There are several methods to make the measurement of the diffusion
coefficient for liquid solutions.
The simplest apparatus, used till today, is the one set up by Clack on
1921
The apparatus consists of a very thin vertical glass capillary
which ends at the two extremities whith expantions as reservoir.
The lower reservoir is filled with the more dense solution, the upper
with the less dense one ( usually the more diluite or pure solvent)
to have
no disturbance from gravity
.
In both the reservoir the solutions are agitated to mantain a uniform
solute concentration.
Both the recipients have to be sufficiently big because the little mass
transport has not to sensibly alter the concentration.
What does it happen filling the cell?
After reaching a stationary state, the flux of diffusing substance depends only on the concentration gradient which is constant in time.
(*)
This means that the concentration difference between two points, whose distance Dx is fixed, is a constant independently of the position of the two points along the capillary.
Upon reaching a stationary state, the flux of diffusing substance depends only on the constant concentration gradient
and the concentration in the capillary shows a linear profile.
The procedure is as follows: measurement of the small concentration variations in the superior reservoir, determination of the flux of the diffusing substance J=dc/dT and derivation of the diffusion coefficient using the First Fick's law equation, with the constant concentration gradient obtained by (*).
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