Spectroscopie UV-visible : dispositif et principe - Physique-Chimie - Terminale S - digiSchool

Introduction

This tutorial provides a comprehensive guide to understanding UV-visible spectroscopy, focusing on the experimental setup and the underlying principles of its operation. This knowledge is particularly valuable for high school students studying physics and chemistry.

Step 1: Assemble the Experimental Setup

To conduct UV-visible spectroscopy, you need to set up the following components:

1. Light Source

   • Use a suitable UV-visible light source. Common choices include a quartz lamp or a tungsten lamp, depending on the wavelength range required.

2. Focusing System

   • Integrate a lens system to focus the light onto the sample. This ensures that the light intensity on the sample is sufficient for accurate measurements.

3. Slit

   • Install a narrow slit to allow a specific wavelength of light to pass through. This helps in isolating the desired wavelength for analysis.

4. Monochromator

   • Use a monochromator to disperse the light into its constituent wavelengths. This device will allow you to select the specific wavelength of light that interacts with the sample.

5. Sample Cell (Cuvette)

   • Place the solution to be analyzed in a cuvette. Ensure that the cuvette is clean and made of materials that do not absorb UV-visible light, such as quartz.

6. Detector

   • Connect a detector (e.g., photodiode or photomultiplier tube) to measure the intensity of the transmitted light after passing through the sample.

7. Display Unit

   • Finally, incorporate a display unit to visualize the data obtained from the detector. This will help you analyze the absorbance and transmittance of the sample.

Step 2: Understand the Principle of UV-Visible Spectroscopy

The principle of UV-visible spectroscopy revolves around the interaction between light and the sample solution. Here’s how it works:

1. Light Interaction

   • Light from the source interacts with the molecules in the sample solution. Depending on the energy of the light (wavelength), certain wavelengths may be absorbed by the sample.

2. Transmittance and Absorbance

   • Measure the intensity of light before and after it passes through the sample:
     • Transmittance (T) is the ratio of transmitted light intensity (I) to incident light intensity (I0):
       • T = I / I0
     • Absorbance (A) is calculated using the formula:
       • A = -log10(T)
   • Absorbance corresponds to how much light is absorbed by the sample at a specific wavelength, providing information about the concentration of absorbing species in the solution.

Step 3: Perform the Spectroscopic Analysis

Once your setup is complete and you understand the principles, follow these steps to conduct analysis:

1. Calibrate the System

   • Use a blank cuvette filled with a solvent to set a baseline for measurements. This will ensure that any absorbance readings are solely due to the sample.

2. Measure Absorbance

   • Place the cuvette with the sample in the path of the light beam. Record the absorbance at different wavelengths.

3. Analyze Data

   • Create an absorbance spectrum by plotting absorbance values against wavelength. Look for peaks in the spectrum, which indicate specific wavelengths absorbed by the sample.

4. Interpret Results

   • Use the absorbance data to deduce information about the concentration of substances in the sample using Beer-Lambert Law, which relates absorbance to concentration.

Conclusion

In this tutorial, you learned how to set up a UV-visible spectroscopy experiment and understand its principles. By assembling the necessary components and grasping the concepts of absorbance and transmittance, you can effectively analyze samples in a laboratory setting. For practical applications, consider exploring the concentration of various substances in solutions or assessing the purity of chemical compounds. This foundational knowledge will enhance your understanding of analytical chemistry and prepare you for more advanced studies.