Osmometry Flashcards
What is Osmometry?
A technique for measuring the concentration of dissolved solute particles that contribute to the osmotic pressure of a solution.
What is Osmossis?
- Movement of water across a membrane
- Due to the osmotic pressure on both sides of the membrane
- Moves from the more dilute to the more concentrated
What is Osmotic Pressure?
- Governs the movement of water across membranes that separate two solutions.
- Selective membranes are permeable to water and small molecules and ions but not to larger molecules e.g. proteins.
- When there is a difference of osmotically active molecules across a membrane that cannot cross, those that can cross move to establish osmotic equilibrium – exerting
- At equilibrium the gravitational pressure of the column of solution in the manometer equals the osmotic pressure and prevents further movement of water.
- The osmotic pressure is defined as the pressure that would have to be exerted on the high solute side to prevent the flow of water.
What is osmolarity and osmolality?
- Osmolarity: Concentration per volume of solution 1 osmolar solution = 1 mmol/L solution
- Osmolality: Concentration per mass of solvent 1 osmolal solution = 1 mmol/kg H2O
What is the advantage of using Osmolality?
- Used in biological systems which contain a mixture of different substances as comparisons can be made between solutions of different strength, irrespective of the molecular weights of the substances dissolved in them.
What is Colligative Properties?
Properties of solutions that depend on the number of molecules in a given volume of solvent rather than the properties of the molecules themselves.
What are the changes to colligative properties?
Changes to colligative properties with increased osmolality:
- Osmotic pressure - increased
- Boiling point - increased
- Vapour pressure - decreased
- Freezing point - decreased
What are osmometry methods?
- Freezing point depression
- Vapour pressure lowering
- Colloid Osmotic pressure
What is freezing point depression?
- The thermistor’s resistance changes with temperature.
- The sample and thermistor are lowered into a cooling bath.
- The sample is cooled to 7˚C below it’s freezing point.
- Crystallisation is initiated either by rapid vibration or solenoid knocking.
- Slush forms around the 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. Measurement taken at this point.
- Freezing point is inversely proportional to osmolality.
What is Vapour Pressure?
Vapour Pressure: The pressure of a vapour in thermodynamic equilibrium with it’s condensed phases in a closed system.
- A liquid will evaporate at all pressures below its vapour pressure and remain stable at all pressures above the vapour pressure.
- Boiling point – when atmospheric pressure= vapour pressure, as temp rises vapour pressure overcomes atmospheric pressure and vapour ensues.
- A substance with a high vapour pressure at normal temperature is volatile.
What is Vapour Pressure osmometer?
- Is related to Dew point rather than vapour pressure
- Dew point = temp at which condensate = vapour
- Dew point decreases with decreased vapour pressure.
- A thermocouple sits inside a small sealed chamber
How is Vapour pressure osmometry carried out?
- 8µl sample on filter paper is inserted into the chamber
- Thermal and Vapour Equilibrium
- Cooled to below dew point of atmosphere
- Cooling stopped, condensation begins to form and warms the thermacouple
- Temperature stablises as no more condensation forms = dew point
What is Colloid Osmotic Pressure?
- Measuring cell consists of 2 chambers separated by a semi-permeable 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.
- The change in the compartment volumes are directly related to the osmolality in the sample.
What are some limitations of Colloid Osmotic Pressure?
- Hard to synthesis a membrane that is selective enough.
- Only good for looking at larger molecules such as proteins so not in routine use in Clinical Labs.
What are advantages and disadvantages of Freezing Point Osmometry?
Advantages
- Performs rapid and inexpensive measurements
- Simple and reliable performance
- Small sample size (nL to µL range)
- Ideal for dilute biological and aqueous solutions
Disadvantages
- Samples must be of low viscosity
- Not ideally suited for high molality or colloidal solutions
