Hi Chang Wen!
I am aware of the possibility to use DSC for evaluating the
Melting point of a solution and then through the Freezing
Point Depresion (FPD) to calculate the osmotic pressure.
However, some times the melting point (melting peak) is influenced
by the cooling/heating protocol and the annealing time-temperature
(see also Ablett et al. 1992, J. Chem. Soc. Faraday Trans., 88:789-94).
It seem that by the use of Isothermal microcalorimeters it is possible to
achieve more accurate values of water activity (Aw) and solute activity
of a solution (binary or muticomponent) irrespectively of how concentrated
the solution is. see Berling et al., 1997, Thermochimica Acta, 3-7.
I suspect that you want to evaluate the osmotic pressure and osmolality
from relatively dilute solutions (FPD: 0.5-5), If so then the use of another
instrument, Milk Cryometer (thermistor cryoscope method, IDF standard
108B:1991), will solve the accurancy problem (+or -0.003C).
It is a very common instrument in every dairy laboratory, it is used for
cheking possible milk dilution with water. The "milk" folks might not agree
in the beginning that the instrument has that high accurancy from the range
of FPD between 1-5 but please take my word for it and if this is not
enough...
see also Chen et, al. 1996 Journal of Food Engineering, 30:239-253.
Having evaluated accurately enough the FPD, there are many good papers
that relate it with the Aw (water activity of the solution) see
(Ferro-Fontan &
Chirife, 1981. Journal of Food Technology, 16:21-30; Lerici et al.,
1983.Journal
of Food Science, 48:1667-1669)
the equation has the form:
-ln(Aw)=9.9693E-3(FPD)+4,761E-6(FPD)^2
in the case you would like to predict rather evaluating the Aw of your
solutions
see (Chen 1989, Journal of Food Science, 54:1318-21; Chen, 1990. Journal
of Food Science, 55:494-497,515.)
Having the Aw (calculated or evaluated) you could evaluate the Osmotic
pressure
of your solution based on the assumption that the molar volume of the water
in
your solution is equal with the molar volume of the pure solution
(relatively true
assumption) based on the equation:
OP=(RT/Vm)*-ln(Aw)
Where OP= osmotic pressure
R= universal gas constant
T= the temperature on which you whant to know the OP
Aw= water activity
I hope that those information could help
Best regards
Nikos
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Nikolaos E. Mavroudis, Tekn. Lic.
PhD-student, Food Engineering,
Center of Chemistry and Chemical Enginering
Lund University, Box 124, S-221 00 Lund, Sweden
fax +46 46 2224622 tel +46 46 2229822
WWW http://www.livstek.lth.se/info/Nikolaos.htm
"If you think education is expensive, try ignorance!"
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