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Study Notes: Choosing an Appropriate Solvent

An important consideration when selecting a solvent for preparation of a sample for spectrophotometric analysis is transparency.

The solvent needs to be transparent to below the wavelength at which measurements are to be made. Just as it applies to sample cells of glass or quartz construction, a solvent should not be used that will significantly absorb at the analytical wavelength (and so interfere with the analysis).

The table below lists the minimum wavelength (for a 1 cm cell) below which a range of common solvents should not be used because of absorption. Note that solvent purity, with regard to the type and level of contaminants, may affect the position of the minimum by pushing it to longer wavelengths.

Solvent
Minimum Wavelength (nm)
Water
190
Ethanol
210
n-Hexane
195
Cyclohexane
210
Benzene
280
Diethyl Ether
210
Acetone
330
1,4-Dioxane
220

Reference: Principles of Instrumental Analysis, D.A. Skoog and J.J. Leary, Fourth Edition, 1971, Saunders College Publishing.

One of the most commonly used solvents is 95% ethanol – it is cheap, has good solvent characteristics and is transparent down to around 210 nm. Others include water, cyclohexane and 1,4-dioxane.

Other considerations that impact the choice of solvent are summarised below (these have previously been discussed in Section 3, Task 1).

  1. The nature of the solvent will influence the position of absorption maxima ( lambdamax). It is important therefore that the same solvent is used when comparing spectra in qualitative analysis. In quantitative analysis, samples, standards and blanks must all be prepared using the solvent that is specified.

  2. Polar solvents such as water and alcohol will remove the spectral fine structure that arises from molecular vibrational movements of samples in the gas phase. These affects may be observed to varying extents in the spectra of samples prepared with non-polar solvents such as hexane.
Broad smooth absorption peaks occur for spectra where the polar solvents, water and alcohol, are employed. Spectra showing a peak with fine structure, more pronounced with the sample in gas phase than dissolved in the non-polar solvent hexane.

 

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