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Related post: has found the widest acceptance among physicists. From the observa-
tions of osmotic pressure by means .of Pfeffer's ferrocyanid of copper cell, Cheap Loratadine
Van't Hoff (1885 and 1887) arrived at the conclusion that the phenomena
of osmotic pressure are governed by the same laws as the pressure of gases,
quantitatively as well as qualitatively. This would imply, that the
pressure in both cases is produced by the same cause, namely, the kinetic
motion of the molecules bombarding the walls of the container. Dis-
solved molecules would behave precisely like gaseous molecules. The
membrane would play a purely passive part; being important only by
making the kinetic motion manifest to the senses.
According to the Buy Loratadine Online Van't Hoff theory, the molecules of both solvent and solute are
in constant motion, moving with the same energy as if they were in the form of a gas.
Every molecule, by striking against the walls of the container, or against other mole-
cules, exerts a certain pressure, which is precisely the same whether the substance Buy Loratadine is in
the form of a gas or of a solution. If the same number of molecules per cubic space
exist in two solutions separated by a semipermeable Purchase Loratadine membrane, the pressure will be
626 MANUAL OF PHARMACOLOGY
equal on each side of the membrane. The nature of the molecules is immaterial in this,
as long as they all penetrate through the membrane with equal readiness. If all the
molecules can pass through the membrane, an exchange will take place; but since as
many pass to one side as to the other, there will be no difference in the pressure in the
two compartments. If, on the other hand, the molecules on the one side can pass
through readily, while some of the molecules on the other side are not capable of diffusing
through the membrane, then the pressure in the two compartments will become unequal.
In an osmotic cell, the non-diffusible molecules will all Loratadine Online rebound from
the membrane, and so remain confined to their own side. The diffusible
molecules will, like gases, tend to pass through, irrespective of the other
molecules, until they exist in equal concentration on each side, i.e., until
the partial pressure of each kind of molecule is the same on both sides.
If a solution consisting of x molecules of salt and (y x) molecules of water per
cubic centimeter were inclosed in a closed vessel and separated from water containing
y molecules per cubic centimeter by a semipermeable membrane, then water would pass
into the solution until it also contained y molecules of water per cubic centimeter.
But it would then also contain x molecules of salt. Its total molecular concentration
per cubic centimeter would therefore be y -f- x; higher by x than that of the liquid
outside of the vessel. The pressure in the vessel would therefore be increased by *
The process might be represented as follows:
Partial Molecular concentration: y HzO x salt y H-f) x salt
- xH-f) y HiO
tends to become-
Total Molecular concentration: y \y y y -+ x
This excess of total molecular concentration constitutes the osmotic
An experiment analogous to osmosis can be performed on gases directly. Palladium
is permeable for H but not for N. Ramsay, 1894, found that when a palladium vessel
is filled with N, closed, and immersed in Order Loratadine an atmosphere of H, the pressure rises within
the vessel until it exceeds the outside pressure by the partial pressure of the H. How-
ever, this experiment could also be explained by the Buy Cheap Loratadine other theories.
Application of Gas Laws to Osmotic Pressure. Van't Hoff claimed as chief support
of his theory, that the osmotic pressure obeys quantitatively the laws of gas pressure.
As applied to osmotic pressure, these would read: Boyle-M ariotte' s Laiv: With constant
temperature, the osmotic pressure of a solution is proportional to its concentration.
Under standard conditions, a gram-molecule distributed over a liter of space, exerts
22.34 atmospheres of pressure.
Gay-Lussac's Law: With equal concentration, the osmotic pressure grows by
73 for each degree C.
Avogadro'sLaw: With equal osmotic pressure and equal temperature, equal volumes
of solutions contain the same number of dissolved molecules, and the same number as
would be contained in an equal volume of gas of the same temperature and presence.
Experimental Evidence. The experimental confirmation Purchase Loratadine Online of these laws is unfor-
tunately very incomplete (Kahlenberg), because of the difficulty of constructing
strictly semipermeable membranes which will resist the high pressures. The experi-
ments which form the original basis of the Van't Hoff theory were very inaccurate.
Since then, only a few rigorous determinations of absolute osmotic pressure have been
attempted (especially by Morse, Holland and co-workers, 1911 and 1912). These
showed a good agreement with the theory; but they were confined to a single type of
membrane (ferrocyanid of copper) and to a single substance (cane sugar), within a
limited range of concentration and temperature. Outside of these limits, there were
considerable discrepancies. The numerous determinations of relative osmotic pressure,
which Order Loratadine Online can be easily made, are inconclusive; for while they agree with this theory,
they also harmonize with the competing theories. On the other hand, Kahlenberg,
using different, but typically semipermeable membranes, finds that the gas laws do
not hold at all; and that the phenomena can be explained quite simply on an entirely
different basis. The evidence for the Van't Hoff theory is therefore incomplete.
Advantages of the Van't Hoff Theory. This is very attractive in that it reduces all
the physical phenomena of gases and solutions to a common basis, and thereby permits
convenient and far-reaching theoretical and practical deductions. For instance, by
PHYSICAL FACTORS OF SALT ACTIONS 627
postulating that osmotic pressure is directly proportional to the molecular concentration,
it justifies the substitution of an easy calculation for the very difficult experimental Generic Loratadine
determination of the pressure. Since it assumes that the membrane is purely passive,
it permits the calculation of osmotic pressure in situations which are inaccessible to
direct experimentation. In a word, if it is correct, it makes the difficult method of
direct experimentation more or less superfluous, substituting for it an easily applied
formula. This would be highly convenient and desirable but before relying on such
deductions, the correctness of the formula must be more securely established.
Kohlenberg's Theory. This has the advantage that it furnishes the
simplest explanation of the nature of osmotic pressure; and that it is in
the best accord with these phenomena as exhibited in living structures.
The theory refers osmosis to solution-affinity, and may be best understood
from an experiment made by L'Hermite in 1854.
L'Hermite's Experiment. A layer of chloroform is poured into a vessel, then a layer
of water, and finally a layer of ether. Even if any mechanical mixing of the strata is
carefully avoided, it will be found that the ether layer gradually diminishes, until finally
there are only two layers, the upper containing mainly the water, the lower the chloro-
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