Chemistry Reference
In-Depth Information
of these effects may depend on drop sizes, solute diffusivity, and the presence of
volatile impurities in the solvent or solute. The vapor phase osmometer is not a
closed system and equilibrium cannot therefore be reached. The system can be
operated in the steady state, however, and under those circumstances an analog of
expression (3-33) is
Δ
T
c
2
5k
s
1
M
n
1Bc
2
1Cc
2
1
?
(3-34)
where
k
s
is an instrument constant. Attempts to calculate this constant
a priori
have not been notably successful and the apparatus is calibrated in use for a given
solvent, temperature, and thermistor pair, by using solutes of known molecular
weight. The operating equation is
k
ðΔΩ=cÞ
c5
0
M
n
5
(3-35)
where
k
is the measured calibration constant and
is the imbalance in the
bridge (usually a resistance) that contains the two thermistors.
There is some question as to whether the calibration is independent of the molec-
ular weight of the calibration standards in some VPO instruments. It is convenient to
use low-molecular-weight compounds, like benzil and hydrazobenzene, as standards
since these materials can be obtained in high purity and their molecular weights are
accurately known. However, molecular weights of polymeric species which are
based on the calibrations of some vapor phase instruments may be erroneously low.
The safest procedure involves the use of calibration standards that are in the same
molecular weight range, more or less, as the unknown materials to be determined.
Fortunately, the low-molecular-weight anionic polystyrenes that are usually used as
gel permeation chromatography standards (
Section 3.4.3
) are also suitable for vapor
phase osmometry standards. Since these products have relatively narrow molecular
weight distributions all measured average mo
lec
ular weights should be equal to each
other to within experimental uncertainty. The
M
v
average (
Section 3.3.1
) of the poly-
styrene should be considered as the standard value if there is any uncertainty as to
which average is most suited for calibration in vapor phase osmometry.
Vapor phase osmometers differ in design details. The most reliable instru-
ments appear to be those incorporating platinum gauzes on the thermistors in
order to ensure reproducible solvent and solution drop sizes. In any case, the
highest purity solvents should be used with this technique to ensure a reasonably
fast approach to steady-state conditions.
The upper limit of molecular weights to which the vapor phase osmometer can
be applied is usually considered to be 20,000 g mol
2
1
. Newer, more sensitive
machines have extended this limit to 50,000 g mol
2
1
or higher. The measurements
are convenient and relatively rapid and this is an attractive method to use, with the
proper precautions.
ΔΩ
