Physical bases of freezing point measurement using ... - Boschung

Proceedings of the 9 th SIRWEC Conference 15-17 March 1998, Luleå, Sweden 280
with m = m + m .
tot NaCl H2O The volumetric properties of aqueous sodium chloride solutions are for instance given
in [Rogers and Pitzer 1982]. For temperatures close to 0 °C, the specific volume ν (in cm 3 /g)
can be fitted by :
2
ν( c) = 0. 653c − 0. 704 c+
0. 999
It is then possible to compute the thickness e of the solution :
e =
c ⋅ m
A
. (8)
ν( ) tot . (9)
It remains to express the freezing temperature Ts corresponding to the considered salt
concentration. Ts is of course given by the phase diagram of the NaCl - H2O mixture [Zaytsev
and Aseyev 1992] represented in figure 5.
T s [°C]
0
-5
-10
-15
-20
-25
c
0 0.05 0.1 0.15 0.2 0.25 0.3
Figure 5 : NaCl - H2O phase diagram. Eutectic coordinates are ceutectic = 0.2331
and Ts min = -21.12 °C.
In order to compute the freezing temperature, the curves have been fitted. The one with
negative slope (c ∈ [0, 0.2331]) is approximated by the following polynomial :
3 2
T ( c) =−470. 65c −32. 625c −57.
232 c
s
while the one with positive slope (c ∈ [0.2331, 0.266]) is approximated by :
, (10)
3 2
Ts( c) =− 531, 791c + 393, 483c − 96, 251c+ 7, 770. 5 . (11)