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Chapter: Clinical Anesthesiology: Perioperative & Critical Care Medicine: Management of Patients with Fluid & Electrolyte Disturbances

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Nomenclature of Solutions

The system of international units (SI) has still not gained universal acceptance in clinical prac-tice, and many older expressions of concentration remain in common use.

Nomenclature of Solutions

The system of international units (SI) has still not gained universal acceptance in clinical prac-tice, and many older expressions of concentration remain in common use. Thus, for example, the quantity of a solute in a solution may be expressed in grams, moles, or equivalents. To complicate matters further, the concentration of a solution may be expressed either as quantity of solute per volume of solution or quantity of solute per weight of solvent.

MOLARITY, MOLALITY, & EQUIVALENCY

 

One mole of a substance represents 6.02 × 1023 molecules. The weight of this quantity in grams is commonly referred to as gram-molecular weight. Molarity is the standard SI unit of concentration that expresses the number of moles of solute per liter of solution. Molality is an alternative term thatexpresses moles of solute per kilogram of solvent. Equivalency is also commonly used for substances that ionize: the number of equivalents of an ion in solution is the number of moles multiplied by its charge (valence). Thus, a 1 M solution of MgCl 2 yields 2 equivalents of magnesium per liter and 2 equivalents of chloride per liter.

OSMOLARITY, OSMOLALITY, & TONICITY

 

Osmosis is the net movement of water across a semi-permeable membrane as a result of a difference in nondiffusible solute concentrations between the two sides. Osmotic pressure is the pressure that must be applied to the side with more solute to prevent a net movement of water across the membrane to dilute the solute.Osmotic pressure is generally dependent only on the number of nondiffusible solute particles. This is because the average kinetic energy of particles in solution is similar regardless of their mass. One osmole equals 1 mol of nondissociable substances. For substances that ionize, however, each mole results in n Osm, where n is the number of ionic species produced. Thus, 1 mol of a highly ionized substance such as NaCl dissolved in solu-tion should produce 2 Osm; in reality ionic inter-action between the cation and anion reduces the effective activity of each such that NaCl behaves as if it is only 75% ionized. A difference of 1 mOsm/L between two solutions results in an osmotic pres-sure of 19.3 mm Hg. The osmolarity of a solution is equal to the number of osmoles per liter of solution, whereas its osmolality equals the number of osmoles per kilogram of solvent. Tonicity, a term that is often used interchangeably with osmolarity and osmolal-ity, refers to the effect a solution has on cell volume. An isotonic solution has no effect on cell volume, whereas hypotonic and hypertonic solutions increase and decrease cell volume, respectively.

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