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Osmolarity measures solute particles in a liter of solution, while osmolality measures them in a kilogram of material. Tonicity measures the relationship between two fluid solutions and determines what moves through a semipermeable membrane. Osmolarity can be useful for analyzing patient samples, including blood and urine, to determine medical problems and medication effectiveness.
Osmolarity is a measure of the number of solute particles, expressed in osmoles, in one liter of solution. This measurement can be difficult to obtain because the liter of solution will be subject to temperature and pressure changes which could cause it to expand and contract. Preferably, some scientists prefer to measure osmolality, which looks at the number of osmoles in one kilogram of material. The weight will remain relatively stable, assuming the scientist doesn’t go into space, and therefore the measurement will be more accurate.
Osmolarity measurements include all solutes in a solution, including those that are capable of moving through a semipermeable membrane and those that are not. It is also an absolute measure. These two characteristics distinguish it from tonicity, a related measure that may be useful in some contexts. The two concepts are sometimes confused because they look like measures of the same thing, as both deal with evaluations of solutes in a mixture of solvents.
When a researcher measures tonicity, he is interested in the relationship between two fluid solutions. If the solutions are isotonic, they have the same tonicity. If one is hypotonic relative to the other, it has a lower solute concentration. The tonicity of two solutions determines what moves through a semipermeable membrane and when. Osmosis plays an important role in the life and function of many organisms and can become a hindrance if cells are immersed in too hypotonic or hypertonic solution, as cells can swell so much that they explode or lose so much water that they die.
Knowing the osmolarity of a solution will provide information about its concentration, but this does not necessarily provide any data about its tonicity. The measurement does not distinguish between different solutes and therefore a mixture could have an osmolarity of three, but this does not tell a researcher which of the solutes, if any, are able to pass through a semipermeable membrane. Osmolarity measurements can be useful for tasks such as analyzing patient samples to check levels of dissolved salts and other compounds.
Blood and urine sample reports often discuss the osmolarity of the total solution. The test can also offer a breakdown of the different compounds found and their levels. This information can help a physician determine the nature of a patient’s medical problem. It can also help in an evaluation to see if a patient is responding to medications.
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