An osmometer is useful for determining the total number of salts (sodium), sugars (glucose), and electrolytes (potassium) found in blood or urine samples. Because of this, osmometers are often employed during cases of dehydration, electrolyte fluctuations, change in glucose levels, sodium and potassium disorders, poisoning, adrenal gland disorders, and even neurological injury.

Osmometers can be used for a variety of applications that include clinical studies and treatment, biopharmaceutical testing and manufacturing, and QA/QC of consumer products.

To better understand how osmometers work, let’s review osmolality and osmolarity.

Osmolality, a type of test, can be defined as the measurement of osmotic particle concentration in a solution by weight. The weight is defined in terms of osmoles per kilogram (Osm/kg).

An osmole is a unit that describes the number of moles present in a solution, and measures at 1 osmole per 1 mole of dissolved solute in a substance. A mole, simply defined, is the SI base unit for the amount of substance. Osmometers are capable of measuring in milliosmoles, or 1/1000 of an osmole, making these devices highly sensitive and precise. Thus, osmolality can be defined as the concentration of dissolved solutes per kilogram in a solution (Osm/kg or mOsm/kg).

It is used to determine the total number of solute particles within the solution that contribute to osmotic pressure, the pressure difference required to prevent osmosis across a semipermeable membrane.

Osmolality measurement works the way it does because during osmosis, water molecules move through the membrane in order to create balance between unequal concentrations of solution. The water will travel from the area that is more concentrated to the area that is less concentrated thus maintaining equilibrium.

This movement is defined by osmotic pressure, which is caused by different levels of concentration of solutes present on either side of the membrane. The water molecules present will migrate to the side of the membrane with less osmotic pressure.

In contrast, osmolarity, the measurement of osmoles per liter of solution, rather than kilogram, is both similar and different to osmolality. Osmolarity depends solely on the total number of particles present, regardless of type, which makes it easier to calculate. However, it is typically less reliable than osmolality.

This is because the solution’s volume changes as solute is added and temperature and pressure change, making the total weight more difficult to discern. With osmolality, the amount of solvent will remain constant despite temperature and pressure, allowing for easier measurement.

Understanding the differences between osmolarity and osmolality is important in regards to osmometry.