Determination of Absorption Coefficient of a Solution

Determination of Absorption Coefficient of a Solution by a Simple Experimental Setup Deepak Gopalakrishnana, C. Akhildeva, P. V. Sreenivasanb, K. K. Leelammaa, Lyjo K. Josepha and E. I. Anilaa,c a

Department of Physics, b Department of Chemistry, Union Christian College, Aluva- 683102.Kerala, India. c Corresponding author: E-mail: [email protected]

Abstract. The absorption coefficients of aqueous potassium permanganate (KMnO 4) solution at 638.8nm for various concentrations are determined using a simple experimental set up. The setup consists of He-Ne laser source (Red, 638.8 nm, 10mW), a glass jar in which the KMnO4 sample is taken, a mirror strip inclined at 450 to direct the laser beam towards the bottom of the glass jar, a traveling microscope to adjust the position of light dependent resistor (LDR) and a digital multimeter to measure the resistance. Keywords: absorption coefficient, uv-vis absorption spectrum, LDR. PACS: 78., 78.20.-e,78.20.Ci,78.40.Ha

INTRODUCTION

THEORY

Quantum physics developed through the first half of the twentieth century, largely by understanding how photons and matter interacted and inter-related. This was viewed however, as a study of the matter involved rather than the light involved. The concept of lasermatter interaction is now one of the rapidly evolving fields in the modern scientific world. When a beam of light i.e., a stream of photons is passed through a solution medium, some of the energy of these photons will lose, may be, during the collision between the photons and the particles in the solution. Evidently, we can say that there is absorption of light by the medium. This absorption is usually quantified in terms of absorption coefficient. There may be a variation in absorption coefficient with the frequency of the incident light, concentration of the solution, and temperature. L Sundar Rao studied the photodecomposition and absorption spectrum of KMnO4 [1]. J. Hodgkinson et al calculated the absorption coefficient of KMnO4 solution by photother‘mal detection using visible laser diodes [2, 3]. Here, this experiment is intended to study the variation of absorption coefficient with concentration of the solution.

The attenuation of light takes place according to the relation [4] I = I0 e-αt where I0 = incident light intensity, x = distance traversed by the beam in the medium and I = transmitted light intensity. α= absorption coefficient.

EXPERIMENTAL SETUP

FIGURE 1. Schematic diagram of the experimental setup.

The experimental set up is shown in Figure 1. Using a plane mirror inclined at 450 with the vertical, the laser beam is reflected towards the bottom of the glass jar containing the KMnO4 solution. The beam,

Optics: Phenomena, Materials, Devices, and Characterization AIP Conf. Proc. 1391, 372-374 (2011); doi: 10.1063/1.3643553 © 2011 American Institute of Physics 978-0-7354-0960-6/$30.00

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after interaction with the solution medium, falls on the LDR. The beam is scanned across its width by the LDR to get the maximum intensity position of the beam profile using a traveling microscope as shown in the diagram. Resistance across the LDR is measured using a digital multimeter. Absorption spectrum of the KMnO4 solutions with different concentration was taken using Shimadzu spectrophotometer.

RESULTS AND DISCUSSION Absorption coefficient is determined by measuring the incident and transmitted intensity of light for various lengths of traversal in the solution. The intensity of the laser beam is measured in terms of the reciprocal of the resistance of the LDR when positioned at the maximum of the beam profile. First, the LDR is set inside the jar at the bottom with no solution and the reciprocal of the resistance of the LDR corresponds to the intensity I0. For determining the absorption coefficient of the solution with a given concentration, the jar is filled with that solution to various heights and for each height x, the LDR is positioned just above the surface of the solution and the least resistance of the LDR while scanned across the transmitted beam profile is noted. The reciprocal of the resistance is taken as a measure of the transmitted intensity I(x) for distance x traversed through the solution. This is repeated for different concentrations. The variation of current (reciprocal of resistance) with height of the solution column for various concentrations is shown in Figure 2. Each graph shows an exponential decay of current with depth of solution.

FIGURE 3. ln (I0/I) Vs height of the solution. (a).0.04 mg/cm3 (b). 0.08 mg/cm3 (c). 0.12 mg/cm3 (d). 0.16 mg/cm3 (e). 0.20 mg/cm3 TABLE 1. Variation of absorption coefficient of KMnO4 with concentration Concentration Absorption coefficient D (cm-1) (mg cm-3) using laser from absorption spectrum setup 0.04

0.0766

0.0729

0.08

0.0987

0.1018

0.12

0.1397

0.1403

0.16

0.1663

0.1691

0.20

0.2672

0.2652

As the intensity of transmitted light varies with the concentration of the solution, ab``sorption coefficient also varies. The absorption coefficient varies linearly as shown in Figure 4. The absorption spectrum of the samples is shown in Figure 5. It is in close agreement with the previous studies [2, 3]. The absorption coefficient values calculated from the spectrum are found to be matching with that obtained from the experiment as shown in Table1.

FIGURE 2. Decay of current with path length in the solution (a). 0.04 mg/cm3 (b). 0.08 mg/cm3 (c). 0.12 mg/cm3 (d). 0.16 mg/cm3 (e). 0.20 mg/cm3

ln (I0/I) versus x graph is a straight line as shown in Figure 3. Slope of the graph gives absorption coefficient of the solution at that particular concentration which is given in Table 1.

FIGURE 4. Graph showing the variation of absorption coefficient with concentration


analysing how a laser beam is attenuated when it is passed through potassium permanganate solution. The same setup can be used to study the effect of temperature, solvents, electric fields, light frequency etc. on absorption coefficients of different solutions without any further modifications. The absorption coefficient of the KMnO4 solution for different concentrations is calculated and they are found to be in agreement with that obtained using absorption spectrum analysis.

REFERENCES FIGURE 5. Absorption spectrum of KMnO4 samples (a). 0.04 mg/cm3(b). 0.08 mg/cm3 (c). 0.12 mg/cm3 (d). 0.16 mg/cm3 (e). 0.20 mg/cm3

1.

CONCLUSIONS

3.

2.

4.

In recent years, the study of laser-matter interaction has been without any doubt one of the most rapidly evolving field in physics. In the present work, a simple and cost effective experimental setup is suggested for

A. L. Sundar Rao, Proceedings Mathematical Sciences 6 (5), 1937, pp 293-300. Jane Hodgkinson, Mark Johnson and John P. Dakin, Sens. Actuators, B 67, 2000, pp 227-234. Jane Hodgkinson, Mark Johnson and John P. Dakin, Meas. Sci. Technol. 9, 1998, pp1316-1323. Brian T. Fisher and David W. Hahn, Appl. Opt.43, 2004, pp 5443-5451
