Solvent Effects on the UV-visible Absorption Spectra

Spectrophotometric measurements can be carried out in gaseous, liquid or solution as well as solid state of a light absorbing analyte. Majority of UV-visible spectrophotometric measurements require the analyte(s) to be dissolved in an appropriate solvent. Water and many organic solvents such as ethanol, acetonitrile, cyclohexane, dimethylformamide (DMF), dimethylsulfoxide (DMSO), etc. are used for this purpose. Solventsshould dissolve the sample well, be optically transparent (not absorb incident radiation), and chemically inert and pure.Although solvents should be, in principle, optically transparent, most of the solvents absorb significant amount of light especially in the UV regions of electromagnetic spectrum. Therefore, one should choose a solvent that allows optimal transmission of the light in the wavelength range of interest. Further, any finite absorption of light by the solvent in the wavelength range of interest should be subtracted as a blank or reference spectrum from the analyte spectrum.

It has been observed that the positions, intensities and shapes of the absorption spectral bands of light absorbing molecules depend on the nature of the solvent and often on the solvent polarity. Based on polarity scales, solvents are classified as polar and nonpolar solvents. Water, ethanol, acetonitrile, diethyl ether, etc. are a few examples of polar solvents. On the other hand, carbon tetrachloride, nhexane, cyclohexane, benzene, dioxane, etc. are a few examples of nonpolar solvents. Polar solvents like water and alcohol tend to obliterate the fine spectral details of the absorbing species unlike nonpolar ones. Therefore, a nonpolar solvent like n-hexane should be used where the fine spectral details are required as in the case of qualitative analysis.