The Similarities Between Osmolarity And Osmolality

Osmolarity and osmolality are both measures of the concentration of solute particles in a solution, but they differ in how that concentration is expressed. Here’s a breakdown of each term and how they’re measured:

Osmolarity

• Definition: Osmolarity is the number of osmoles of solute per liter of solution (osmoles/L).

• Units: Osmol/L or mOsmol/L (milliosmoles per liter)

• Volume-based: It is affected by changes in temperature and pressure, since these can change the volume of a solution.

• Use: More common in chemistry and IV fluid formulations.

Osmolality

• Definition: Osmolality is the number of osmoles of solute per kilogram of solvent (osmoles/kg).

• Units: Osmol/kg or mOsmol/kg (milliosmoles per kilogram)

• Mass-based: Independent of temperature and pressure.

• Use: More commonly used in physiology and clinical medicine (e.g., assessing plasma or urine concentration).

How They’re Measured

Measured Osmolality:

• Method: Typically measured using an osmometer, which determines the freezing point depression or vapor pressure of a solution.

  • Freezing point depression is the most common clinical method.

  • The more particles in solution, the lower the freezing point.

• Application: Used to assess serum or urine osmolality in diagnosing dehydration, SIADH, diabetes insipidus, etc.

Calculated Osmolarity (or “calculated osmolality”):

• Often estimated from known concentrations of major solutes like sodium, glucose, and urea in plasma:

Osmolarity≈2[Na+]+{[Glucose]/18}+{[BUN]/2.8}​

• Units:

  • Na⁺ in mEq/L

  • Glucose and BUN (blood urea nitrogen) in mg/dL

• This equation gives a calculated plasma osmolarity in mOsmol/L.

Osmolarity vs Osmolality – Which is better?

• Osmolality is more accurate for physiological fluids because it’s not affected by temperature or pressure.

• Osmolarity is simpler for lab-prepared solutions where volume is easy to control.

Summary Table