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What is an Osmometer?
An instrument for measuring solute particles in a solution
What properties have to be considered in osmometry?
Colligative; how each would need to change to incr. osmolality.
What are the 4 colligative properties? Which concern osmometry?
1. Osmotic pressure
2. Vapor pressure - this.
3. Boiling point
4. Freezing point - & this.
How does Vapor pressure vary with increasing osmolality?
For every mOsm, Pvap decreases 0.3 mm Hg
How does Freezing pnt vary with increasing osmolality?
For every mOsm, freezing pnt decreases 1.86 ' C.
What IS vapor pressure?
The pressure needed to prevent molecules from escaping from a liquid.
What happens to Pvap when you add solute?
Decreases; need less pressure to keep from evaporating.
On what principle does an osmometer operate?
the principle of freezing point depression.
What is the name of the most common brand of osmometer?
What are the 5 components of an osmometer?
1. Cooling bath
2. STirrer
3. Thermister probe
4. Readout
5. Measuring potentiometer
What's in the cooling bath? How does it work?
Antifreeze; the patient sample gets lowered into it.
What 2 things stick into the patient sample?
-Stirrer to agitate vigorously
-Thermister probe
How does a thermistor probe work?
Its resistance changes with temperature changes, giving a readout that is proportional.
On what point of the temp versus time curve is the osmolarity reading taken?
at the plateau region - the heat of fusion, equilibrium temp between thawing and freezing.
On the osmometer, what is the
-Input transducer?
-Output transducer?
Input = thermistor
Output = Wheatstone bridge
How do you calibrate the osmometer?
By first using a standard of 300 mOsm; setting one potentiometer on the wheatstone brdg at 300;
Use 500 mOsm standard and do same w/ other POT.
Use Calibrated pot to set zero;
Measure patient sample, adjust final Pot for value.
How does the Wescor Vapor pressure osmometer work?
Same concept as the osmometer, but you replace the graph values of heat of fusion for "Heat of condensation", between liquid and pressure.
What are 2 problems with osmometry?
-Sample volume the holder holds 2-3 ml of plasma
-Vat of Antifreeze maintanence.
What are 3 advantages of Osmometry?
-Small sample size
-Short time period of measurement.
-Sample is not altered in anyway.
What is the Dew point?
The temperature at which vapor in air goes into the liquid phase because of atmospheric pressure.

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