Why is it important to calibrate the UV spectrophotometer before use?
To reduce errors from the instrument and to obtain absorbance values for the correct wavelength
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What does UV spectroscopy measure?
The transitions from ground to excited state
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What is a chromophore?
Part of a molecule responsible for the colour of a compound
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How does conjugation in a chromophore affect the extinction coefficient?
The more extensive the conjugation in a chromophore, the greater the extinction coefficient (or absorbance) and the longer the wavelength of absorption maximum (lower energy)
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What factors influence UV absorbance?
Nature of the solvent, pH effects
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Describe the bathochromic shift
If the excited state is more polar than the ground state, then the excited state is stabilised by polar solvents. Less energy needed for transition and wavelength is longer e.g. pi to pi*
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Describe the hypsochromic shift
If the ground state is more polar than the excited state, then the ground state is stabilised by polar solvent. More energy needed for transition and wavelength is shorter e.g. n to pi*
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Why does pH have an effect on UV spectroscopy?
Ionisation can lead to changes in electronic structure of the chromophore
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What is the Beer-Lambert Law?
A = ecl
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What are the units for absorbance?
No units
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What are the units for the molar extinction coefficient?
L mol-1 cm-1
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What are the units for pathlength?
(1) cm
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What is the specific absorption coefficient?
The specific absorbance is simply the numerical value of A when measuring a 1% w/v solution contained in a transparent cuvette of 1 cm path length.
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When is the specific absorption coefficient used?
When the molecular weight of a compound is not known e.g. polymers (value can vary depending on conditions e.g. aqueous acid)
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How is the specific absorption coefficient substituted into the Beer-Lambert equation?
c = A/A(1%, 1cm) where c has units of g/100 mL
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What is the linear equation for the Beer-Lambert law?
y = [A (1%, 1cm) . l . c] + 0
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What is the optimal range for absorbance values?
0.1-1.0
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What would happen if a sample had an absorbance value beyond the optimal range?
Sample should be diluted or become more concentrated
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Describe how a calibration curve can be used to determine the unknown concentration of a sample
Measure absorbances of samples of known concentration. Construct calibration curve (absorbance vs concentration). Measure absorbance of sample and use curve to determine concentration
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What are the possible errors in UV spectroscopy?
Constant error due to calibration of UV spectrometer. Pipetting errors when making the standard solutions
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What are the chromophores in aspirin?
Benzene ring, COOH, COO
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How would you measure the absorbance of a very concentrated sample without dilution?
Increase the path length in the spectrometer
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What is a rare earth element?
Element which exists in nature and is difficult to separate e.g. holium
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What is the relationship between the colour of potassium dichromate solution and its UV/Vis absorption spectrum?
+6 to +3 bathochromic shift, +3 to +6 hypsochromic shift
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Why is a fused silica cuvette used in the far UV instead of a glass or plastic cell?
Glass and plastic are opaque to UV radiation (affects results)
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Other cards in this set
Card 2
Front
What does UV spectroscopy measure?
Back
The transitions from ground to excited state
Card 3
Front
What is a chromophore?
Back
Card 4
Front
How does conjugation in a chromophore affect the extinction coefficient?
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