APPENDIX
D
Special Application Notes
OSMOMETRY WITH MULTI-SOLVENT SOLUTIONS
Biological solutions, in general, are aqueous in nature. Most speci-
mens submitted to the clinical laboratory for testing, both pathologic
and normal, will exhibit characteristic properties that are essentially
attributable to the cardinal properties of water, as modified by the
dissolved solute particles. Such solutions, which can be represented
by a simple model, i.e., water as solvent with nonvolatile solutes,
will have a linear, uniform relationship among all of the colligative
properties (vapor pressure, freezing point, boiling point, etc.). In
addition, most of these same solutions can be uniformly frozen with
few artifacts arising from the freezing process. Thus, one can
expect to obtain very similar results, if not exact duplication,
between freezing point and vapor pressure measurements on the
vast majority of clinical specimens.
Aside from this broad category of solutions, there is a small but
important class of solutions that may be encountered in clinical work
where the colligative relationships do not necessarily hold. These
are solutions in which non-physiological volatile solutes–actually
solvents–are present. In such cases, the interactions among the
various molecules cause the properties of such solutions to be more
complex. They generally do not follow linear relationships, as in
solutions having only a single solvent. It must be remembered that
osmometers for clinical applications, whether based on freezing
point or vapor pressure methodology, determine the osmolality of
solutions by indirect means. When complex solutions are encoun-
tered, the results obtained by either of these instruments may not
faithfully represent the osmolality of the solution. Each instrument
will respond to the parameter it is designed to measure, and the
resultant indications must be interpreted accordingly.
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