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?

UV Spectroscopy and Qualitative Analysis

1) UV-vis spectroscopy is usually not very useful for qualitative analysis because there are few absorption maxima and minima

2) Solvents:a. Must be transparent in region of

interestb. Should not interfere with absorbing

species (but usually it does). Polar solvents tend to obliterate fine structural detail in molecular spectra

UV Spectroscopy and Qualitative Analysis

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

UV-vis spectroscopy and Quantitative Analysis

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

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

Construction of Calibration Curves is often done on the absorption

max. Why?

Sample and Reference cells should be matched

ExampleThe 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

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?

Relaxation Processes

1. Radiationless loss of energy in small steps; excitation energy converted into kinetic energy by small collisions with other molecules, small increase in temperature

2. 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

Stokes Shift 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

Results from thermal energy losses

More about fluorescence

Why do some molecules fluoresce and others don’t?

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

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

More about excess energy loss

2.Thermal radiation a.k.a blackbody radition1. 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

3. Phosphorescence (another way to lose excess energy)

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

Draw the spectra for Li and acetone

Relevent energy transitions for Li:• p p* 1.33 x 10-18 J• N s* 1.06 x 10-18 J• N p* 7.12 x 10-19 J

• Relevent energy transitions for acetone3.63 x 10-19 J3.26 x 10-19 J2.96 x 10-19 J

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.

Riboflavin Demo