Does a Universal Spectroscopic Solvent exist?

Does a Universal Spectroscopic Solvent exist?
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Spectroscopic grade solvents (Image Courtesy : http://3.imimg.com/)

The answer to this question is NO. Water has been termed as a universal solvent because of its natural abundance and its ability to dissolve most substances. Though it finds use in either pure form or in combination with acids, bases or specific reagents in spectroscopic studies it cannot be termed as a universal spectroscopic solvent as it shows absorption bands in NIR region. Likewise each solvent has its merits and demerits concerning applicability for spectroscopic analysis. In this article we shall examine the properties that make a solvent a good choice for spectroscopic studies

Solubility

Solubility of a solute in a solvent is a prerequisite for spectroscopic analysis. A solid will dissolve when solute – solute forces become lower than solute – solvent forces in presence of the solvent. In the solution the forces that come into play are dipole – dipole interactions, hydrogen bonding and van der Waal forces. The solid must be fully soluble otherwise insoluble solid particles will contribute to errors due to scattering of incident light beam.

The universal solubility principle provides us valuable guidance. Remember polar solvents will dissolve polar compounds and non-polar solvents will dissolve non-polar compounds.

Solvents can also influence spectral characteristics. In general a nonpolar solvent does not hydrogen bond with the solute and the spectrum resembles the pure solute spectrum in the gaseous phase. On the other hand polar solvents can change spectral characteristics due to hydrogen bonding. Such interactions can shift the position of the chromophoric absorbance bands or wipe out the vibrational fine structure of the spectrum.

Non-Toxicity

The chosen solvent should be preferably non- toxic. It is advisable to use gloves while preparing solutions to prevent skin contact. Though spectroscopic analysis does not require large volume handling but if you are doing repetitive analysis using toxic solvents day after day toxicity can become an issue. It is also necessary to follow safety instructions provided on the label as well as in the Material Safety data sheet.

Non-Flammability

As in the case of toxic solvents flammable solvents handling also requires handling care. Large volumes of the solvents should be stored in separate areas away from the instrument room.

Inertness

The selected solvent should be inert towards sample handling cuvettes. Acid mixtures such as aqua regia will dissolve and damage the sample cells in no time. Strong acids or bases can also lead to leaching and early decay of sample cells. Similarly organic solvents should not be used with plastic cuvettes.

Transparency over the spectral region

Spectral analysis should be conducted over the wavelength range over which the solvent does not have its absorption bands. A prior knowledge of the cut-off point of the solvent is a great help. It is defined as the point at which the solvent has a transmittance of 10% (1.0 absorbance) in a cuvette of path length 1cm. Below the cut-off point the solvent becomes opaque and the reported results become misleading.

Spectroscopic solvents are available in different purity grades and the cut-off wavelength which generally falls in the UV region is stated on the bottle label. Some approximate cut-off points for solvents commonly used are provided for guidance purpose. Use the solvents always above the cut-off wavelengths.

SolventCut-off wavelength (nm)
Acetonitrile190
Water190
Hexane195
Cyclohexane200
Heptane200
Methanol205
Isopropanol205
 Ethyl alcohol 210
 Trifluoroacetic acid 210
 Tetrahydrofuran 210
 n-Butyl alcohol 215
 1,4 –dioxane 215
 Dichloromethane 233
 Chloroform 245
 Toluene 254
 Ethyl acetate 256
 Dimethylsulfoxide 268
 Methyl Ethyl ketone 330
 Acetone 330

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