Colligative Properties & Solutions

Colligative Properties: Concentration-Dependent Effects

Colligative properties depend on the number of solute particles in a solution, not their chemical identity. The four classical colligative properties are: vapour pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure. In clinical medicine, osmotic pressure — and its related concepts of osmolarity, osmolality, and tonicity — is the most important.

Osmolarity vs Osmolality

Osmolarity (units: mOsm/L) measures the number of osmoles of solute per litre of solution. It depends on volume and therefore changes with temperature.

Osmolality (units: mOsm/kg H₂O) measures osmoles per kilogram of solvent (water). It is independent of temperature and volume. The laboratory measures osmolality directly using freezing point depression. Normal plasma osmolality: 275–295 mOsm/kg.

The calculated plasma osmolality:

Posm = 2[Na⁺] + [glucose]/18 + [urea]/2.8

(all in conventional units: Na⁺ in mmol/L, glucose and urea in mg/dL)

The osmolar gap = measured Posm − calculated Posm. A gap >10 mOsm/kg suggests an unmeasured osmole (methanol, ethanol, ethylene glycol toxicity).

Tonicity

Tonicity describes a solution's effect on cell volume relative to normal plasma. It depends only on solutes that cannot freely cross cell membranes (effective osmoles).

• Isotonic: same effective osmolality as plasma (~285 mOsm/kg). No net water movement. Example: 0.9% NaCl (normal saline), 5% dextrose in water (D5W is isotonic in the bag but becomes hypotonic once glucose is metabolised).
• Hypertonic: higher effective osmolality. Water moves out of cells — cells shrink. Example: 3% NaCl (used in severe hyponatraemia, cerebral oedema).
• Hypotonic: lower effective osmolality. Water moves into cells — cells swell. Risk of cerebral oedema if hypotonic fluids are given too rapidly.

Clinical IV Fluid Contexts

Normal saline (0.9% NaCl): 154 mmol/L Na⁺ and Cl⁻. Isotonic. Large volumes cause hyperchloraemic metabolic acidosis.

Hartmann's solution (Ringer's lactate): balanced isotonic fluid with lactate (metabolised to HCO₃⁻). Preferred for surgical fluid replacement in NZ.

5% dextrose (D5W): isotonic osmolality in the bag, but dextrose is metabolised — effectively delivers free water. Used for hypernatraemia correction. Do NOT use for hypovolaemia.

Mannitol 20%: hypertonic osmotic diuretic. Draws water from brain tissue into plasma. Used in raised intracranial pressure. Must be given via central line.

Osmotic Pressure

The van't Hoff equation: π = iCRT

Where i = van't Hoff factor (number of particles per formula unit), C = molar concentration, R = 8.314 J mol⁻¹ K⁻¹, T = temperature in Kelvin. NaCl dissociates into 2 ions (i ≈ 1.86 in physiological saline), so it exerts nearly double the osmotic pressure of a non-electrolyte at the same concentration.