http://en.wikipedia.org/wiki/Osmometer

Years ago when I worked in chemistry, we used a freezing point depression osmometer for serum and urine samples. I'm not sure if this is current methodology or not. Anyone out there in chemistry land know?

Here are some more URL's of interest......

http://www.clinchem.org/cgi/reprint/14/1/38

http://www.aicompanies.com/fsk_products/210/210_faq.htm

http://tinyurl.com/2zu46m

Judy Dilworth, M.T. (ASCP)
Microbiology

Judy gave some good references there. Of all the four ways to measure
osmolality, freezing point is used clinically because of possibility
of volitile alcohols being present which would impact vapor pressure
methods.  As far as ease of measurement then yes it would be also.
There are other ways to measure colligative properties including
decreases in vapor pressure, increases in boiling point and increases
in osmotic pressure.

Molarity and osmolarity are related but not identical and so
osmolarity and osmolality are not the same either. Osmolarity is
calculated based on the molarity of the substance. Osmolality is a
measured value and so in reality we report out osmolality in terms
used by freezing point depression of mOsmo/Kg of water.
An ideal osmolarity of one would decrease the freezing point by
-1.86C.

Vapor pressure can be calculated from a form of Von't Hoffs law. In
biological fluids at 310K degrees, plasma with an osmolality of 0.300
Osmoles/Kg of water the osmotic pressure must be 0.3 osmo/kg x 0.082
kg atm/osmol k x 310K = 7.63 atms. This is equivalent to the pressure
exerted by a column of water 76 meters high.

The above example was stolen from somewhere on the net. I did give an
inservice on the topic and so it's a short reply to a 25 page doc that
I wrote pertaining to osmometry.
Hope it helps

On 6/6/07 5:06 PM, in article
kpOdnfgCwJa90frbnZ2dnUVZ_oytnZ2d@buckeye-express.com, "JEDilworth"
<bactitech@nospamhortonsbay.com> wrote:

Thank you for the information. I was thinking of measuring freezing point
depression. Considering how sloppily I work, I doubt that I would get much
supercooling.

In fact, I was looking at eBay to see if I could pick up a cheap Beckman
thermometer. I saw one like that at a swap meet for about $30 but thought
that I would never be interested in it.

I note that the first URL (not wiki) given in your reply referred to the
Beckmann method. I wonder whether the Beckmann method used a differential
thermometer. Arnold Beckman (one n) of pH meter fame probably did not make
such a thermometer according to the Beckman Institute¹s Beckman Room Museum
at Caltech. I used what I thought was a Beckman differential thermometer
many years ago. I thought, at the time, that it had been made by Beckman
instruments. It was a mercury thermometer with graduations of about 0.002°C,
but was not suitable for absolute measurement.

Bill

June 9

I have been having some problems with my news reader so I did not respond
sooner. I also saw a response today by Robert <Goldentouchman@yahoo.com>
that had useful information

Bill