Ultraviolet and Visible Spectroscopy Chemical Ideas 6.8

Why are carrots orange? Carrots contain the pigment ‘Carotene’. Carotene absorbs blue light strongly leaving the orange-red light to reach your eyes. If you were to use a spectrometer you could plot the intensity of absorption against wavelength. This is called an absorption spectrum.

Why are carrots orange? This is the absorption spectrum for carotene run in hexane solution.

Absorption Spectra Two identical beams of light are shone from the source. One passes through the sample and the other through pure solvent. The light in the two emerging beams is compared to give the absorption spectrum of the sample.

Absorption Spectra Most spectrometers will scan in the UV and visible regions. These are the areas associated with changes in the electronic energy of the molecule and therefore spectra are often called ‘Electronic Spectra’. As our eyes cannot detect UV light, if a compound absorbs in this region it does not affect its colour. E.g. Benzene appears colourless.

UV/Vis vs. IR In IR spectra the peaks ‘hang down’ from a baseline but in UV/Vis they rise. IR uses wave number as its units for radiation absorbed but UV/Vis uses wavelength (nm).

Using ultraviolet and visible spectra In UV/Vis spectra you will often find broad absorption bands which are characteristic of general structural features rather than specific functional groups. A colorimeter is a simple version of a visible spectrophotometer. It is used to measure the intensity of absorption of coloured compounds over a narrow range of frequencies, finding the ‘concentration’ of a coloured compound.

Interpreting the Spectrum Colour chemists are interested in three main features of the spectrum: 1.The wavelength of the radiation absorbed. 2.The intensity of the absorption 3.The shape of the absorption band

Interpreting the Spectrum When recording the spectrum, chemists will often record the wavelength of the maximum absorption. For organic molecules with delocalised electron systems, the longer the conjugated carbon chain, the more intense the absorption and the longer the wavelength of λ max. The intensity of the absorption is important commercially because it determines the amount of pigment or dye required to produce a good colour. The shape and width of the absorption is important because it gives the shade and purity of the colour seen.

Reflectance Spectra To measure the absorption spectra you need to make a solution of the coloured substance but this is not always possible e.g. if the pigment is on the surface of a painting. Instead chemists use a different type of spectra – a reflectance spectra. They shine light directly onto the paint surface and examine the composition of the reflected light. This is the light not absorbed, so a type of ‘negative absorption spectra’.

Monastral Blue

Practise Time Now have a go at CI 6.8 Problems 1-4 to check your understanding