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Plasma osmolality measures the body's electrolyte–water balance. [1] There are several methods for arriving at this quantity through measurement or calculation. Osmolality and osmolarity are measures that are technically different, but functionally the same for normal use.
From this measurement, the clinician can calculate the plasma osmolality of a patient's blood. A second vial is also sent to the laboratory. This vial is put in an instrument that measures the freezing point depression of all the solutes in the plasma. This measurement gives the true plasma osmolality.
Note: NaCl does not dissociate completely in water at standard temperature and pressure, so the solution will be composed of Na+ ions, Cl- ions, and some NaCl molecules, with actual osmolality = Na+ concentration x 1.75] Another example is magnesium chloride (MgCl 2), which dissociates into Mg 2+ and 2Cl − ions.
To calculate an estimation, the total amount of substance in the body before the loss is first estimated: = where: n b = Total amount of substance before fluid loss; Osm b = Body osmolarity before loss (almost equal to plasma osmolality of 275-299 milli-osmoles per kilogram [4])
[2] 290 mOsm/kg is the presumed stool osmolality, and the measured concentration of sodium and potassium cations is doubled to account for the corresponding anions which must be present. [citation needed] A normal gap is between 50 and 100 mOsm/kg, [3] corresponding to the concentration of other solutes such as magnesium salts and sugars ...
Osmometry is widely used in pharmaceuticals, quality control laboratories, and clinical chemistry to measure the osmolality in aqueous solutions accurately. It is commonly used in medical clinics to assist with various pharmaceutical practices, [ 10 ] including the development of lens care solutions and eye drops.
For liquid solutions, the osmotic coefficient is often used to calculate the salt activity coefficient from the solvent activity, or vice versa. For example, freezing point depression measurements, or measurements of deviations from ideality for other colligative properties, allows calculation of the salt activity coefficient through the osmotic coefficient.
In the physiology of the kidney, free water clearance (C H 2 O) is the volume of blood plasma that is cleared of solute-free water per unit time. An example of its use is in the determination of an individual's state of hydration.