Exp. 8 Diffusion of Sodium Chloride in Water

Mass & Heat Transfer Lab

BKF3721 BKF37 21

Faculty of Chemical & Natural Nat ural Resources Engineering

Experiment 8 DIFFUSION OF SODIU C!"ORIDE IN #$%ER 

Name atric No 'roup (rogram Section Date

Semester II ) Session *+,-.*+,/

E0(ERIEN% 81 DIFFUSION OF SODIU C!"ORIDE IN #$%ER 

O23EC%I4E

To determine determine the liquid diffusion coefficient c oefficient of NaCl solution in distilled/de-ionized water.

Mass & Heat Transfer Lab

BKF3721

IN%RODUC%ION

Diffusion is the transport matter from one point to another point by kinetic energy of random molecular motion. The most common driing force of diffusion is a concentration gradient of  diffusing fluids. Concentration gradients tend to moe the fluid in such a direction as to equalize concentrations and destroy the gradients. Diffusion also can force by an actiity gradient! pressure gradient! temperature gradient or e"ternal force fluid. Diffusion is not restricted to molecular  transfer through stagnant layers of solid or fluid.

Diffusion in liquid is sensitie to the composition change but relatiely insensitie to changes in pressure. Diffusion of high iscosity! syrup liquids and macromolecules is slower. #hen solutes molecules diffuse through a solution! solent molecules must be pushed out of the way. $or  this reason! liquid-phase inter diffusion coefficients are inersely proportional to both the iscosity of the solent and the effectie radius of the solute molecules.

When a concentration gradient exists within a fuid consisting o two or more components, there is a tendency or each constituent to fow in such a direction as to reduce the concentration gradient. This is known as mass transer. It takes place in either a gas or a liquid phase or in both phases simultaneously.

Rate o diusion is gi!en by"  J 

Where mol(cm s D

 J

= − D

∂C  ∂ x

#

%$& diusion fux across unit area to the x 'direction #right side%,

)

∂C  ∂ x

& diusi!ity, cm (s )

mol  / cm

%

cm

& concentration gradient in the x 'direction,

 The negati!e sign indicates that the fow is rom high to low concentration. *y expanding +quation #$% using respecti!e terms constituti!e relations, we get

Mass & Heat Transfer Lab

BKF3721

  = − D π  d  dt    &

V  dk  C  M 

'

   M      N   x   #)%

Rearranging gi!es  D

&Vx =

π  d 

dk 

'

 NMC  M  dt 

#% where - & !olume o water in diusion !essel,  x & length o capillaries, cm d & capillaries diameter, cm / & number o capillaries 0 & molar concentration o /a1l solution, mol( 10 & conducti!ity change per unit molar concentration change, 23(mol('$ dk  dt 

& rate o conducti!ity change o!er time

 The slope obtained rom the plot o conducti!ity as unction o time can be used to calculate the diusi!ity.

EQUIPMENT/ APPARATUS/ MATERIAL

3')$4'* iquid diusion apparatus

5igure $" iquid 6iusion 1oe7cient 8pparatus

Mass & Heat Transfer Lab

BKF3721

 This liquid diusion coe7cient apparatus is used to determine the diusi!ity o /a1l solution in distilled water. 8 known concentration o /a1l solution is placed in a diusion cell immersed in distilled water. 8 magnetic stirrer and a conducti!ity meter are pro!ided to monitor the progress o  diusion o!er time. 8 plot o conducti!ity against time will allow or the determination o the liquid diusi!ity. The concentration at the chosen lower ends is taken to be constant while the concentration at top end is eecti!ely 9ero during experiment.

EXPERIMENTAL PROCEDURES

$. :repare

standard

salt

solution

and

get

its

conducti!ity

by

using

conducti!ity meter. #0ust ensure the standard cur!e data co!ers the experimental sample data%. ). :repare the solution o $;;m, 0 /a1l. . 6etermine the number o capillaries #holes% o the :-1 round plate in the <' tube. #The diameter is ) mm while the height is $;.; mm%. =. 1lamp the higher end o the <'tube to the <'tube clamper. *e careul when clamping the <'tube. >. :our /a1l solution to the <'tube until the liquid ?ust reaches the tops o the capillaries. @indly wipe away the excess solution with a sot tissue. 4. Insert the connecti!ity probe to the liquid !essel. +nsure the cable is connected to the digital conducti!ity meter. A. :our $;;; ml o deionised water into the test !essel.

B. (witch )N the magnetic stirrer and set the speed at %** rpm. C. 3witch D/ the conducti!ity meter. +nsure there is reading shown in the meter. $;.

:lace the <'tube into the test !essel. 1are must be taken in this

procedure. /o solution should be dropped to deionised water. $$.

5ill the test !essel with small amount o water until the capillary tops

are submerged approximately > mm below the surace o the water. Record the amount o water added to the test !essel.

Mass & Heat Transfer Lab

$).

BKF3721

8t the same time, when the capillary tops are submerged, start the

stop watch. $.

5or e!ery ) minutes, take the conducti!ity !alue or a time period o 

=; minutes. $=.

Dnce the experiment done, kindly remo!e all the glassware and rinse

with deionised water.

RESULT

Table 1: Standard curve data !r "!d#u$ c%l!r#de NaCl c!ncentrat#!n &M' ;.;;;; ;.;;$; ;.;;$> ;.;;); ;.;;; ;.;;=; ;.;;>; ;.;$;; ;.;$>;

C!nduct#v#t(

Table ): E*+er#$ental Data T#$e &$#n'

C!nduct#v#t(

DISCUSSIONS

6iscuss all your results. The questions below only ser!e as a guideline. Eour discussion should not only limit to these questions.

Mass & Heat Transfer Lab

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$. :lot a graph o conducti!ity !ersus time. 6etermine the liquid diusi!ity o  /a1l solution rom the obtained slope. '. 1ompare the experimental !alue with theoretical !alue #can be rom the

literature re!iew or calculation using liquid diusion equation, e.g" Wilke' 1hang etc.%.