A Complete Guide to Osmometer

• Hypertonic - Concentration of solutes is higher in the outside solution causing the cell to lose water.
• Hypotonic - Concentration of solutes is lower in the outside solution causing the cell to gain water. Cells without cell walls swell and burst due to excessive water intake.
• Isotonic - Concentration of solutes is same inside and outside the cell maintaining equilibrium.

• Osmolality is a measurement of the total number of solutes in a liquid solution.
• It is expressed in mOsm/kg H20.
• Osmolality, along with pH, is one critical factor while preparation of cell culture medium.

1. Vapor pressure depression osmometers
2. Membrane osmometers
3. Freezing point depression osmometers

• Determines the concentration of osmotically active particles that reduce the vapor pressure of the solution.
• Disadvantage:- Volatile compounds exert their own vapor pressure.

• Measures osmotic pressure of a solution separated by a semi-permeable membrane.
• Disadvantage:- Lack of availability of a proper semi-permeable membrane

• Determines the osmotic strength of a solution using freezing point depression.
• Most commonly used method in clinical, pharmaceutical and quality control laboratories.
• Rapid, sensitive and accurate. Hence it is widely preferred over the other two types.
• Requires very less sample size (very few microlitres).
• Easy to use. Does not require sound technical knowledge of the instrument for handling.

• When a solute is added to a solvent, the freezing point of the solution is lower than that of the solvent.
• As the concentration of the solute increases, there is further decrease in the freezing point of the solution.
• The osmolality of the solution can be determined by measuring the freezing point of that solution.

• A cuvette containing the test solution is loaded into the cooling chamber. 
• The solution is freeze below its expected freezing point with constant stirring by the wire.
• The temperature of the solution is monitored throughout the process by a thermistor connected to the wire.
• Stirring leads to the formation of ice crystals and thermal heat is generated in this process.
• As a result, an equilibrium is achieved and the temperature of the solution remains constant. 
• A plateau is achieved which indicates the real freezing point of the solution.

• Calibrate the instrument with a known standard before use to ensure proper working of the instrument.
• Use clean cuvettes for freezing (preferably new).
• Wash the cuvettes properly if later use is expected.
• Keep extra cuvettes covered to avoid contact with dust.
• Never keep the freezing chamber empty.
• When not-in-use, place an empty cuvette in the freezing chamber.
• Always wipe the probe and the wire with lint-free tissue to remove traces of the previous solution.
• Check the level of heat transfer fluid at regular intervals. Change the fluid when the solution reaches its warning mark.
• Annually or six-monthly calibrate the osmometer with different ranges of standards to check the efficiency of the instrument.
• Cover the instrument when not-in-use.