Interpretation of UV–Visible Spectrum and Calculation of λmax of Anthracene

 

Experiment: Interpretation of UV–Visible Spectrum and Calculation of λmax of Anthracene

Aim

To record and interpret the UV–Visible spectrum of anthracene and determine its maximum absorption wavelength (λmax).

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Principle

Anthracene is an aromatic hydrocarbon containing three fused benzene rings, which creates a highly conjugated π-electron system. When UV light passes through anthracene, π-electrons absorb energy and undergo π → π* electronic transitions.

Due to the extended conjugation, the absorption shifts toward longer wavelength compared with benzene and naphthalene.

The absorbance of the solution follows Beer–Lambert law:

$A = \varepsilon c l$ 

Where

• A = Absorbance

• ε = Molar absorptivity

• c = Concentration

• l = Path length of the cuvette

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Apparatus

• UV–Visible spectrophotometer

• Quartz cuvette

• Volumetric flask

• Pipette

• Beaker

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Chemicals / Reagents

• Anthracene sample

• Ethanol or hexane (solvent)

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Procedure

1. Prepare a dilute solution of anthracene in ethanol or hexane.

2. Fill a quartz cuvette with solvent and set it as the blank.

3. Fill another cuvette with the sample solution.

4. Scan the spectrum in the range of 200–400 nm using the spectrophotometer.

5. Record absorbance values at different wavelengths.

6. Plot a graph of Absorbance vs Wavelength.

7. Identify the wavelength corresponding to maximum absorbance (λmax).

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Observation Table

Sr. No.	Wavelength (nm)	Absorbance
1	240	0.32
2	260	0.48
3	280	0.64
4	300	0.78
5	320	0.88
6	341	0.96
7	360	0.72
8	380	0.50

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Calculation of λmax

$${\lambda }_{max}=\text{Wavelength at which absorbance is maximum}\text{<br>}\text{From the observation table:}$$ $$\lambda_{max} = 341 \, nm$$

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Interpretation of Spectrum

• Anthracene shows π → π* electronic transitions due to its conjugated aromatic system.

• The presence of three fused benzene rings increases conjugation, causing absorption at longer wavelength than benzene and naphthalene.

• The strong peak near 341 nm represents the main electronic transition in anthracene.

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Result

The UV–Visible spectrum of anthracene was recorded and the maximum absorption wavelength (λmax) was found to be approximately 341 nm.