Chemistry Reference
In-Depth Information
∂
∂
c
x
∂
∂
c
x
∂
∂
c
x
- (
J
1
) =
123
(
D
11
)
v
+
123
(
D
12
)
v
+
123
(
D
13
)
v
(5)
∂
∂
c
x
∂
∂
c
x
∂
∂
c
x
- (
J
2
) =
123
(
D
21
)
v
+
123
(
D
22
)
v
+
123
(
D
23
)
v
(6)
∂
∂
c
x
∂
∂
c
x
∂
∂
c
x
- (
J
3
) =
123
(
D
31
)
v
+
123
(
D
32
)
v
+
123
(
D
33
)
v
(7)
where the main diffusion coefficients
123
D
11
,
123
D
22,
and
123
D
33
, give the flux of each
solute produced by its own concentration gradient. Cross diffusion coefficients,
123
D
12
,
123
D
13
,
123
D
21
,
123
D
23
,
123
D
31
,
and
123
D
32
give the coupled flux of each solute driven by a
concentration gradient in the other solute.
4.2 EXPERIMENTAL TECHNIQUES: CONDUCTIMETRIC AND TAYLOR
DISPERSION TECHNIQUES
Experimental methods that can be employed to determine mutual diffusion coef-
ficients [20, 21]: Diaphragm cell (inaccuracy 0.5-1%), conductimetric (inaccuracy
0.2%), Gouy and Rayleigh interferometry (inaccuracy <0. 1%), and Taylor Disper-
sion (inaccuracy 1-2%). While the first and second method consume days in experi-
mental time, the last one imply just hours. The conductimetric technique follows
the diffusion process by measuring the ratio of electrical resistances of the electro-
lyte solution in two vertically opposed capillaries as time proceeds. Despite this
method has given us reasonably precise and accurate results, it is limited to stud-
ies of mutual diffusion in electrolyte solutions, and like in diaphragm cell experi-
ments, the run times are inconveniently long (~days). The Gouy method also has
high precision, but when applied to microemulsions they are prone to gravitational
instabilities and convections. Thus, the Taylor dispersion has become increasingly
popular for measuring diffusion in solutions, because of its experimental short time
and its major application to the different systems (electrolytes or non electrolytes).
In addition, with this method it is possible to measure multicomponent mutual dif-
fusion coefficients.
Mutual differential diffusion coef¿ cients of binary (e.g. [6-8]) and pseudo binary
systems (such as, e.g., cobalt chloride in aqueous solutions of sucrose [9]), have been
measured using a conductimetric cell and an automatic apparatus to follow diffusion.
This cell uses an open ended capillary method and a conductimetric technique is used
to follow the diffusion process by measuring the resistance of the solution inside the
capillaries, at recorded times. Figure 1 shows a
schematic representation of the open-
ended capillary cell.
Search WWH ::
Custom Search