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# Osmometry Q and A’s

Q. What is Osmometry ?

A. The measurement of the freezing point of a solution is related to the osmotic concentration of that solution. Osmotic concentration can be considered as the concentration of particles of solute per unit amount of solvent.

The manner in which the solute and solvent interact is called “activity”. This refers not only to the degree of dissociation of the solute but also to a number of other factors including the degree to which the water molecules are free to enter the solution.

Q.How is it measured?

A. One option to measure Osmolarity is to use use a freezing point Osmometer. A sample is introduced to a cooling chamber where it is cooled to a temperature just below freezing point. A needle is introduced into the sample temporarily to initiate the formation of crystals. The heat generated from this process brings the sample to a temperature that is equal to its freezing point. A standard freezing curve is generated and the osmolarity is calculated from from the freezing point depression, using a calculation.

Q. What is the difference between Osmolarity and Osmolality and the measuring units of each?
A. Measurements of osmolar concentration are often expressed in Osmolarity and Osmolality.

Osmolarity is a measure of the osmoles of solute per liter of solution. A capital letter M is used to abbreviate units of mol/L. Since the volume of solution changes with the amount of solute added as well as with changes in temperature and pressure, osmolarity is difficult to determine.

Osmolality is a measure of the moles (or osmoles) of solute per kilogram of solvent expressed as (mol/kg, molal, or m). Since the amount of solvent will remain constant regardless of changes in temperature and pressure, osmolality is easier to evaluate and is more commonly used, and often preferred, in practical osmometry. Most commercially available osmometers report results using osmolality units mOsm/kg

Q. What sort of samples can you measure the Osmolarity of?

A. Commonly used in clinical laboratories to measure the osmolarity of urine and serum, but anything can have an osmolarity if it is a solution.

Osmolality of blood increases with dehydration and decreases with overhydration. In normal people, increased osmolarity in the blood will stimulate secretion of antidiuretic hormone (ADH). This will result in increased water reabsorption, more concentrated urine, and less concentrated blood plasma. A low serum osmolality will suppress the release of ADH, resulting in decreased water reabsorption and more concentrated plasma.

Increased osmolarity frequently occurs following illness due to chronic neurotoxic diseases such as Lyme disease.

The normal value for human serum is 285-290 mOsm/kg

Q. Can i measure sodium concentration using an Osmometer?

A. Sodium and osmolarity together are helpful in distinguishing between water intoxication, true sodium depletion, progressive renal disease, primary water loss and false hyponatremia  (low sodium levels) due to liquids or artifacts occupying serum space. When used in conjunction with a flame photometer, an osmometer is useful in distinguishing between various types of low sodium conditions. Since an osmometer measures the concentration of solute per total water rather per total volume (as a flame photometer), it will not be confused by the presence of something which changes the water content of the solution.

What other applications are there which use an Osmometer?

Some substances raise blood osmolarity markedly like drugs and alcohol, so it can be a marker of use in acute situations, along with clinical observations.

Infant diarrhoea is a major cause of paediatric death, and osmometry can be used to accurately indicate the degree of dehydration . For small sample volumes Camlab have the Type 15M Osmometer from LÖser Gmbh which utilises small volume sizes of 25-50µl. The  LÖser Type 15 uses samples in the range 50-150µl

Since osmometers measure only dissolved particles in solution the number of applications is almost unlimited; dialysis, laboratory reagent checks, fixing solutions, contact lens manufacturing, cell research, milk analysis, sports science hydration studies.

For more information on Loser Osmometers see our quick guide or watch this video on how to carry out a two point calibration with the Type 15 Osmometer