470 0.94 0.0268

2 02 460 0.92 0.0362

3 03 440 0.88 0.055

4 04 430 0.86 0.0655 05 410 0.82 0.086

6 06 390 0.78 0.107

7 07 370 0.74 0.130

8 08 360 0.72 0.142

9 09 350 0.70 0.154

10 10 340 0.68 0.167

16


17/38

Table - 3.4:





ObsNo


1 01 450 0.90 0.0457

2 02 400 0.80 0.0969

3 03 330 0.66 0.180

4 04 300 0.60 0.221

5 05 250 0.50 0.301

6 06 210 0.42 0.3767 07 180 0.36 0.443

8 08 150 0.30 0.522

9 09 130 0.26 0.585

10 10 100 0.20 0.698

17


18/38

Table - 3.5:


1 % methyl orange aqueous solution



ObsNo


1 01 300 0.60 0.221

2 02 280 0.56 0.251

3 03 250 0.50 0.301

4 04 220 0.44 0.3565 05 210 0.42 0.376

6 06 190 0.38 0.420

7 07 180 0.36 0.443

8 08 140 0.28 0.552

9 09 120 0.24 0.619

10 10 90 0.18 0.654

18


19/38

Table - 3.6:





ObsNo


1 01 290 0.58 0.23

2 02 270 o.54 0.26

3 03 240 0.48 0.31

4 04 220 0.44 0.35

5 05 190 0.38 0.42

6 06 180 0.36 0.44

7 07 150 0.30 0.52

8 08 130 0.26 0.58

9 09 110 0.22 0.65

10 10 80 0.16 0.79

19


20/38

Table - 3.7:





Obs No Length (cm) Current (A) Transmittance Absorbance

1 01 280 0.56 0.25

2 02 250 0.50 0.30

3 03 230 0.46 0.33

4 04 210 0.42 0.37

5 05 180 0.36 0.44

6 06 160 0.32 0.497 07 140 0.28 0.55

8 08 120 0.24 0.61

9 09 110 0.22 0.65

10 10 95 0.19 0.73

20


21/38

Table - 3.8:





ObsNo


1 01 260 0.52 0.28

2 02 240 0.48 0.31

3 03 220 0.44 0.35

4 04 200 0.40 0.395 05 170 0.34 0.46

6 06 150 0.30 0.52

7 07 130 0.26 0.58

8 08 110 0.22 0.65

9 09 90 0.18 0.74

10 10 80 0.16 0.79

21


22/38

Table - 3.9:





ObsNo


1 01 240 0.48 0.31

2 02 235 0.47 0.32

3 03 210 0.42 0.37

4 04 190 0.38 0.42

5 05 150 0.30 0.52

6 06 140 0.28 0.55

7 07 120 0.24 0.61

8 08 100 0.20 0.69

9 09 80 0.16 0.79

10 10 70 0.14 0.85

22


23/38

Table - 3.10:


2 % methyl orange aqueous solution



ObsNo


1 01 210 0.42 0.37

2 02 190 0.38 0.42

3 03 150 0.30 0.52

4 04 115 0.23 0.63

5 05 110 0.22 0.65

6 06 80 0.16 0.797 07 70 0.14 0.85

8 08 25 0.05 1.30

9 09 20 0.04 1.39

10 10 10 0.02 1.69

23


24/38

Table - 3.11:


all concentrations of methyl orange aqueous solutions

Length =1 cm.

ObsNo

Concentration (%)

Densitygm/ml

Current(A)

Absorbance Transmittance

1 0.01 1.7088 490 0.087 0.98

2 0.2 1.7112 480 0.0177 0.96

3 0.5 1.7136 470 0.0268 0.94

4 0.7 1.7180 450 0.0457 0.90

5 1 1.7201 300 0.221 0.60

6 1.2 1.7228 290 0.23 0.58

7 1.4 1.7251 280 0.25 0.56

8 1.6 1.7284 260 0.28 0.52

9 1.8 1.7304 240 0.31 0.48

10 2 1.7324 210 0.37 0.42

24


25/38

25


26/38

26


27/38

27


28/38

28


29/38

29


30/38

30


31/38

31


32/38

32


33/38

33


34/38

34


35/38

35


36/38

36


37/38

Study of tables & figures shows that as the length of the liquid medium is

increased in cuvette the absorbance is increased, at same time the variation of

transmittance decreasing as the length of the medium is in the cuvette increased.

Figure show that there is linear increase and decrease of absorbance and

transmittance of laser light in acqueous solution of methyl orange.

Figure 3.11 shows variation of absorbance with concentration of acqueous

solution of methyl orange. According to Beers it seems that this dependence of

absorbance should linear with concentration but this solution does not seems to verify

the Beers law. Study of fig 3.11 shows that in the region 0 - 0.6% of acqueous solution

of methyl orange verifies Beers law but after that from 0.8 2% concentration Beers

law does not verified. That is only at low concentration Beers law is verified.

The over all study of the results shows that for each concentration of acqueous

solution of methyl orange, the length of the medium increases absorbance increases

and transmittance decreases. That is absorbance of light is directly proportional to

length and concentration of liquid in cuvette, and transmittance is inversely

proportional to the concentration and length of liquid in the cuvette. One can not verify

Beers and Lamberts law in the high concentration region.

37


38/38

Absorbance and Transmittance

Documents

Transcript of Absorbance and Transmittance