Happy new Year,

Nicheal Kanes explanation of osmolarity is not yet complete. It should be
realized that it holds only for infinitely diluted solutions in which the
activity coefficient is 1 i.e activity and concentration are equal. In all
other cases, and certainly in physiological solutions which are
physico-chemically speaking rather concentrated, to calculate osmolarity
from concentration, a correction factor (osmotic coefficient, g, 0<g<1)
should be applied. This correction factor can be found in physical
chemistry tables, but most often only for monospecific solutions. When
osmolarity really is of importance in your experiments use an osmometer.
They are often based on the proportionality between osmolarity and LOWERING
of vapour pressure of the solvent (water in biological cases).

Habo Jongsma

habo j. jongsma
department of medical physiology
utrecht university
pobox 80043
3508 TA Utrecht
the Netherlands
tel: +31-30-2538900
fax: +31-30-2539036