Re: mass vs. volume measurement

"Old Man" <nomail@xxxxxxxxxx> wrote:

>
>"Andy Resnick" <andy.resnick@xxxxxxxxxxx> wrote in message
>> Yeah, I still don't really understand how osmometers work. Something
>> about freezing point depression, but it seems kinda voodoo.
>
>Not Voodoo. Lots of good physics !! It depends
>upon the non-equilibrium water evaporation rate
>from the thermocouple. Extremely small temperature
>drop. You can roughly calculate the vapor pressure
>over a solution from ion concentration. if NaCl is
>1 / 2, then CaCl2 is 1 / 3. However, tables in the
>Handbook of Chemistry & Physics are the standard.
>
>Works great until some slob mucks-up the thermo-
>couple with his solution. For fast response, the
>thermocouple is about the size of a dust mote. The
>electrical leads are virtually invisible. Dust motes get
>brushed away by untutored slobs.

There are also freezing point depression osmometers that work by
chilling a solution (about 0.5 - 5 ml depending upon the model used)
below the freezing point, seeding ice with a vibrating rod, and then
measuring the temperature plateau as ice grows with a thermocouple.
The freezing point depression (compared to pure water) is then
converted to osmoles by dividing by 1.86. More robust than vapour
pressure osmometers but the vibrating rods are prone to breakage and
solutions containing fish antifreeze proteins give wildly wrong
answers (whereas a vapour pressure osmometer gives the correct
osmolality).

Ken Muldrew
kmuldrezw@xxxxxxxxxxxx
(remove all letters after y in the alphabet)
.

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