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Osmometers, Vapor Pressure

Definition : Osmometers designed for indirect determination of a solution's osmotic pressure (i.e., osmolality), a variable related to the particle concentration in the solution, by measuring the vapor pressure (or dew point) depression without changing the physical state or the temperature of the sample, based on one of the colligative properties of solution. These instruments consist of a sealed chamber with an electronic temperature sensor (e.g., a thermocouple) that is electronically nulled and a Peltier element that cools the thermocouple to a temperature below the dew point, causing the water to condense on the thermocouple's surface. An abnormal ratio between the osmolality in urine and serum may indicate renal disease, diabetes insipidus, or secretion of abnormal levels of antidiuretic hormone; water and electrolyte imbalances are also present in patients with acute poisoning and shock trauma. Vapor pressure/dew point osmometers cannot be used if the sample has volatile components (e.g., ethanol); some instruments include a sweat-collecting device to make iontophoresis determinations.

Entry Terms : "Vapor Pressure Osmometers"

UMDC code : 23794

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