Instrumental Analysis, ChFE 5503
Refractive Index of a Liquid
A. The Refractive Index (R.I.) is an important property of the molecular structure of a liquid. For
pure hydrocarbons, one can get an idea of aromatic content of a liquid using R.I.
R.I.
aromatics
1.50
naphthenes
1.44
saturates
1.37
B. Theory:
When a light beam passes from one substance into another, the beam is refracted (bent)
so that it travels in a different direction. The extent of refraction depends on:
1) the relative concentration of atoms or molecules
2) the structure of the atoms or molecules
A refractometer measures this property (R.I.) using the D line of sodium.
N/n = sin i1/sin r1
N = R.I. of prism
n = R.I. of liquid
N/na = sin i/sin r
na = R.I. of air (= 1)
since r + rl = 90°,
sin r = cos r1 = (1-sin2r1)
Hence, n =  (N2 - sin2i) and
n is determined from angle i and N
R.I. is reported as nD =
.
C. The R.I. is measured by the critical angle method with a Bausch and Lomb or Abbe
Refractometer using monochromatic light.
R.I.
=
(Sine of Angle of Incidence/sine of angle of refraction) as
light passes from air into the substance.
Refractive Index of a Liquid - 2
In the Abbe Refractometer liquid is placed between two
prisms, which are rotated to obtain grazing refraction angle.
- This is achieved by a focusing lens, which gives a dark field
and a light field in the viewpiece.
- When the two fields are equal (as determined by a set of
crosshairs), the angle is measured.
- A calibrated scale gives the R.I. directly for this angle and
the R.I. of the prism.
Procedure: (ASTM 1218, 1747)
1.
Set the bath temperature to 20C.
2.
Open the prism and place approximately two drops of the
liquid sample on the top of the bottom prism. Use care not to
touch the prism surface with the dropper.
3.
Close the prism case and place the lamp close to the prism.
Turn on the lamp.
4.
Adjust the hand wheel on the right side of the instrument until
the viewer appears light above and dark below the center
horizontal line.
5.
Adjust the compensator dial on the front until the image in the
viewer shows a sharp line between light and dark.
6.
Adjust the hand wheel on the right until the horizontal line in
the viewer passes between the crosshairs.
7.
Depress and hold the switch on the left and read the R.I.
value.
8.
Clean the prism with acetone and a Kimwipe.
Refractive Index of a Liquid - 3
D. Molar Refraction, Mr
Refraction of light is an additive property, but also depends upon the structural arrangement
of atoms in a molecule. From the electromagnetic theory of light, the molar refraction, Mr, is given
by:
MW(n2-1)
Mr =
ρ(n2+2)
The molar refraction can be calculated from the sum of the individual atomic refractions of
the atoms and groups present. This can sometimes be used to determine the structure of an unknown
compound whose molecular formula is known. It is not useful for mixtures.
Table I. Atomic Refractions
Group
Mr
Group
Mr
CH2
4.618
Cl
5.967
H
1.100
Br
8.865
C
2.418
I
Double bond (C=C)
1.733
N (pri-amines)
2.322
Triple bond (CC)
2.398
N (sec-amines)
2.499
O (carbonyl) (C=O)
2.211
N (tert-amines)
2.840
O (hydroxyl) (OCH)
1.525
CCN
5.459
O (ethers) (RCOCR)
1.643
13.900
Example:
CH3
A substance having the analysis C3H6O might be either acetone
CO
CH3
OR
H
allyl alcohol
C=CCCCOH.
H H
Determine which of these two chemicals the substance is from the fact that
its molar refraction, Mr, is 16.971.
Refractive Index of a Liquid - 4
Answer to example:
Acetone
Allyl Alcohol
3 carbons
=
7.254
3 carbons
=
7.254
6 hydrogens
=
6.600
6 hydrogens
=
6.600
2.211
1 double bond
(C=C)
=
1.733
1 hydroxyl oxygen =
1.525
1 carbonyl
oxygen
=
CCC
16.065
Since the experimentally determined molar refraction agrees more
closely with that calculated for ally alcohol, the substance probably is
ally alcohol rather than acetone.
CCC
17.112