Chemistry Reference
In-Depth Information
Fig. 2.19
Schematic
of a high speed osmometer
(from ref [
68
])
Fig. 2.20
Extrapolation
to zero concentration
π
/C
0
C [g/l]
The results obtained from either method must still be extrapolated to zero concentration for van't
Hoff's law to apply. Such extrapolation is illustrated in Fig.
2.20
. Because all molecules contribute
equally to the total pressure, osmotic pressuremeasurements yield a number average molecular weight.
Light scattering
measurement is a technique for determining the weight average molecular weight.
When light passes through a solvent, a part of the energy of that light is lost due to absorption,
conversion to heat, and scattering. The scattering in pure liquids is attributable to differences in
densities that result from finite nonhomogenuities in the distribution of molecules within adjacent
areas. Additional scattering results from a presence of a solute in the liquid. The intensity or amplitude
of that additional scattering depends upon concentration, the size, and the polarizability of the solute
plus some other factors. The refractive index of pure solvent and a solution is also dependent upon the
amplitude of vibration. The turbidity that arises from scattering is related to concentration:
X
X
turbidity
t ¼
t
1
H
C
i
M
i
¼ H
c
M
w
2
4
3
n
o
2
H ¼
32
p
ð
d
n=
d
cÞ
=
3
l
N
o
where
n
o
is the refractive index of the solvent,
n
is the refractive index of the solution,
l
is the
wavelength of the incident light,
c
relationship is obtained by measuring the slope of the refractive index as a function of concentration.
It is constant for a given polymer, solvent, and temperature and is called the
specific refractive
increment
.
N
o
is Avogadro's number, and
c
is the concentration. The d
n
/d
