What is osmometry?
Technique for measuring the conc of dissolved solute particles that contribute to the osmotic pressure of a solution
What is osmotic pressure?
- Governs movement of water across membranes
- The pressure that would have to be exerted on the high solute side to prevent the flow of water
What are colligative properties?
Properties of solution that depend on the number of molecules in a given volume of solvent rather than the properties of the molecules themselves
What are some osmometry methods in the clinical setting?
- Freezing point depression
- Vapour pressure lowering
- Colloid osmotic pressure
Note 229 labs of 236 on UKNEQAS used freezing point depression osmometers
How does the freezing point depression work?
Thermistors resistance changes with temperature
- Sample and therminstor is lowered into cooling bath
- Sample cooled to 7 degrees below freezing
- Crystallisation is initiated either by rapid vibration or solenoid knocking
- Slush formed around thermistor, releasing latent heat of fusion and raising the temperature
- Plateau in temperature = freezing point - when latent heat of fusion and abstraction of heat from the cooling bath are in equilibrium. Here takes measurement
- Freezing point is inversely proportional to osmolality
Describe the graph of freezing point depression.
Temp against time
- Begin with a fast cool which reaches below freezing point and rate begins to slow until the sample freezes
- When the sample freezes there is a straight line up to the freezing point where it plateaus
- After the plateau the readout is made
What is vapour pressure?
The pressure of a vapour in thermodynamic equilibrium with its condensed phases in a closed system
Describe how vapour pressure affects the liquid?
- A liquid will evaporate at all pressure below its vapour pressure and remain stable at all pressures above the vapour pressure
The more concentrated a sample is the more readily it will evaporate
How does a vapour pressure osmometer work?
- It is related to Dew point rather than vapour pressure
- Dew point= temp at which condensate=vapour
- Cooled down below dew point of atmosphere
- Cooling stopped, condensation begins to form and warms the thermacouple
- Temp stabilizes as no more condensation forms = dew points
What is colloid osmotic pressure?
- Measuring cell consist of 2 chambers seperated by a semi-permebale membrane
- Sample is placed on one side of the membrane and colloid free physiological saline solution is placed on the other reference side
- Solvent will move via osmosis from the saline side
- Change in compartment volumes are directly related to osmolality in the sample
- Hard to synthesis a membrane that is selective enough
- Only good for larger molecules like proteins
What effects can be seen in low osmotic solutions, using vapour pressure lowering?
Vapour pressure will be less negative, opposite for high osmotic concentration
- More condensation and a higher TD(dew point temp)
Clinical applications of osmometry
- Investigation to electrolyte disordes: hyponatraemia and hypernatraemia
- Urine conc ability: Patient with poluria or oliguria
- Screen for toxin ingestion: e.g. salicylate, ethanol, methanol
- Measurement of the faecal osmotic gap may be helpful in differentiating secretory from osmotic diarrhoae
- Hydration levels of athletes
What is normal ranges of urine osmolality?
No reference range available, depedent on: serum results
- If serum osmolality is high, urine osmolality should be high (conc urine, water reabsorbed)
- If serum osmolality is low, urine osmolality should be low (no ADH released and water lost in the urine)
How does increased osmolality affect the colligative properties?
Changes in colligative poperties with increased osmolality:
- Osmotic pressure - increased
- Boiling point: increased
- Vapour pressure: decreased
- Freezing point: decreased
What does the plateau on the freezing point depression represent?
Freezing point - latent heat of fusion and abstraction of heat from cooling bath are at equilibrium
True or False. Freezing point is directly proportional to osmolality
False it is inversely proportional
How does vapour pressure affect liquids?
Liquid evaporates below vapour pressure, but is stable above it
Describe boiling point in terms of vapour pressure.
When atmospheric pressure=vapour pressure, as temp rises vapour pressure overcomes atmospheric pressure and vapour ensues
What is the dew point?
Temp at which condensate=vapour
How is dew point affected by vapour pressure?
As vapour pressure decreases the dew point decreases
Using the vapour pressure osmometer, when is dew point reached?
Temp stabilises as no more condensation forms = dew point
Depeding on the serum osmolality what can you make of the urine osmolality?
Serum osmolality is high, urine osmolality should be high
If serum osmolality is low then the urine osmolality is low
