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OSMOTIC PRESSURE. 3
have the same osmotic pressure, corresponds to Avogadro’s law, that
equal volumes of different gases under the same pressure contain the
same number of molecules.
From Pfeffer’s results of the osmotic pressure of cane-sugar solu-
tions van’t Hoff has calculated that it is the same as the pressure exerted
by any gas of the same molecular concentration and temperature. In
general the following is true:
Dissolved bodies exert in solution the same osmotic pressure they would
exert if they were gases at the same temperature and in equal volume.
Recently Morse, Frazer and collaborators have brilliantly substan-
tiated the theory of van’t Hoff for solutions of cane-sugar and glucose,
by making use of Pfeffer’s method but using a very refined technique. 1
From what has been given, the osmotic pressure of a solution, sepa-
rated from the surrounding pure solvent by a semipermeable membrane,
exerts its effects in two ways. First the pure solvent tries to enter
the solution and secondly the dissolved substance presses upon the
membrane with a force equal to the gas pressure. According to whether
we consider either one or the other of these ways, the osmotic pressure
of a solution can be considered as its ability to attract the solvent, or as
a pressure directed toward the outside. This last conception seems prob-
ably for the present to be the most acceptable, nevertheless, the fact that
the pure solvent enters through the unmovable semipermeable membrane
(as in Pfeffer’s experiments) is difficult of reconciliation with this mode
of explanation. Obviously, and for physiological purposes, it seems best
to make use of the former explanation, in which the osmotic pressure
is considered as a measure of the force with which a solution attracts
the solvent.
Pfeffer’s above-described method of directly determining the
pressure can only be used in exceptional cases, first because the prepara-
tion of the semipermeable membrane is connected with difficulties, and
second, because there are only a few crystalline bodies for which imper-
meable membranes have been found. There are other quicker and easier
ways of determining the osmotic pressure.
Solutions of non-volatile substances boil at a higher temperature
than the pure solvent. This is due to the fact that the dissolved sub-
stance, because of the osmotic pressure, holds on to the solvent with
a certain force. As in boiling a part of the solvent is separated from
the dissolved body, and as the osmotic pressure can be considered as a
measure of the attractive power between the solvent and the dissolved
substance, then it is clear that solutions which are prepared with the
same solvent and have the same osmotic pressure (isosmotic solutions)
1
Amer. Chem. Journ., 37, 425, 558 (1907); 41, 1, 257 (1909).
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