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.
Conjugated systems:
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 λ_{max} values of such compounds are relatively very simple and is exemplified below:
Calculation of X_{max} values
- For a, P unsaturated ketones:
(i) In order to calculate the absorption (λ_{max} ) 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.