Principle and Application of Osmotic Pressure Measurement
1. Osmotic pressure
Osmotic pressure refers to the attractive force of solute particles in solution to water. The size of the solution osmotic pressure depends on the number of solute particles in the unit volume of the solution: the more solute particles, that is, the higher the solution concentration, the greater the attraction to water, and the higher the solution osmotic pressure; conversely, the less solute particles, that is The lower the concentration of the solution, the weaker the attraction to water and the lower the osmotic pressure of the solution.
2. Application of osmotic pressure
It is mainly used to detect the osmotic pressure of the solution, the osmotic pressure of human blood, urine, and feces, the osmotic pressure of eye drops, and the osmotic pressure of cell culture fluid (in the various inorganic salt ions that make up the extracellular fluid, the content occupies The obvious advantages are Na+ and Cl-, and more than 90% of the osmotic pressure of extracellular fluid comes from Na+ and Cl-. At 37°C, the osmotic pressure of human plasma is about 770kPa, which is equivalent to the osmotic pressure of intracellular fluid), etc., The osmotic pressure of biochemical reagents, the screening of ingestion poisoning, the monitoring of the concentration of osmotic active substances, the determination of the water content state in athletes, the osmotic pressure of food and beverages, etc.
3. Osmotic pressure detection principle Physical principle of osmotic pressure
When a solute is dissolved in a pure solvent, the solvent undergoes the following changes:
(1) Freezing point depression △Tf＝Kf×m
(2) Vapor pressure drop △Pv＝Kv×m
(3) Boiling point rise △Tb＝Kb×m
(4) Osmotic pressure rises △Po＝Ko×m
In the formula, Kf, Kv, Kb, and Ko are all constants, and m is the weight molar concentration. It is only related to the number of particles (molecules, ions) of the solute in a certain amount of solution, and has nothing to do with the nature of the solute. These properties are called the "colligative property" of the dilute solution.
5. Osmotic pressure calculation
The unit of osmolality is usually expressed in milliosmoles of solute per kilogram of solvent, ie mOsmol/kg. Milliosmol concentration (mOsmol/kg) = [grams of solute dissolved in each kilogram of solvent (g/kg)/molecular weight (g)] × n × 1000, where n is the number of particles formed when a solute molecule dissolves , in an ideal solution, such as glucose n=1, sodium chloride or magnesium sulfate n=2, calcium chloride n=3, sodium citrate n=4. In the physiological range and very dilute solution, the osmolarity has a small deviation from the calculated value under the ideal state; with the increase of the solution concentration, compared with the ideal value, the actual osmolarity decreases, such as 0.9% chloride For sodium injection, the ideal osmolality is 2×9/58.4×1000=308mOsmol/kg, but actually at this concentration, the n of sodium chloride solution is slightly less than 2, and the actual measured value is 286mOsmol/kg; complicated The theoretical osmolality of mixtures such as hydrolyzed protein injection is not easy to calculate, so it is usually expressed by the actual measured value.
