The Theory of Freezing Point Osmometer

Osmolality is an important indicator of the homeostasis of the biological internal environment, and plays an important role in biological processes such as diffusion and transport of substances in organisms. It is of great significance for the study of water-salt metabolic balance, evaluation of renal dysfunction, monitoring of changes in critical surgical conditions such as diabetes mellitus, as well as the pharmacological analysis of artificial dialysis and drugs. Next, let’s learn more about the working principle and application of Freezing Point Osmometer.

What is osmotic pressure

Osmolality is a measure of the osmotic concentration of a solution and reflects the total number of solute particles in a solution that affect the osmotic pressure. Expressed as the number of osmolalities per kilogram of solute in the solvent, it is usually expressed as Osmol/kg, mOsmol/kg.
When a solute is dissolved into a solution, the solution undergoes the following changes:
Vapor pressure decreases
Boiling point increases
Freezing point decreases
Increase in osmotic pressure
The changes in these properties depend only on the number of solute particles, the total number of molecules and ions, and are independent of the type and particle size of the solute. Theoretically, any of these four properties can be used to determine the number of solute particles in a solution.
From this come the freezing point osmolometer, vapor pressure osmolometer, membrane osmolometer, and so on.

Principle of Operation of Freezing Point Osmometer

The principle of operation of the Freezing Point Osmometer is based on the proportional relationship between the freezing point drop and the molar solubility of a solution. The freezing point of the solution is measured by means of a semiconductor measuring sensor, a highly sensitive temperature sensing element, which is converted into osmolality units (m0sm/kg).
The test tube of the sample to be measured is placed in a semiconductor cooler, where a non-freezing liquid is used as a conduction medium to cool the solution inside the test tube. The heat-absorbing side of the semiconductor cooler absorbs heat from the non-freezing liquid to cool it down.

The process of temperature change of the sample under test is as follows:

Freezing point is the temperature at which a solution changes from liquid to solid. In the subcooling state (the temperature has reached the freezing point and does not freeze the state) under the liquid state is extremely unstable, any perturbation can be caused by its immediate crystallization and become a solid state. When the liquid becomes solid, due to the sudden change of molecular energy from a high-energy state to a low-energy state, the excess energy will be released in the form of heat, which is called “heat of crystallization”. The presence of the heat of crystallization will cause the temperature of the supercooled solution to rise at the moment of icing formation.

Freezing point falling method

Determination of the osmotic pressure of the solution is mainly measured using four methods: vapor pressure reduction method, boiling point elevation method, freezing point depression method and osmotic pressure semi-permeable membrane assay. In practice, due to the direct measurement of osmolality is more difficult, the freezing point drop is more popular in the clinical and research industry.

Measurement Methods	Freezing point depression method	Dew point method
Advantages	Wide measuring range Easy to operate Accurate and reproducible results	Suitable for samples with suspended particles or high viscosity. Direct measurement of osmotic pressure in plant and animal tissue sections.
Disadvantages	Not suitable for formulations with high viscosity, many suspended particles and multi-solvent mixtures.	Not suitable for volatile and heat-decomposed samples.