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Determination of absorption coefficient of cuso4 by optical method Abstract:This experiment has been performed to find out the true relationship between absorbance and concentration of colored solutions, by optical method. By using He-Ne laser of wavelength 632.8 nm, the absorbance of 9 different solutions of CuSo4 , ranging from 0.125 to 1.1 M, was found using light dependent resistor (LDR). As absorbance and resistance are inversely proportional (in the process of LDR) we have introduced a constant of proportionality n, which is equal to 9.89 x 104, by reference of report we followed. This factor was found to be consistent with the range of concentration used for this project. In accordance with the knowledge from formulas, the graphical analysis revealed that the relationship between absorbance and concentration was directly proportional with a molar absorptivity of 2.9 M-1cm-1 for CuSo4

Introduction:To understand the absorption coefficient, we first need to understand what happens when light shines through a material. Light that encounters a material can be reflected, transmitted, or absorbed. Reflection occurs when light bounces off a material; you can see your reflection in a mirror, because mirrors reflect a lot of light. Light that passes through a material has been transmitted, so the light you see through a window has been transmitted through the window. Absorbed light is light that is neither reflected nor transmitted; this light goes into a material, but never comes out. So where does absorbed light go? When a material absorbs light, it takes the energy of the light and changes it into another form of energy such as heat. The absorption coefficient describes how much light of a given color is absorbed by a material of a given thickness. The absorption coefficient is often represented by the Greek letter alpha. It has units of 1/cm (1/length), because it describes the amount of light absorbed per thickness of material. The more light a material absorbs, the higher its absorption coefficient will be. Because materials often absorb some colors better than others, the absorption coefficient is a function of color. A material that absorbs red light while transmitting blue light will have a high absorption coefficient for red light and a low absorption coefficient for blue light. Its S.I unit is M−1⋅cm−1. In order to find absorption coefficient we use beer-lambert law. The Beer-Lambert law (or Beer's law) is the linear relationship between absorbance and concentration of an absorbing species. The general Beer-Lambert law is usually written as: A = a(lambda) * b * c where A is the measured absorbance, a(lambda) is a wavelength-dependent absorptivity coefficient, b is the path length, and c is the analyte concentration. When

working in concentration units of molarity, the Beer-Lambert law is written as: A = epsilon * b * c where epsilon is the wavelength-dependent molar absorptivity coefficient with units of M -1 cm -1 .

Experimental Procedure:-

    

CuSo4 Solution Multimeter LDR He-Ne Laser Glass Container

Nine samples were prepared at different concentration of CuSo4 ranging from 0.125 to 1.1 M to analysis the absorption coefficient. After preparing the solutions attached the LDR with glass container perfectly so the chances of error will be reduced, both legs of LDR connected to the multimeter. Initially we take the background readings when white light is turn ON which is about 8.0kΩ in order to reduce errors and making the reading precise. Now we place the container below the He-Ne laser and pour samples in it after one and other and note down the reading which appears on multimeter correctly then subtract the background reading from them which is shown in table 2. After this by using the N factor we calculate the absorbance at each corresponding values of resistance then wo plot a graph between absorbance and concentration which shows the linear behavior and justifying the Beer-Lambert Law.

Result:After calculating the absorbance by using the N factor at each corresponding value of resistance which are shown in table 1 we plot a graph between concentration and absorbance which is the function of resistance. According to Beer-Lambert law absorbance is directly proportional to the concentration. Generally absorbance is the log of intensities but here we use absorbance as a function of resistance which also shows the linear behavior in between concentration and absorbance so our conversion from intensity to resistance on the basis of principle of LDR is verified by this graphical representation as shown in figure 1.

S.No 1. 2. 3. 4. 5. 6. 7. 8. 9.

Concentration (M) 0.125 0.25 0.376 0.502 0.626 0.752 0.877 1.0 1.1

Asorbance 14.00 14.059 14.180 14.30 14.386 14.513 14.53 14.59 14.729

(Table 1)

Absorbance 14.8

14.729

14.7

14.59

Absorbance

14.6

14.513

14.5

14.53

14.386

14.4

14.303

14.3 14.18

14.2 14.059

14.1

14

14 13.9 0

0.2

0.4

0.6

Concentratio (M)

0.8

1

1.2

(Figure 1)

14.386 − 14.180

Slope = m = 𝜀 =

𝐴

𝑚

=

𝑐𝑙

= 0.824

0.626 − 0.376

𝑙

=

0.824 𝑀⁻¹ 0.282𝑐𝑚

= 2.921 𝑀−1 𝑐𝑚⁻¹

Discussion:Our purpose of performing this experiment is to determine the absorption coefficient of CuSo4 by optical technique. It was observed that there is a linear relationship between concentration and absorbance by this it is proved that our experiment obeys BeerLambert Law. According to Beer-Lambert law 𝐼 = 𝐼ₒ𝑒 −𝛼𝑡 𝐼 𝐼ₒ

= 𝑒 −𝛼𝑡

log (𝐼⁄𝐼ₒ) = −𝛼𝑡 log (𝐼ₒ⁄𝐼 ) = 𝛼𝑡

A = 𝛼𝑡

where; A = log (𝐼ₒ⁄𝐼 )

(A = absorbance, 𝛼 = 𝑎𝑏𝑠𝑜𝑟𝑡𝑖𝑣𝑖𝑡𝑦, t =thickness)

𝛼 = 𝐴⁄𝑡 𝐴 𝑡

= 𝜀𝑐

A = 𝜀𝑐𝑙

where; 𝛼 = 𝜀𝑐 (𝜀= molar absorptivity/absorption coefficient, c= concentration)

But here we calculate absorbance as a function of resistance. From the working principle of LDR we know that absorbance is inversely proportional to resistance so, 𝐴 ∝

1 𝑅

𝑁

A=𝑅 A=

𝑛⁄ 20 𝑅

where ; N = 𝑛⁄20

Where n is the proportionality constant, calculated by report which we used as a reference at c = 0.25 M, 0.703 absorbance obtained by using this data we can calculate n factor 𝑛 20 𝑛 20

= AR = (0.703) (7040)

n = (4.9 x 103) (20) n = 9.89x104 by using this we calculate absorbance at every value of resistance, and its graph shows linear relationship between absorbance and concentration shown in figure 1

S.No CuSo4 (g) 1. 0.2 2. 0.4 3. 0.6 4. 0.8 5. 1.0 6. 1.2 7. 1.4 8. 1.6 9. 1.8

Moles (mol) 1.25x10-3 2.5x10-3 3.76x10-3 5.02x10-3 6.26x10-3 7.52x10-3 8.77x10-3 0.01 0.011

Concentration (M) 0.125 0.25 0.376 0.502 0.626 0.752 0.877 1.00 1.1

Resistance (kΩ) 0.45 0.48 0.54 0.60 0.64 0.70 0.71 0.74 0.80

Corrected Count After (-8kΩ) 7550Ω 7520Ω 7460Ω 7400Ω 7360Ω 7300Ω 7290Ω 7260Ω 7200Ω

(Table 2)

Appendix 1:

Background Readings

White light on LDR = 8.0kΩ Laser + White Light absorb by container = 0.48kΩ 

Thickness of container = 2.22mm

Appendix 2:Path length: diameter = D = 6.715

Corrected Count After (-0.48kΩ) 7070Ω 7040Ω 6980Ω 6920Ω 6880Ω 6820Ω 6810Ω 6780Ω 6770Ω

radius = R =

6.715

= 3.357

2

V = 𝜋𝑟 2 ℎ

( h =𝑙 = path length)

V = 10ml = 10cmᶾ 10𝑐𝑚ᶾ

𝑙 = (3.142)(3.357𝑐𝑚)² = 0.282 cm

Appendix 3:LDR:An LDR or light dependent resistor is also known as photo resistor, It is a one type of resistor whose resistance varies depending on the amount of light falling on its surface. When the light falls on the resistor, then the resistance changes. These resistors have a variety of functions and resistance. For instance, when the LDR is in darkness, then it can be used to turn ON a light or to turn OFF a light when it is in the light. A typical light dependent resistor has a resistance in the darkness of 1MOhm, and in the brightness a resistance of a couple of K Ohm

Working Principle:This resistor works on the principle of photo conductivity. It is nothing but, when the light falls on its surface, then the material conductivity reduces and also the electrons in the valence band of the device are excited to the conduction band. These photons in the incident light must have energy greater than the band gap of the semiconductor material. This makes the electrons to jump from the valence band to conduction. These devices depend on the light, when light falls on the LDR then the resistance decreases, and increases in the dark. When a LDR is kept in the dark place, its resistance is high and, when the LDR is kept in the light its resistance will decrease.

Reason Of Using LDR:•

Initially we used photodiode because it has a linear relationship b/w concentration & absorption so it satisfy Beer-Lambert Law but our output results were very small and difficult to measure so we had to amplify our results to measure them properly.

•

We had to make a circuit with photodiode in order to amplifier results and we successfully made it but, we were not able to align it properly.

•

Because of these complications we replaced photodiode by LDR, phenomenon of LDR is different as compare to photodiode but it gives amplified results as compare to photodiode so in LDR there is no need of amplification.

Appendix 4:Reference:https://pdfs.semanticscholar.org/d547/882a5e2c89bf977b004bf817983153b4a218.pdf