The iron content of runoff from a banana ranch is a necessary analytical parameter to analyze. A 25.0mL sample of the runoff was acidified with HNO3 and treated with excess KSCN to form a red complex. (KSCN itself is colorless.) The solution then was

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• The iron content of runoff from a banana ranch is
  a necessary analytical parameter to analyze. A
  25.0mL sample of the runoff was acidified with
  HNO3 and treated with excess KSCN to form a red
  complex. (KSCN itself is colorless.) The
  solution then was diluted to 100.0mL and put in a
  variable pathlength cell. For comparison, a
  10.0mL reference sample of 6.80x10-4 M Fe3 was
  treated with HNO3 and KSCN and diluted to 50.0mL.
  The reference was placed in a cell with a 1.00cm
  light path. The runoff sample exhibited the same
  absorbance as the reference when the pathlength
  of the runoff cell was 2.48cm. What was the
  concentration of iron in the banana runoff?

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UV Spectroscopy and Qualitative Analysis

• UV-vis spectroscopy is usually not very useful
  for qualitative analysis because there are few
  absorption maxima and minima
• Solvents
• Must be transparent in region of interest
• Should not interfere with absorbing species (but
  usually it does). Polar solvents tend to
  obliterate fine structural detail in molecular
  spectra

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UV Spectroscopy and Qualitative Analysis

• 3) UV-vis spectroscopy does provide some
  information on functional groups

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UV-vis spectroscopy and Quantitative Analysis

• 1)Scope is huge
• a.)95 of all quantitative analyses in health
  care field are done by UV-vis spectroscopy
• b.) wide applicability to organic and inorganic
  analyses
• c.) even non-absorbing species can be used by
  doing colorimetric reactions (reactions must go
  near to completion)

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UV-vis spectroscopy and Quantitative Analysis

• 2. L.O.D. is low, typically 10-4 ? 10-5M but can
  be as low as 10-6 ? 10-7M
• 3. Moderate to high selectivity
• 4. Accuracy to within 1-3 with minimal training

• 5. Easy and accurate data acquisition

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Construction of Calibration Curves is often done
on the absorption max. Why?
Sample and Reference cells should be matched
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Example

• The ultraviolet absorbances of a series of 9
  standards having different nitrate concentrations
  were determined at 220nm using a 1.0cm cell 8
  samples of river water were taken downstream from
  a chemical plant, avg. absorbance 0.642. What is
  the nitrate content of the river in mg/mL?

NO3 (mg/mL 0 .004 .015 .025 .035 .04 .05 .06 .07
Abs. .003 .10 .211 .350 .453 .556 .623 .671 .691
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The n ? p (T1) transition occurs at 397nm, the n
? p (S1) transition occurs at 355nm. What is
the difference in energy between the n ? p (T1)
state?
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Relaxation Processes

• Radiationless ? loss of energy in small steps
  excitation energy converted into kinetic energy
  by small collisions with other molecules, small
  increase in temperature
• Fluorescence radiative form of relaxation
• Resonance fluorescence no change in wavelength
  from excitation to emission Lowest e- state,
  vibrational, rotational state of each excited
  state produces resonance ? mostly in atoms

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Stokes Shift
Results from thermal energy losses

• Molecules see more non-resonance fluorescence
• Not all excited energy is transmitted as
  radiation (some is non-radiative)
• Makes emission spectrum look like mirror image of
  excitation spectrum
• This shift in the spectrum toward longer
  wavelengths is called Stokes Shift

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More about fluorescence

• Why do some molecules fluoresce and others dont?
• Want as fast of way as possible to get down to
  the ground state, generally non-radiative
  internal conversion is fastest but sometimes due
  to molecules configuration, fluorescence may be
  faster

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More about excess energy loss

• 1. Emission of radiation
• Excited particles (ions, atoms, or molecules)
  relax to lower energy level by giving up excess
  energy as photons
• Excitation brought about by bombardment with e-,
  exposure to high potential current, or heat
  treatment by arc or flame

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More about excess energy loss

• 2.Thermal radiation a.k.a blackbody radition
• 1. radiation emission lmax prop. To 1/T
• 2. Energy emitted varies as the 4th power of
  temperature
• 3. Emissive power varies 1/l5
• Heated solids produce IR, vis, and longer l UV

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3. Phosphorescence (another way to lose excess
energy)

• Radiative form of relaxation, involves
  inter-system crossing where an e- flips spin.
  Long lifetime!

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Riboflavin Demo
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• You are in graduate school and just synthesized a
  new organic complex that absorbs UV light. Your
  advisor is really excited and tells you that you
  can write a paper about it for submission to the
  Journal of Organic Chemistry but first wants more
  information about the new compound for inclusion
  in the paper. One piece of useful information
  would be the e. Design an experiment to do this.