J.RADIOANAL.NUCL.CHEM.,LETTERS 94 /4/ 233-240 /1985/

TRACER DIFFUSION OF ]31I- IONS IN A FEW 1-2 ELECTROLYTES IN AGAR GEL

/Miss/ S. Baluja, R.N. Singh, R. Tripathi, B.M. Shukla

Nuclear and Radiation Chemistry Laboratory, Department of Chemistry, Banaras Hindu University,

Varanasi - 221 005, India

Received 8 October 1984 Accepted 22 October 1984

Tracer diffusion of 131I- ions is studied at different temperatures /20 ~ - 50 ~ in 2,5% agar gel containing sodium and potassium sulphate solutions over a wide range of con- centrations. The results are inqualitative agreement with the theoretical values in the concentration range IO-6-10-IM. Agar gel shows an obstruction effect to diffusional flow. The activation energy for tracer dif- fusion was fgund to be of the order of 16.3 kJ mol -~.

INTRODUCTION

Extensive work has been done on diffusion, self-dif-

fusion and tracer diffusion of a number of ions in

various electrolyte solutions by using different tech-

niques I-5 Several workers 6'7 have used agar gel for

diffusion studies employing radiotracers. The present

paper deals with the tracer diffusion studies of 131I-

233

BALUJA et al. : DIFFUSION OF 131I- IONS

ions in various concentrations /lO-6-10-1M/ of Na2SO 4

and K2SO 4 electrolyte solutions, ~mmobilized in 2.5%

agar gel, at different temperatures /20 ~ ~ under

boundary conditions of zone diffusion. The results are

compared with the theoretical values calculated from the

Gosting-Harned equation 8 .

EXPERIMENTAL

The tracer diffusion coefficients of 131I- ions were

determined at different temperatures and concentrations

of sodium and potassium sulphate solutions in 2.5% agar

gel by zone diffusion technique.

A clean, dry pyrex tube of 30 cm length and of uni-

form diameter /i cm/ was taken. A 1-2 ~ zone of 2.5%

gel containing desired electrolyte solution labelled

with 131I- ions was allowed to set in the centre of two

columns of identical gel containing the same concentration

of electrolyte solution as in the zone. For the study

of obstruction effect on tracer diffusion, the ager per-

centage was varied from 0.5 to 5.0%, keeping the electro-

lyte /Na2SO4/ concentration /IO-2M/ constant. After the

gel had set, both ends of the tube were tightly corked

and kept in a thermostat at a desired temperature.

After allowing the diffusion to proceed for a definite

time /24 h/, the gel columns on both sides of the zone

were sliced into 0.5 cm samples. These samples were trans-

ferred to aluminium planchettes and carefully dehydrated.

The radioactivity of each sample was then measured by

an end-window GM counter.

234

BALUJA et al. : DIFFUSION OF 131I- IONS

RESULTS AND DISCUSSION

Under the conditions of present experiment, Fick's

IInd law takes the form

a o exp l-x2/4Dt/ Iii

a/x't/ - /4~Dt

where a/x,t / is the radioactivity at a distance x from

the zone at time t and a o is the total radioactivity in

the initial zone at time t = O. Experimental values of

tracer diffusion coefficients of 131I- ions were calculated 2

from the slope of the plot of ig a vs. x .

1 Slope = 2.303 x 4Dt /2/

whereas theoretical values were calculated from Gosting-

Harned equation 8

Dj = I ZJ I F2~ _ 33 N~3 2.694xi016 47 i- d(~j )

/3/

d(~j) for 131I- ions in 1-2 electrolyte solutions where

is given by:

] d(ej)

~[I~ + 12 i I + 13 o 2 o o

141

0 I 1 is the limiting equivalent conductance of the diffus-

o and o ing ion and 12 13 are the limiting equivalent con-

ductances of cations and anions of the supporting electro-

lyte, respectively. Other symbols in Eq. /3/ have their

usual significance.

235

BALUJA et al. : DIFFUSION OF 131I- IONS

21 '~ 1.5~ enius pt~

1.9~ ~ "~0~.06 3.20 3.1.0

?, %

IL �9 Dexpt

D ~ 2.01.91.8 ~ Dtheo

13 b) l i "kk'h, 1"~1 0 0.1 0.2 0.3 0.4 0.5 0.6

V'6') mol.dm-3 Fig. i. Concentration dependence of tracer diffusion

of 131I- ions in /a/ Na2SO 4 and /b/ KgSO 4 ~ c solutions immobilized in 2.5% ager gel as 25

A plot of D vs. /C /Fig. i./ shows the effect of

concentration of supporting electrolyte on tracer dif-

fusion of 131I- ions. From Fig. la and ib, it is observed

that in both cases, experimental values of tracer dif-

fusion coefficients are higher than those of theoretical

values. Further, tracer diffusion coefficient decreases

continuously with an increase in electrolyte concentration 9

as predicted by the Onsager-Fuoss theory . However, a i0

large deviation was observed by us for tracer diffusion 32 -

of H~ PO 4 ions. Activation energy for tracer diffusion l_l-

of I ions was calculated from Arrhenius plot

/Fig. ic/ and a value of 16.3 kJ mol -I was obtained.

Experimental and theoretical values of activation ener-

gy are in good agreement showing that the mechanism of

236

BALUJA et al. : DIFFUSION OF 131I- IONS

TABLE 1

Variation of tracer diffusion coefficient of 131I-ions with gel concentration at 25 ~

Sample Concentration -i No. of gel, % D x 105 cm 2 sec

1 0.5 2.18

2 i.O 2.13

3 1.5 2.09

4 2.5 2.00

5 3.5 1.92

6 5.0 1.78

diffusion of 131I- ions in gel is the same as in free

aqueous solutions.

Table 1 contains tracer diffusion coefficient of 131I-

ions in different gel concentrations at 25 ~ The ob-

served decrease in tracer diffusion with gel percentage

can be explained as mechanical obstruction of macromole-

cules of the gel in the path of diffusing ions II. The

obstruction effect is attributed to lengthening of the

effective path of the diffusing ions due to irregular

network of macromolecules. This obstruction effect, re-

ferred to as formation factor /F/ is defined as:

i= g - F D x

s 1 + ~

151

where D x is the extrapolated value of D x to zero agar s g content, ~ is the weight fraction of agar gel and ~ is

the slope of the plot of D x g vs. ~ divided by DM s

The experimental value of e is found to be 3.82. Long-

don and Thomas II assumed gel as a collection of random-

237

BALUJA et al. : DIFFUSION OF 131I- IONS

ly oriented, heavily hydrated needles and obtained the

following expression:

F = 1 + p/B-l/

where p is the effective volume fraction of gel, which

is given by ~/d,~ is the weight of gel and d is the den-

sity of gel /d = 1.6]; 8 the shape factor /5]3 for needlesl.

Thus, the theoretical value of ~ was found to be 0.42,

whereas the experimental value of ~ is 3.82. This shows

that gel has the obstruction effect for the motion of

131I- ions. The difference between experimental and

theoretical values of ~ is most likely due to specific

binding between diffusing ions and gel as well as electro-

static effects of polar sulphonyl groups of agar molecules.

M

One of us /S.B./ is thankful to the University Grants

Commission for financial support.

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BALUJA et al. : DIFFUSION OF 131I- IONS

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