Ultraviolet-Visible Spectroscopy
Definition and meaning of Ultraviolet-Visible Spectroscopy in chemistry.
Ultraviolet-visible spectroscopy, often abbreviated as UV-Vis, is an analytical technique used to measure how much ultraviolet and visible light a chemical substance absorbs. This method helps chemists determine the exact concentration of molecules in a sample.
In more detail
The fundamental principle behind UV-Vis spectroscopy is that different molecules absorb specific wavelengths of light based strictly on their unique atomic structures. When a liquid chemical sample is exposed to light in the ultraviolet and visible regions of the electromagnetic spectrum, the energy from the light rapidly excites the valence electrons inside the molecules.
These electrons absorb the photon's energy and jump from their stable ground state to a higher energy, unstable excited state. Because every distinct molecule has a specific energy gap between these two states, it will only successfully absorb certain exact wavelengths of incoming light. During a typical UV-Vis laboratory experiment, a computerized machine called a spectrophotometer passes a focused beam of light through a clear glass or quartz cuvette containing the sample solution.
The instrument accurately measures the intensity of the light before it enters the sample and compares it to the intensity of the light that successfully passes all the way through. The difference between these two numerical values represents the precise amount of light absorbed by the sample.
The resulting data is plotted on a graph called an absorption spectrum, showing absorbance versus wavelength, which acts like a unique optical fingerprint for the chemical substance being tested. One of the most common applications of UV-Vis spectroscopy is determining the precise concentration of a known substance in a complex liquid mixture.
This feat is achieved using Beer's Law, a mathematical formula which states that there is a direct, linear relationship between the absorbance of the light and the concentration of the absorbing molecules. UV-Vis is widely used across many fields, from monitoring reaction rates in physical chemistry laboratories to identifying transition metal complexes and testing water quality in environmental science.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Standard Acronym | UV-Vis |
| Measures | Light Absorbance |
| Particle Excited | Valence Electrons |
| Governing Law | Beer's Law |
| Chart Produced | Absorption Spectrum |
| Cuvette Material | Quartz or Glass |
A chemist can use ultraviolet-visible spectroscopy to determine exactly how much copper is dissolved in a water sample by measuring how much blue light the solution absorbs.
Frequently asked questions
Why do different chemicals absorb different colors of light?
Every chemical has a unique arrangement of electrons, creating specific energy gaps that only match the energy of certain colors of light.
Can UV-Vis spectroscopy identify an unknown chemical completely?
While it provides a useful optical fingerprint, it is usually combined with other techniques like mass spectrometry to definitively identify a completely unknown molecule.
What is the purpose of Beer's Law?
Beer's Law allows chemists to mathematically calculate the exact concentration of a chemical in a liquid based purely on how much light it absorbs.