Lecture Notes Monday March 12th

Answers to quiz

Molecular absorption spectrocopy in solution.

Mostly UV and visible parts of the spectrum (electronic spectroscopy). Once we get into the IR, the solvent absorbs (and this effect swamps the absorption by dissolved solutes at low concentrations). Need an absorbing molecular species or a reaction to create one. Usually the reagent absorbs radiation and so the product of the reaction must have different absorption characteristics: most likely a shift in absorbance maximum to longer wavelength. Molecules that absorb have a chromophore, which is often associated with pi-bonds and resonance structures (alternating single and double bonds), and aromaticity. Some transition metal species are highly colored because of charge transfer between a ligand and the metal (for example permanganate MnO_4_^-^).

Instruments give a linear relationship between absorbance A and concentration up to the point where stray light is more intense than light coming through the sample. For a good quality instrument this can be as high as A = 3 or 4. At A = 3, the percent transmission is only 0.1%, so not much light is getting through the solution.

At the other end of the concentration scale, a good quality instrument might be able to measure 0.01 above the noise on the blank absorbance and so an estimate of the lowest concentration that can be measured can be made by assuming a path length (1 cm) and a molar absorptivity value (say 10^4^ L/mol/cm). Thus we estimate that a solution containing 10^-6^ M can be detected. For a species of molar mass 100 g/mol, this is a 100 ppb solution (10^-6^ g/L). The instrument is doing the equivalent of looking a two bright lights (of power P_o_ and P) and trying to see a difference between them.

There are lots of published methods, so there are lots of reagents known. The issue often is what are the other species on the sample that (a) absorb at the wavelength you want to measure and (b) react with the reagent as well to give the same sort of colored product as the reaction with the analyte species? And then deciding how to achieve the desired separation.

You should be able to look at a molecular sturcture and decide whether it absorbs or not. Saturated structures (no double bonds) do not absorb very well, and structures with isolated double bonds don't absorb well either.

Absorbance is additive, so you can analyse mixtures by setting up simultaneous equations based on the absorbance at different wavelelengths and known calues of the molar absorptivities.