Applications of UV in analytical chemistry
Applications of UV in analytical chemistry:
The working formula:
A = ε b c
is derived from Lambert-Beer’s law, where A is the absorbance, E is molar extinction coefficient, b is the path length in cm and c is molar concentration (mol l–1). The absorbance values of the standard solutions of known concentrations are determined from the spectrophotometer and a calibration graph is constructed. Absorbance value of the test solution is determined from the instrument and the concentration of the test solution is determined from the calibration groups.
Fig. 1 Energy diagram for the electronic transitions.
If two or more chromophoric groups are present in a molecule and they are separated by two or more single bonds, the effect on the spectrum is additive. But there is little electronic interaction between isolated chromophoric groups. The X values for a, p unsaturated ketones, a, P unsaturated aldehydes and conjugated dienes bear an excellent numerical correlation depending on the number of substituent groups. The rules for calculation of λmax values of such compounds are relatively very simple and is exemplified below:
Calculation of Xmax values
- For a, P unsaturated ketones:
(i) In order to calculate the absorption (λmax ) of an a, P unsaturated ketones a base value of 215 mμ is assigned.
(ii) For each a-substituent 10 mμ are added to the base value.
(iii) For each p substituent 12 mμ are added to the base value.
(iv) For each ring system (6 or 5 membered) to which the carbon-carbon double bond is exocyclic 5 mn are added to the base value.
(v) If the carbon-carbon double bond and the carboxyl group are in a five-membered ring 10 mμ subtracted from the calculated value and if only the carbon-carbon double bond is in a five membered ring 5 mμ is added.
The correlation of calculated values and experimental values is within a permissible limit.