Ultraviolet/Visible Spectroscopy (UV-Vis)

Summary

This video explains the fundamentals of ultraviolet-visible (UV-Vis) spectrometry, which analyzes electronic transitions in atoms and molecules. The technique involves measuring the absorption of light as electrons move between energy levels. The process uses two light sources—tungsten for visible light and deuterium for ultraviolet light—and passes light through a sample to determine its absorbance. UV-Vis spectra are used to study solutions, with results showing how much light is absorbed at various wavelengths. The script demonstrates this process using a green food dye, which absorbs orange and blue light, appearing green as a result.

Takeaways

• 😀 Ultraviolet-visible spectrometry analyzes electronic transitions in atoms and molecules by measuring absorbed energy during electron movement between energy levels.
• 😀 Compounds that absorb light in the visible region, such as some transition metal compounds and organic dyes, are colored, while those that only absorb in the UV region are colorless.
• 😀 A UV-Visible spectrometer typically uses two light sources: a tungsten lamp for the visible region and a deuterium lamp for the UV region.
• 😀 A diffraction grating in the monochromator splits light into its constituent wavelengths, which correspond to different colors of light.
• 😀 Wavelengths in the visible range are from 400 to 700 nanometers, while wavelengths shorter than 350 nanometers are classified as ultraviolet.
• 😀 The spectrometer scans through the spectrum, passing different wavelengths of light sequentially through the sample, thanks to the rotating grating.
• 😀 A double beam setup splits the light into two beams—one through the sample and the other through a reference cell—using mirrors and a modulator.
• 😀 The detector compares the intensity of light from both beams and sends a signal to the computer, which calculates the absorbance.
• 😀 Absorbance is calculated as the logarithmic ratio of the light intensity passing through the sample versus the reference.
• 😀 UV-Visible spectra are usually run on solutions, as light does not typically pass through solid samples.
• 😀 In experiments, a sample is placed in a cuvette, and a solvent blank is used to calibrate the instrument. Cuvettes must be made of quartz for UV range measurements, as glass and plastic absorb UV light.

Q & A

What does ultraviolet-visible spectrometry reveal?

Ultraviolet-visible spectrometry provides information about electronic transitions in atoms and molecules, specifically how electrons move between different energy levels, absorbing energy corresponding to the gap between those levels.

Why are compounds that absorb visible light colored?

Compounds that absorb light in the visible spectrum, such as some transition metal compounds and organic dyes, appear colored because the absorbed wavelengths correspond to the visible colors we can see.

What happens when compounds only absorb ultraviolet light?

Compounds that absorb only ultraviolet light are colorless because ultraviolet light is beyond the visible spectrum, and no visible color is produced.

What is the function of the two light sources in a UV-Visible spectrometer?

A UV-Visible spectrometer typically has two light sources: a tungsten lamp for visible light and a deuterium lamp for ultraviolet light. These provide the appropriate wavelengths of light for the analysis of samples.

How does the diffraction grating in the monochromator work?

The diffraction grating in the monochromator splits light into its constituent wavelengths, much like a CD's surface. It helps separate the light into different colors or wavelengths for analysis.

What is the significance of different wavelengths in the context of the spectrometer?

Different wavelengths of light correspond to different colors, with red light having a wavelength of about 700 nanometers and blue light around 400 nanometers. Wavelengths shorter than 350 nanometers are considered ultraviolet.

What role does the modulator play in the spectrometer?

The modulator chops the light into two beams: one passes through the sample cell, and the other through a reference cell. This setup is essential for measuring and comparing the absorbance of light by the sample and reference.

Why is the instrument called a 'double beam instrument'?

The spectrometer is called a 'double beam instrument' because it splits the light into two beams—one for the sample and one for the reference—and compares their intensities.

What is absorbance and how is it calculated?

Absorbance is a measure of how much light is absorbed by the sample at a particular wavelength. It is calculated by taking the logarithm of the ratio of the intensities of the sample and reference beams.

Why are quartz cuvettes used for ultraviolet measurements?

Quartz cuvettes are required for ultraviolet measurements because glass and plastic absorb ultraviolet light, which would interfere with the measurement. Quartz allows for the passage of UV light without significant absorption.