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An osmometer is an analytical instrument used to measure the osmolality or osmolarity in different aqueous samples, such as plasma, urine, serum, and biologic drugs.
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.
Osmometry techniques vary based on their method of measurement, and the instrument’s functionality is affected by this as well. There are two important types of osmometry methods used today, which include freezing-point depression osmometry and vapor-pressure osmometry.
A freezing point depression osmometer provides both efficient and accurate determinations of a solution’s osmotic strength, or concentration, using a sample’s freezing point. Freezing-point depression is the decrease of the freezing point of a solvent by adding another solute. The mixed liquid solution thus has a lower freezing point than the pure solvent or solute would have. An example of this is salt in water.
Freezing point depression osmometers are typically used for medical, chemical, and pharmaceutical applications and processes. They are useful in measuring osmolality of eye drops and contact lens solutions as well. A Clifton nanolitre osmometer is an example of a freezing point depression osmometer which measures the melting and freezing point of an aqueous sample. It requires only nanoliters of a sample to perform this function.
A vapor pressure osmometer, also known as a vapor phase osmometer, functions by measuring the decrease in vapor pressure as a solute is added to a solvent. This type of measurement determines the concentration of osmotically active solutes in the solution. Molecular weight of a sample or solution is also determined. Vapor-pressure osmometry is generally used for analyses of oligomers, a type of polymer with few repeating units, and short-chain polymers.
Membrane osmometers, which employ membrane osmometry, measure the osmotic pressure of a solution that is separated by a semipermeable membrane. It is used to determine the molecular mass of a polymer by means of osmosis.
Osmotic pressure is generated as the solvent in the solution pass through the semipermeable membrane, which indirectly measures the number average molecular weight of the polymer sample.
There are several advantages and disadvantages to the various osmometry methods used in laboratories.\n\nFreezing-point osmometry, a preferred method by many scientists and researchers, can be used to perform quick and inexpensive measurements with small sample sizes. It’s easy to use and reliable. However, it may not be optimal for colloidal solutions with high molality. Furthermore, the samples tested must have a low viscosity.
Vapor-pressure osmometry can also be used to perform tests quickly and cost-effectively, and only requires a small amount of sample to operate. But, it isn’t nearly as accurate as the freezing-point method, nor can it be used on more volatile solutions, such as alcohols and other organic solvents. That said, it is an ideal method for testing biological and aqueous solutions that have been diluted.
Lastly, membrane osmometry, unlike freezing-point and vapor-pressure, is time consuming and difficult to perform. It requires a large sample volume to operate, and cannot handle small molecules and aggressive solvents. Testing also renders the membrane unusable, so results cannot be reproduced.
As you can see, there are several factors you have to consider when acquiring a osmometer, from the tests you’re running and the needs of your lab to the type of osmometry that fits these requirements best.
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