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147
Π
c
=
RT
1
M
+
A
2
c
2
+
....
⎛
⎝⎜
⎞
⎠⎟
Important: Osmometry provides M
n
– the number average molecular weight
(Proof given in Section 3.3.3.)
3.3.2. Using osmometry
to determine M
n
First, samples with different (predetermined) concentrations (c
1
, c
2
, …, c
i
) (in
g/ml) are produced.
Then, the corresponding osmotic pressures (
Π
1
,
Π
2
, …,
Π
i
) are measured in
an osmometer. Then, for each sample, the ratio
Π
i
/c
i
is calculated, and plotted
as a function of c
i
.
According to the equation
Π
i
/c
i
is a linear function of c
i
. Thus, a linear
regression is performed obtaining:
Intercept = RT/M
Slope = RTA
2
(R gas constant, T is absolute temperature (K))
3.3.3. Polydispersity: Osmometry provides M
n
.
We use an argument that later will also be used for intrinsic viscosity and light
scattering. The important point is to consider the additive step for a
c
(mg/ml)
Π
/c
Regression line
c
1
c
2
c
3
c
4
c
0
= 0
(blank)