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Types of Osmometers

Types of osmometers
An osmometer is a device for measuring the osmotic strength of a solution, colloid or compound. There are three major types of osmometers commerically available, each leveraging a particular colligative property to achieve their analytical results:

Freezing Point Osmometers – determine the osmotic strength of solution by utilizing freezing point depression

Vapor Pressure Osmometers – determine the concentration of osmotically active particles that reduce the vapor pressure of the solution

Membrane Osmometers – measure the osmotic pressure of a solution separated by a semi-permeable membrane

  Advantages and disadvantages of the differerent osmometer technologies are explained

 

Osmometer Type: Advantages: Disadvantages: Comments:
Freezing Point Osmometry (FPO)
  • Performs rapid and inexpensive measurements
  • Simple and reliable performance
  • Industry preferred FP method
  • Small sample size (nL to µL range)
  •  Ideal for dilute biological and aqueous solutions
  • Samples must be of low viscosity
  •  Not ideally suited for high molality or colloidal solutions
FPO provides rapid and inexpensive results with the industry preferred freezing point method. Requires small sample size and ideally suited for most biological and aqueous applications.
Vapor Pressure Osmometry (VPO)
  • Performs rapid and inexpensive measurements
  • Small sample size (nL to µL range)
  •  Ideal for dilute biological and aqueous solutions
  • Less accurate than FPO
  •  Cannot be used for volatile solutes like alcohols or other organic solvents
  • Not ideally for high molality or colloidal solutions
VPO provides fast an inexpensive measurements requiring a small sample size, although not as fast or reliable as FPO. Volatile solutes are not amenable to VPO limiting its utility in many applications
Membrane Osmometry (MO)
  • 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
  • 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
MO provides a direct measurement of osmolality and is suitable for high molality and colloidal samples. Long analysis time and requires a large sample volume. Not applicable for small molecule applications.