Lab2 - John Abbott College Department of Chemistry...

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John Abbott College Department of Chemistry Chemistry NYA Lab 2 Week of January 23rd, 2006 Spectrum of the Dichromate Ion Objectives: 1. To determine λmax for Cr 2 O 7 2- (aq) in the 340–580 nm range 2. To determine the molar absorptivity coefficient, ε, of Cr 2 O 7 2- (aq) at λmax . Theory: You will be using Spectronicc-20, an instrument used to measure the amount light absorbed by a colored sample at different wavelengths. This method involves the interaction of electromagnetic radiation (EMR) and matter. The most common region of the EM spectrum used for the analyses are UV, visible and IR regions. Remember that the visible region lies between a wavelength of 350(violet) – 650(red) nanometers. Every chemical substance possesses its own unique set of electronic, vibrational and rotational energy states. When EM radiation falls on the atom or molecule, the atom absorbs energy equivalent to the difference between two energy states and now the atom or molecule is in an ‘excited’ state. . The remainder of the radiation(complementary radiation)which is not absorbed called the ‘transmitted’ light passes through the sample and can be detected by the detector. The colour we see is the colour that is transmitted by the sample. (colour of the transmitted wavelength of light) Remember: Absorbance is dependent on concentration, but lambda max depends only on the colour of the solution(species present) and epsilon( ε , absorptivity co-efficient) is dependent on lambda max.
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THE HIGHER THE CONCENTRATION OF THE ABSORBING SPECIES in the sample, the more the radiation is absorbed and more intense is the complementary beam that is transmitted. In this experiment the visible region is used to measure the
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Lab2 - John Abbott College Department of Chemistry...

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