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Theory of Osmolality

Osmolality is a general measure of the particle concentration in a solute. It´s not depending on the nature of molecules but just their number. Therefore, a two molar solution of a non-dissociating molecule (A) has the same osmolality as a one molar solution of a fully dissociating salt composed of two ions (B). The osmolality of a solution is the same, even when molecules vary in shape (C) as well as in size (D). Therefore, all solutions containing the same number of osmotically active particles — regardless of their chemical properties — exhibit the same osmolality.

Theory of Osmolality

Freezing Point Osmometry

The measurement principle of the K-7400S Semi-Micro Osmometer is based on the colligative property of freezing point depression. If a solute is added to a liquid this results in a decreased freezing point of the solution. The depression is 1.858 K per 1 mole of ideally solved compound in one liter of water. This effect depends only on the number of particles in the liquid and not on the physical or chemical properties of the solutes. Due to this linear correlation, the osmolality of a sample can be determined by precisely measuring its freezing point.

Mechanism of Freezing Point Osmometry

Knauer's K-7400s Freezing Point Osmometer

Semi-Micro Osmometer K-7400
Semi-Micro Osmometer K-7400

KNAUER is one of the pioneers in the field of osmometry and known for its reliable and user friendly instruments for many decades. The newest freezing point Osmometer K-7400S allows for the easy and fast determination of the osmolality of various aqueous solutions. In addition, the freezing point depression of such samples can be measured.

  • Sample volume: 50 – 150 µl
  • Osmolality range: 0 – 2000 mOsmol/kg
  • Test time: ~ 2 min
EuroOsmo 7400 Software
EuroOsmo 7400 Software

EuroOsmo 7400 is a user-friendly software developed for the freezing point Osmometers K-7400 and K-7400S. It allows control of the instruments and recording of the measured data. The software automatically plots the temperature curve for each measurement and calibration and allows saving of the measured values.

Osmometry Spares

K-7400 Measuring Head
K-7400 Measuring Head

Spare Measuring Head, Part No. A0840-2

NaCl Calibrating Solutions
NaCl Calibrating Solutions
  • 300 mOsmol/kg
  • 400 mOsmol/kg
  • 850 mOsmol/kg
  • 2000 mOsmol/kg

Osmolalities of the World

Dr Till Myrach of Knauer has been creating the first “Osmo-World Map”, showing the osmolalities of water sources around the world, including Australia. Take a look at

If you would like us to find out the osmolality of your water  sample, all you need to do is send us about 1 ml in a sample tube (note, not polyethylene) to P.O. Box 450, Waverley, NSW 2024 with a note giving your contact details, where the sample came from and its Google Map coordinates. We will inform you when your sample appears on the map.

How "salty" is the Parramatta River, NSW?

The Parramatta River is the main tributary of Sydney Harbour. It is defined as an intermediate tide dominated, drowned valley estuary. The river is tidal up to Parramatta, approximately 30 kilometres (19 mi) from the opening of Sydney Harbour into the Tasman Sea.

As part of the “Osmolalities of the World” project we thought we would take samples from the river at three different places, to see how the salinity of the river is affected by tidal flows: next to Sydney Olympic Park Wharf, Mort Bay Park in Balmain and outside the harbour at Bondi Beach.

Osmolalities Along Parramatta River

Osmolality Readings (mOsmol/kg)

  • Sydney Olympic Park: 450
  • Balmain: 912
  • Bondi Beach: 1058

Note: a reading of 0 (zero) mOsmol/kg indicates pure water with no particles disolved in it.

Parramatta River Sampling Sites


The osmolality of the middle harbour at Balmain is twice as high as that further up the river at Olympic Park. Whereas the osmolality in the open sea at Bondi Beach is only about 14% higher than at Balmain. This non-linear drop in osmolality is probably due to the strong effect of the tide on the wider stretches of the Parramatta River from the harbour mouth up past the middle harbour.