UV-Visible Spectroscopy

Question	Answer
Which block of the periodic table contains the metal ions that can absorb energy to promote electrons to the outer energy levels of the metal?	D block.
Other than emitting energy quanta that correspond to certain wavelengths of light, what other property can be a result of electrons returning to their original energy levels in D block metal ions?	Magnetic properties.
What are conjugated bonds?	Multiple repeats of single, double or triple bonds.
What are chromophores?	Bonds in organic molecules that allow free electron movement once energy is absorbed so that delocalisation of the electrons gives rise to colour. These may be conjugated bonds or delocalised networks of electrons.
Why do conjugated bonds result in chromophore properties?	The multiple repeats of double and single bonds allow electrons to move along the carbon chain or around a ring having absorbed energy in the form of light.
What word, used in Art too, describes the relationship between the visible wavelength at which energy is absorbed, and the visible wavelength that is produced?	Complementary.
What energy levels do long and short wavelengths of light have?	Low and high, respectively.
How are wavelength and energy related, demonstrated in Planck's Law: E = (h x c)/ λ ?	Wavelength is represented by λ (lambda) so if it is larger then (h x c) is being divided by a larger denominator so E (energy) must be lower. The reverse is true for short wavelengths and high energy.
At which wavelengths does chlorophyll absorb?	Violet (420nm) and red (670nm).
What is the Beer-Lambert Law and what does it state?	The Beer-Lambert Law, Absorbance = Molar Absorptivity x Pathway Length (cm) x Concentration (M) (or A = E x L x C), states that concentration is proportional to absorbance.
What is molar absorptivity?	A constant for a certain element or compound that relates its light absorbance at a certain wavelength to its concentration at 1M.
In what range are UV wavelengths?	200-400nm
What structural properties could an organic substance have if it absorbs UV light?	Arene rings or unsaturated bonds.
In a colorimeter for measuring the absorption of visible light by a substance, what provides the white light source that is diffracted?	A tungsten filament.
In a colorimeter for measuring absorption of visible light by substances, what two pieces of equipment may be used to diffract the white light source?	A prism or diffraction grating (a monochromator).
What is used to ensure that only one wavelength of light is assayed at a time?	A monochromatic filter.
In a colorimeter for measuring absorption of visible wavelengths by substances, how is a beam of monochromatic light split so that one can be used for a reference cell and one for the sample?	Using a half mirror which transmits half the beam and reflects half the beam equally on to two other mirrors. The receiving mirrors will then reflect the light to the reference and sample cuvettes.
What type of detector measures the transmission of wavelengths in a colorimeter to measure the absorption of visible light by substances?	A photomultiplier detector. The signal response will be a percentage of the intensity that was transmitted through the reference cell.
How many nm is the limit at which your solvent should absorb within the region you're interested in?	0.3nm
What polar, saturated solvents could be used in UV-Visible spectroscopy?	Water, ethanol, acetone, or methane trichloride.
In a colorimeter, why might you choose to use a glass cuvette rather than a plastic cuvette for your sample?	The sample may degrade/dissolve the plastic cuvette.
What is the disadvantage to using a single beam colorimeter and how might this be solved?	When using a single beam colorimeter, the instrument must be calibrated and then opened again to replace the reference cuvette with the sample cuvette, or to switch between sample cuvettes. This can result in variation between source intensity which can give inaccurate readings. It is better to use a double beam colorimeter in which the reference and sample cuvettes are assayed simultaneously.
How might one conduct a quantitative analysis of a substance using a colorimeter?	Prepare a series of standards of a known concentration of your substance and record their absorbance through a 1cm path length cell. Plot a calibration graph of concentration (x) against absorption (y) and read off the absorption of your unknown sample.
Why might a concentration-absorption (calibration) graph be linear?	Because the Beer-Lambert Law states that concentration is proportional to absorbance so the two values will increase proportionally.
How might one carry out an analysis of an unknown biological specimen?	One would have to record the entire absorption spectrum and look up the different λmax absorptions in reference books of structures.
Why might your results be inaccurate/there be deviations from Beer's Law? How might this be minimised?	If the incident beam is wide, or has stray radiation from dirty reflective surfaces, which wider the waveband, absorbance is decreased. Keeping equipment spotlessly clean, regularly servicing the instrument, using narrow wavebands, and keeping the instrument warm to avoid condensation are all techniques that can be used to minimise this.
Why is Beer's Law not obeyed at high concentrations? How might this be resolved?	There is internal absorption of light by the high concentration of the substance, so a weaker signal is transmitted to the detector and it appears that more light is being absorbed than should be by that concentration in a linear relationship. To produce more accurate results, dilution of the sample should be carried out so that the value falls within the straight portion of the calibration graph.

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UV-Visible Spectroscopy

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	Created by sophietevans almost 11 years ago