Instant Notes: Analytical Chemistry

one substituent present, and tables are given in standard spectrometry texts

listing these.

Identification of unknown organic samples can be considerably aided by

considering the UV-visible absorption spectra. The following general rules may

be used as a guide:

Observation Possible conclusion

NoUV absorption present. sbonds or lone pairs only

Isolated double bond

Strong absorption between 200 Aromatic ring

and 250 nm (e~1000)

Weakabsorption near 300 nm (e~1) Carbonyl compound.

Example

An organic compound, C 7 H 14 O, gave a UV spectrum with a peak at 296 nm and

e=3.7 m^2 mol-^1. Is it more likely to be a ketone or an alkene?

The formula allows the possibility of only one double bond. It must therefore

be an alkene with an isolated double bond, absorbing below 200 nm, or a ketone

with a weak n-p* transition near 300 nm. The value of both the absorption

maximum and of the absorptivity suggest a ketone.

Fluorescence is also related to structure. The most intense fluorescence is found

in compounds with conjugated structures, especially polycyclic aromatic

compounds. Fluorescence is particularly favored for rigid structures, or where

the sample is adsorbed onto a solid surface, or in a highly structured metal ion

complex. This is useful for the quantitative determination of metals.

Metal complexes involving organic or inorganic ligands are important in

analytical determinations.

The transitions which are responsible for visible and UV absorption by

complexes may be classified as follows.

(i) d–d transitions due to a transition metal ion. These give rise to the color of

many compounds of transition metals and are modified by changing the

ligands that are bound to the central atom. The spectrochemical series

relates the increasing strength of the ligand field to the shift of the absorp-

tion band towards the UV. However, these transitions often have low

absorptivity constants, which makes them suitable only for determining

high concentrations of metals.

(ii) The absorption bands of an organic ligand may be modified when it is

complexed with a metal. The complexes formed with dyes such as eriochrome

black T, used in complexometric titrations as an indicator, and metals such

as magnesium, have high absorptivities. Complexing agents such as

dithizone (C 6 H 5 .NH.NH.CSN=N.C 6 H 5 ) form highly colored complexes with

several metals.

(iii)Charge-transfer bandsarise because of transitions between the levels of an

electron donor, often the sor porbitals of the ligand, and electron acceptor

levels, such as the empty orbitals of a transition metal. The intense red color

of the iron(III) thiocyanate complex and the purple color of the perman-

ganate ion both arise as a result of such transfers.

Examples of the quantitative determination of metals by the direct absorptio-

metry of complexes formed by the addition of chromogenic reagents and also

230 Section E – Spectrometric techniques