EP 2.2.35 Osmolality

Ph. Eur. 2.2.35 Osmolality defines the official European Pharmacopoeia procedure for determining the osmolality of solutions—critical for products such as parenterals and ophthalmics where tonicity matters. The chapter specifies the principle (freezing-point depression), apparatus, verification/calibration using sodium chloride reference solutions, and the measurement workflow to obtain results expressed in mosmol/kg, the unit used by the European Pharmacopoeia.

Ph. Eur. 2.2.35 and USP <785>, which both cover the determination of osmolality, have been harmonized under the Pharmacopoeial Discussion Group (PDG).

Use 2.2.35 whenever a Ph. Eur. monograph requires osmolality. The method is generally applied to aqueous solutions intended for administration, though the chapter is a general method and not limited to any one dosage form.

Core principle. Osmolality is measured by freezing-point depression osmometry: solutes lower the solvent’s freezing point in proportion to the total number of dissolved particles; the osmometer detects the freezing plateau and converts the temperature depression to mosmol/kg.

Apparatus. A freezing-point osmometer providing: (i) a controlled cooling system, (ii) a temperature-measurement system, and (iii) a means to induce crystallization (e.g., seeding/vibration).

Verification/Calibration. The chapter provides reference solutions using sodium chloride (Table 2.2.35-1) for instrument verification across an expanded range (now described up to 4000 mosmol/kg). Water (0 mosmol/kg) is used for the zero check; then verify with at least two sodium chloride reference solutions of different osmolality values, chosen to be relevant to the expected range of the samples.

Measurement. Introduce a defined volume, supercool, seed to initiate crystallization, read the freezing plateau, and report osmolality. Repeat as directed to confirm the result.