Uv vis how does it work

It is vital in chemistry to understand the difference between amount of a sample and the concentration of a sample. An amount is a quantity of something; a concentration is an amount of something in a volume of something else. So 10 grams of sugar is an amount, while 10 grams of sugar dissolved in milliliters of water is concentration. Amounts in chemistry are usually expressed as moles; whereas concentrations use the term molar.

A mole of something is the molecular weight for the compound expressed as grams. So if the molecular weight of a compound is , then one mole of that com- pound would be grams. A one molar solution would be grams dissolved in one liter of solvent water. An example, now, suppose we have a solution of copper sulphate which appears blue because it has an absorption maximum at nm.

We look at the way in which the intensity of the light changes as it passes through the solution in a 1 cm cuvette. We will look at the reduction every 0. For our illustration, we will suppose that this fraction is 0. This data can be graphed to better see the relationship.

However, for absorbance the relationship is a straight line lower left. Scientists love straight lines rather than curves. Because it is easier to mathematically model a straight line as opposed to a curve.

If we plot absorbance at the peak wavelength against concentration, we get a straight line passing through the origin 0,0. With absorbance we can use spectroscopy to measure how much of something is present. This high energy light, compared to near infra-red, has enough energy to cause electronic transitions. These types of transitions move electrons from low energy levels, in an atom or molecule, to higher energy levels.

First, we will explain what happens when organic compounds absorb UV or visible light, and why the wavelength of light absorbed varies from compound to compound. Put simply the energy from the light is imparted to the electrons involved in chemical bonding.

This causes the electrons to be promoted moved to a higher energy level; thereby, absorbing the light energy. These energy levels are associated with the bonding and anti-bonding orbitals of conjugated double and triple bonded carbon atoms or aromatic benzene type ring systems in organic molecules.

When light passes through the compound, energy from the light is used to promote an electron from a lower energy bonding or non- bonding orbital into higher energy empty anti- bonding orbitals. The possible electron jumps that light can cause are seen in this slides figure upper right. The length of the arrow is proportional to the amount of energy needed to promote the electron between energy levels. So, if you have a bigger energy jump, you will absorb light with a higher frequency, which is the same as saying that you will absorb light with a lower wavelength.

Real world analogy: The promotion of an electron to a higher energy level is just like throwing a ball into the air. The energy from the muscles of your arm light is imparted promotion to the ball electron causing it to rise against the force of gravity ground state.

The ball then reaches a certain height excited state that is determined by the amount of effort light wavelength you used in the throw. This is the process of light absorption in a nutshell, except!

The ball electron does not just hang out at the higher energy levels, it falls back to the ground level from where it was originally promoted. The same thing happens to the promoted electron when the light energy is shut off. The electron drops back down to the ground state. The potential energy in the falling electron is released as heat or it can be re-emitted as light fluorescence or phosphorescence. This re- emitted light can be studied by luminescence spectroscopy. That means that in order to absorb light in the region from to nm which is where the spectra are measured , the molecule must contain either pi bonds Double bond or aromatic ring systems or atoms with non- bonding orbitals.

Remember that a non- bonding orbital is a lone electron pair on, say, oxygen, nitrogen or a halogen. Groups in a molecule which absorb light are known as chromophores. In addition to organic molecules, individual inorganic at- oms as well as metallic ionic complexes can absorb as well. Inorganic complexes tend to yield sharper spectral peaks with narrow half band widths. An example of a rare earth inorganic compound, holmium oxide, is shown at bottom left.

These conjugated systems have a large influence on peak wavelengths and absorption intensities. The top figure shows the structures of benzene, naphthalene, and anthracene. The bottom figure shows the absorption spectra obtained by dissolving these compounds in ethanol and analyzing the resulting solutions. The concentrations were adjusted so that the absorption intensities of the components were roughly the same.

It can be seen in the figure that peak wavelengths tend to be shifted toward the long wavelength region as the conjugated system gets larger. Outlining UV-Vis Spectrophotometers A UV-Vis spectrophotometer measures the intensity of light transmitted through a sample compared to a reference measurement of the incident light source. Related Product. Share This Post:. Please wait while you are redirected to the right page Please share your location to continue.

The most commonly used solvents are water, ethanol, hexane and cyclohexane. Solvents having double or triple bonds, or heavy atoms e. Because the absorbance of a sample will be proportional to its molar concentration in the sample cuvette, a corrected absorption value known as the molar absorptivity is used when comparing the spectra of different compounds. For the spectrum on the right, a solution of 0. Note that the absorption extends into the visible region of the spectrum, so it is not surprising that this compound is orange colored.

Transoid Diene nm R- Alkyl Group The y-axis vertical shows the dependent variable; the absorbance. UV-Vis spectrum Note: The wavelength region covered by a regular UV-Vis spectrophotometer is nm 1 nanometer, 10 -9 meter from the ultra violet UV: nm , the visible nm , into the near infra red IR: above nm. This is only a very small part of the total EM electromagnetic spectrum. By measuring and comparing a series of standard solutions -with known concentrations - of the analyte, the concentration of the analyte in the sample can be determined.