REFRACTOMETRY
principle
 measures the extent to which light is bent
(i.e. refracted) when it moves from air into a
sample and is typically used to determine the
index of refraction (aka refractive index or
n) of a liquid sample.
Figure (1)
 The refractive index is a unitless number,
between 1.3000 and 1.7000 for most
compounds, and is normally determined to
five digit precision.
 Since the density of a liquid usually decreases
with temperature, it is not surprising that the
speed of light in a liquid will normally increase
as the temperature increases. Thus, the index
of refraction normally decreases as the
temperature increases for a liquid. For many
organic liquids the index of refraction
decreases by approximately 0.0005 for every
1 °C increase in temperature
Factor effecting refracting
index:
 1-temperature of the sample .
 2-wavelength of light used .
 Ex.:
 The italicized n denotes refractive index, the
superscript indicates the temperature in degrees
Celsius, and the subscript denotes the
wavelength of light (in this case the D indicates
the sodium D line at 589 nm).
 Whenever light changes speed as it crosses a
boundary from one medium into another it is
refracted (Figure 1). The relationship between
light's speed in the two mediums (vA and vB),
the angles of incidence (qA) and refraction
(qB) and the refractive indexes of the two
mediums (nA and nB) is shown below:
where M is the molecular
weight expressed in ml.mol,
n it is the refraction index
and r the density.
Instrument:
figure (2)
Cross hair:
figure (3)
figure(4)
the cross hair should be half dark & half light if not
adjust it using the knob.
Why the refracting prism would show
a light region to the left and a
dark region to the right?
Figure(5)
 Inspection the Figure(5) shows that light
traveling from point A to point B will have the
largest angle of incidence (qi) and hence the
largest possible angle of refraction (qr) for
that sample. All other rays of light entering
the refracting prism will have smaller qr and
hence lie to the left of point C
What is dispersion?
 dispersion would result in the light and dark
borderline being in different places for
different wavelengths of light (appears as
"fuzziness" of the borderline)
How to over come dispersion?
 "compensating prisms" into the optical path
after the refracting prism. These
compensating prisms are designed so that
they can be adjusted to correct (i.e.,
compensate for) the dispersion of the sample
Applications:
 Qualitative analysis: in which the refractive
index of each element is constant.
 Quantitative analysis: we can get the
concentration of an element from the
refractive index.
 HPLC: as a detector.
 In the testing the purity of a solvent.
POLARIMETRY
POLARIMETRY:
 It is the re study of the rotation of light by
different substances.
 Theory:
Light( electromagnetic radiation) is randomly
distributed, it consist of electrical and
magnetic vectors perpendicular to each
other.
Polarized light is the light which vibrates in one
direction and it can be obtained by passing
the light through a polarizer.
substances
Optically active
(chiral center)
Optically
inactive
Dextrorotatory
Levorotatory
(clockwise)
Anti-clockwise
 In optically active cpds. Rotation occurs in a
certain angle known as the angle of
rotation(α°)Which defined as the number of
degrees of rotated plan .
 It depends on:
1- temperature.
2-concentration.
3-solvent (in which the sample is dissolved)
4- λ
N.B: it’s the reading I get from the instrument.
Specific rotation:
Equation(1)
[α] = specific rotation with the unit of
degree*ml/dcm*gm, T = temperature,
λ = wavelength of Na= 589.3, α = optical rotation,
c = concentration in g/ml, l = optical path length in
dm ( usually 2 dm).
N.B: in case of pure liquids we use density D instead
of the concentration C .
Specific rotation:
 Defined as the no. of degrees of rotation
observed when using 1 dcm. tube and 1
gm/ml of the substance.
 It depends on:
1- temperature.
2-solvent.
3- λ.
Applications:
 Qualitative analysis: in which each material
has its own specific rotation( at a specific T
and λ).
 Quantitative analysis :we can get the
concentration from the equation
Molecular rotation:
t

t
( M) = (α ) *M / 100
λ
λ
* Where M is molecular weight.