Question

Alkanes such as n-hexane are used as solvents in UV spectroscopy. N-hexane absorbs at the wavelength of 1.38x10^-7m (or 138nm) which is within the UV spectral range (300nm-10nm). Why is n-hexane a good choice as a solvent for UV/vis spectroscopy?

Answer 1

The choice of the solvent to be used in UV spectroscopy is quite important.

The first criterion for a good solvent is that it should not absorb UV radiation in the same region as the substance whose spectrum is determined. Usually solvents which do not contain conjugated system are most suitable for this purpose, although they vary as to the shortest wave length at which they remain transparent to UV radiation.

n-hexane is non conjugated and transparent in the regions of the UV spectrum where interesting absorption peaks from sample molecules are likely to occur.

A second criterion for a good solvent is its effect on the fine structure of the absorption band. n-hexane is a nonpolar solvent. A nonpolar solvent like n-hexane does not hydrogen bond with the solute and the spectrum of the solute closely approximates the spectrum that would be produced in the gaseous state where fine structure is often observed. In a polar solvent, the hydrogen bonding forms a solute-solvent complex and the fine structure may disappear.