Handbook of Solvents - George Wypych - ChemTech - Ventech!

2.3 Basic physical and chemical properties 47
Refreactive index
1.55
1.5
1.45
1.4
1.35
alcohols
aromatic CH
esters
ketones
1.3
0 50 100 150 200 250
Molecular weight, Daltons
Figure 2.3.8. Refractive index for four groups of
solvents as the function of their molecular weight.
Refractive index
1.32
1 2 3 4 5 6 7 8
too heavy to handle. When metering by volume, temperature correction should always be
used because solvent specific gravity changes substantially with temperature.
2.3.4 REFRACTIVE INDEX
Refractive index is the ratio of the velocity of light of a specified wavelength in air to its velocity
in the examined substance. When the principle of measurement is used it may be defined
as the sine of the angle of incidence divided by the the sine of the angle of refraction.
The absolute angle of refraction (relative to vacuum) is obtained by dividing the refractive
index relative to air by a factor of 1.00027 which is the absolute refractive index of air. The
ratio of the sines of the incident and refractive angles of light in the tested liquid is equal to
the ratio of light velocity to the velocity of light in vacuum (that is why both definitions are
correct). This equality is also referred to as Snell’s law.
Figures 2.3.8 and 2.3.9 show the relationship between the molecular weight of a solvent
and its refractive index. Figure 2.3.8 shows that there is a general tendency for the refractive
index to increase as the molecular weight of the solvent increases. The data also
indicates that there must be an additional factor governing refractive index. The chemical
structure of the molecule also influences refractive index (Figure 2.3.9). Normal alcohols
have a slightly higher refractive index than do branched alcohols. Cyclic alcohols have
higher refractive indices than the linear and branched alcohols. For example, 1-hexanol has
refractive index of of 1.416, 4-methyl-2-pentanol 1.41, and cyclohexanol 1.465. Aromatic
hydrocarbons are not dependent on molecular weight but rather on the position of
substituent in the benzene ring (e.g., m-xylene has refractive index of 1.495, o-xylene
1.503, and p-xylene 1.493).
The data also show that the differences in refractive indices are rather small. This imposes
restrictions on the precision of their determination. Major errors stem from poor instrument
preparation and calibration and inadequate temperature control. The refractive
index may change on average by 0.0005/ o C. Refractive index is useful tool for determination
of solvent purity but the precision of this estimation depends on relative difference be-
1.44
1.42
1.4
1.38
1.36
1.34
normal
branched
Number of carbon atoms in alcohol
Figure 2.3.9. Refractive index of normal and branched
alcohols as the function of number of carbon atoms in
alcohol.