Osmometry

Question 1

What is Osmometry?

Answer 1

A technique for measuring the concentration of dissolved solute particles that contribute to the osmotic pressure of a solution.

Question 2

What is Osmossis?

Answer 2

• 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

Question 3

What is Osmotic Pressure?

Answer 3

• 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.

Question 4

What is osmolarity and osmolality?

Answer 4

• 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

Question 5

What is the advantage of using Osmolality?

Answer 5

• 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.

Question 6

What is Colligative Properties?

Answer 6

Properties of solutions that depend on the number of molecules in a given volume of solvent rather than the properties of the molecules themselves.

Question 7

What are the changes to colligative properties?

Answer 7

Changes to colligative properties with increased osmolality:

• Osmotic pressure - increased
• Boiling point - increased
• Vapour pressure - decreased
• Freezing point - decreased

Question 8

What are osmometry methods?

Answer 8

• Freezing point depression
• Vapour pressure lowering
• Colloid Osmotic pressure

Question 9

What is freezing point depression?

Answer 9

• 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.

Question 10

What is Vapour Pressure?

Answer 10

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.

Question 11

What is Vapour Pressure osmometer?

Answer 11

• 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

Question 12

How is Vapour pressure osmometry carried out?

Answer 12

• 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

Question 13

What is Colloid Osmotic Pressure?

Answer 13

• 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.

Question 14

What are some limitations of Colloid Osmotic Pressure?

Answer 14

• 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.

Question 15

What are advantages and disadvantages of Freezing Point Osmometry?

Answer 15

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

Question 16

What are advantages and disadvantages of Vapour Pressure Osmometry?

Answer 16

Advantages

• Performs rapid and inexpensive measurements
• Small sample size (nL to µL range)
• Ideal for dilute biological and aqueous solutions

Disadvantages

• Less accurate than FPO
• Cannot be used for volatile solutes like alcohols or other organic solvents
• Not ideally for high molality or colloidal solutions

Question 17

What are advantages and disadvantages of Membrane Osmometry/Colloid Osmotic Pressure osmometry?

Answer 17

Advantages

• Provides potentially unlimited direct measurement of osmotic pressure and solution osmolality
• Good for colloidal solutions
• No limitation on sample concentration
• Can determine MW of macromolecules

Disadvantages

• Time consuming and difficult to operate
• Requires large sample volume
• Not applicable for small molecules and aggressive solvents due to membrane porosity and compatibility
• Irreproducible results due to clogging of membrane pores

Question 18

What are clinical applications of Osmometry?

Answer 18

• Investigation of electrolyte disorders - Hyponatraemia and Hypernatraemia
• Urine concentrating ability - In a patient with polyuria or oliguria
• Screening for toxin ingestion e.g. Salicylate, Ethanol, Methanol, Ethylene Glycol
• Hydration levels of athletes - To assess whether the athlete needs to drink more water or should not exercise

Question 19

How is ADH controlled?

Answer 19

• Increase in blood osmolarity stimulate osmoreceptors in the hypothalamus to release ADH and stimulate thirst
• Drinking reduces blood osmolarity to set point
• Increase in ADH from the pituitary gland leads to increase permeability in the collecting duct. H20 reabsorption in distal tubule helps further osmolarity increase

Question 20

What are normal ranges for osmolality?

Answer 20

• Serum Osmolality: 275-295 mmol/kg
• Urine Osmolality: No reference range

Question 21

Why does urine osmolality have no reference ranges?

Answer 21

• Depends on the serum result
• If serum osmolality is high, urine osmolality should be high (concentrated urine, water reabsorbed) usually >600-750mmol/kg
• If serum osmolality is low, urine osmolality should be low (no ADH released and water lost in the urine) usually <50 mmol/kg

Question 22

What are different formulae used for Osmolality?

Answer 22

• 1.86(Na + K) + Urea + Glucose
• 1.86Na + Urea + Glucose + 9
• 2(Na+K) + Urea + Glucose
• 2Na + Urea + Glucose

Question 23

What is osmolar gap?

Answer 23

Osmolar Gap = (measured osmolality-calculated osmolality)

• Abnormal gap (>10) indicates presence of unmeasured osmotically active substances – often alcohols