UV-Vis Spectroscopy and its applications in Chemistry

UV-Vis spectroscopy is a branch of spectroscopy is deals with absorption of light in ultraviolet and visible region.

Hi Friends, in this article we will learn about basics of UV visible spectroscopy and also see its applications in chemistry. Here we will discuss transition metal complexes and Beer-Lambert Law. 

We have discussed various applications of spectroscopy in chemistry, medicine and environmental science in another article. Please check out for more details. [Link]

Introduction

UV-Vis spectroscopy is a branch of spectroscopy is deals with absorption of light in ultraviolet and visible region. The atoms and molecules absorb light and undergo electronic transitions. The absorption spectroscopy refers to measurement of transitions from the ground state to the excited state.

Various spectral regions of electromagnetic spectrum (light) are listed below;

Figure1: Regions of electromagnetic spectrum

The uv-spectroscopy deals with absorptions of light in the range of 200-800 nm. The electrons of atoms or molecules absorb precise light in the ultraviolet and visible region to go from one level to another energy level.

The possible electronic transitions that light might cause are given below. In each of the case; an electron from filled orbital excited to go in empty high energy orbital.

Figure 2: Electronic transitions

Every transition requires precise amount energy. The larger the gap between the energy levels, the greater the energy required to promote the electron to the higher energy level. The ultraviolet and visible lights can cause only two transitions; that is

• p-bonding to p*-(anti-bonding)
• non-bonding to p*-(lone pair)

Therefore to absorb light between the range 200-800 nm, the molecular must consists of p bond or any atom with lone pair (oxygen, nitrogen or halogen).

What is p bond system?

A p bond is formed due to side overlap of half-filled p orbitals. For example; ethene molecule is having one p bond which is formed by the overlap of p orbitals of two carbon atoms. In the ground state both the electron found in p –bonding energy level. These electrons absorb light and excites to p* –anti bonding energy level.

Figure 3: Orbital diagram of ethene

Some examples of p-bond systems given below;

Figure 4: p-bond systems

If the energy gap between π bonding orbitals and π* anti-bonding orbitals is more; the molecule absorbs light of higher energy and shorter wavelength. Similarly, if the engery gap between π bonding orbitals and π* anti-bonding orbitals is less; the molecule absorbs light of lower energy and longer wavelength.

In conjugated p-systems as the amount of delocalisation increases; the energy gap between the π bonding orbitals and π* anti-bonding orbitals gets smaller. Therefore the molecule absorbs light of lower energy and longer wavelength.

Transition metal complexes

The transition metal complexes tend to absorb UV and visible light. This is due to splitting of d orbital of central metal ion. When the legand forms bond with metal ion, in this process some of the d orbitals gets energy and some lose energy. Hence the d orbital gets divide by energy. The amount of splitting or energy gap is depending upon legand and central metal ion.

For example; Hexaaquacobolt(II) ion complex [CO(H₂O)₆]. 

Figure 5: Hexaaquacobolt(II) ion complex

In this case five 3d orbitals of cobalt ion divide in to 3 low energy orbitals and 2 high energy orbitals. The electrons from low energy orbital absorb light and excites to higher energy orbital.

Figure 6: Representation of orbital structure of Hexaaquacobolt(II) ion complex

Why we see colour?

When visible light fall on a substance, then characteristic portion of light get absorbed.  And remaining portion of light is reflected. The reflected light is appears as colour of that substance. It is also known as complementary colour of absorbed wavelength of light. This relationship is can be understood by colour wheel. Here it is seen that complementary colours are diametrically opposite each other. That means if a substance absorbs “red” light then it will appears as “green” colour.

Figure 7: The color wheel

How does UV-Visible Spectrum look like?

The UV-Visible spectrum is a graph which is plotted between absorbance (vertical axis) and wavelength (horizontal axis). The wavelength which corresponds to maximum absorption is referred as “lambda max” (l max). Each substance or compound has specific l max value.

Figure 8: Typical UV -spectrum

Maximum absorbance values of some organic compounds as given below;

Figure 9: Maximum absorbance values of some organic compounds

What is Beer-Lambert Law?

The Beer-Lambert Law states that “absorbance is directly proportional to the concentration of the compound in solution”. 

According to Beer-Lambert Law the UV-visible spectroscopy can also be used to measure the concentration of a sample. If we know the absorption value (A) and molar extinction constant (e) of a compound then we can calculate the concentration of unknown solution.

Applications of UV Spectroscopy

To measure concentration of unknown solution:

A plot of absorbance verses concentration of series of samples is linear if they follow Beer-Lambert Law. This graph is known as Calibration graph. And based on absorbance value of the unknown sample it is easy to find concentration of respective solution.  

Figure 10: The Calibration graph

To study Reaction kinetics:

Concentration of reactant or product changes in the course of reaction. Hence absorbance also changes as the reaction progress. Then a plot of absorbance verses time can give the idea about order of the reaction with respect to the reactant or product. This also helps to study the reaction mechanism.

To determine Dissociation constants (pKa) of acids and bases:

 It is possible to determine dissociation constant of compound if the acid / base form of the compound absorbs UV light.

pKa of compound can be calculated by using Henderson–Hasselbalch equation

pH = pKa + log [A^-] / [HA]

The ratio [A^-] / [HA] can be calculated by plotting the graph between absorbance versus wavelength at respective pH value.

Detection of Impurities:

The UV spectrum of sample is compared with that of standard reference. Here Additional peaks can be observed due to impurities. Hence we can identify amount of impurities in the sample.

Structure elucidation of organic compounds:

Cis-trans isomers , presence or absence of unsaturation can be identified by using UV-spectroscopy. 

Summary

To summarize this article, we have learned basics of UV-spectroscopy and its applications. Here electronic state transitions due to ultra violate and visible light are responsible for absorption of light. If the compound absorbs light in the visible region we see its complementary color. UV-spectroscopy can be utilized by measurement of concentration of compound in solutions, and structure elucidations of organic compounds.

That's all for this topic. Thank you..!