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The osmotic pressure of a solution is only proportional to the total number of solute molecules or ions in the solution, and is independent of the nature and size of the solute molecules or ions.
The osmotic pressure of a solution is only proportional to the total number of solute molecules or ions in the solution, and is independent of the nature and size of the solute molecules or ions. In other words, for any solution, as long as the number of solute molecules or ions is equal, whether these solute molecules or ions are small molecular electrolytes or large molecules such as proteins, they all exhibit the same osmotic pressure (force). It is easy to see that the osmotic pressure (force) of a solution, from its essential origin, merely reflects the change in the concentration of solute particles in the solution. For this reason, the osmotic pressure (force) commonly referred to is more accurately called osmotic concentration.
According to Avogadro’s number, 1 mol of any substance contains 6.023×10^23 particles. Therefore, 1 g of H⁺, 23 g of Na⁺, 35.5 g of Cl⁻, 46 g of ethanol, or 180 g of glucose in 1 L of solution all correspond to a concentration of 1 mol/L, and each contains the same number of particles, i.e., 6.023×10^23 particles. Depending on the amount of substance contained per liter of solution, concentrations are commonly expressed as 1 mol/L, 0.5 mol/L, or 0.1 mol/L. Since osmotic concentration is also proportional to the total number of particles in the solution, specialized terms like osmole (Osm) and milliosmole (mOsm) are used when expressing osmotic concentration. For nonelectrolyte solutions, such as glucose solutions, 1 mol = 1 Osmol, whereas for electrolyte solutions, such as sodium chloride solution, 1 mol = 2 Osmol; for calcium chloride 1 mol = 3 Osmol, and for sodium citrate 1 mol = 4 Osmol.
We know that concentration units are generally expressed in two ways: molarity (amount of substance concentration) and molality (mass concentration). Correspondingly, the units expressing osmotic concentration are generally also of two types: osmolarity (volume osmotic concentration) and osmolality (mass osmotic concentration). Molarity refers to dissolving 1 mole of solute in solvent, and then diluting the solution to 1 L; the concentration unit is expressed as 1 mol/L. Molality refers to dissolving 1 mole of solute in 1000 g of solvent, with concentration expressed as 1 mol/kg. Correspondingly, for osmolarity, the concentration unit is 1 Osmol/L; for osmolality, the concentration unit is 1 Osmol/kg, abbreviated as 1 osmole per kilogram. Earlier, the unit (Osm/kg H2O) was used. Since H2O is not a unit of measurement, recent foreign instruments and literature mostly adopt Osm/kg. In medical clinical practice, the unit is usually expressed as one-thousandth of this, i.e., 1 milliosmole per kilogram, abbreviated as 1 mOsm/kg.
Currently, in clinical measurements of body fluid osmotic concentration, mass osmotic concentration (mOsm/kg) is used due to the fact that when osmolarity is used, the volume of solvent water in the solution is affected by temperature, increasing with rising temperature. The increase in solvent volume leads to a corresponding decrease in osmolarity. Since mass does not vary with temperature (e.g., 1 kg of water is 1 kg at both 0℃ and 50℃), using osmolality to express osmotic concentration of a solution avoids influence by temperature changes.
Since the 2005 edition of the Pharmacopoeia, the national pharmacopoeia has used mOsmol/kg as the unit to express osmotic molar concentration, which does not differ in understanding from the internationally accepted mOsm/kg. However, the author believes there is still a minor flaw, mainly regarding the length of characters used in the expression. For example, people usually use “kg” rather than “kilogram” to represent 1 kg of mass. The osmotic pressure unit “mOsm/kg” is actually composed of four parts: “m”, “Osmolality”, “/”, and “kg”, and should not be misunderstood as consisting of five parts: “m”, “Osmolality”, “mol”, “/”, and “kg”. To simplify the expression, “Osmolality” is abbreviated as “Osm,” which is the mainstream and universal expression used by current international osmometry instruments.