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Isotopic exchange between precipitated antimony oxide and antimony ions in aqueous solution
J. inorg,nucl.Chem., 1966.Vol.28, pp. 2603to 2608. Pergamon Press Ltd. Printedin NorthernIMaad
ISOTOPIC
EXCHANGE BETWEEN PRECIPITATED MONY OXIDE AND ANTIMONY IONS IN AQUEOUS SOLUTION* I. H.
ANTI-
QURESHIand M. SHABBIR
Atomic Energy Centre, Ferozepur Road, Lahore, Pakistan
(Received 11 January 1966; in revised form 13 April 1966) Al~'act--Heterogeneous isotopic exchange between freshly precipitated antimony oxide and antimony ions in aqueous solution has been studied at different temperatures. The temperature has a marked influence on the exchange reaction, which appears to be composed of two main processes, viz. adsorption and self-diffusion. The rate constants and apparent activation energies for the two processes have been determined.
IN SOLID-LIQUID, heterogeneous isotopic exchange reactions, rapid initial exchange at the interface is followed by other, relatively slower, processes of self-diffusion and recrystallization in which the adsorbed material is incorporated into the solid. The initial exchange is also governed by diffusion through the adsorbed solution layer. By vigorous and uniform shaking diffusion in the liquid phase can be made very rapid and may then be neglected. The initial exchange depends on the surface area of the crystal, while subsequent processes are controlled by particle size, crystal perfection, solubility, the self-diffusion, coefficient and the temperature. The total exchange may thus be regarded as composed of an exchange reaction at interface of the crystal, diffusion into the interior of the crystal and recrystallization, and the over-all rate will depend not only on the rate of exchange of radioactive atoms at the interface but also on the rate at which the adsorbed material is incorporated in the interior of the crystals by means of self-diffusion and/or recrystallization. The mechanism by which the adsorbed material is incorporated into the crystal appears to vary for different systems, cl~ In the systems PbSO4-Pb ~-, PbCrO4-Pb ~+ and AgBr-Br--, KOLTHOFFet aL found that recrystallization was the controlling mechanism, whereas LANGERfound that self-diffusion was the main process in the incorporation of adsorbed radioactive silver ions into silver chloride crystals. IONESCUet aL ~ studied the exchange between zinc oxide and zinc ions in solution, and concluded that the over-all exchange reaction consisted of the superposition of two processes, adsorption and self-diffusion, but anion exchanges studies of the Zn3(PO4)2-HsPO4 system, in which anion diffusion is very slow, recrystallization is the controlling step. Isotopic exchange between freshly precipitated antimony oxide and antimony ions * This work was supported in part by the U.S. National Bureau of Standards under International Research Grant Program, Contract No. NBS(G)-46. ~1~O. E. MYEF,s and R. J. PRESTWOOD,Radioactivity Applied to Chemistry (Edited by A. C. WAHL and N. A. BONNER)(2rid Ed.) p. 37. J. Wiley, New York (1958). ~2~S. IONESCU,I. NEGOESCUand I. GAINAR,Proc. 2nd U.N. Int. Conf. Peaceful Uses Atomie Energy, Geneva Vol. 20, p]1279 Romania, p. 123. Sept. (1958). ~s~I. H. QtmF..sm and M. SHABBIR, Talanta 13, 847 (1966). 2603
2604
I.H. QuREsm and M. SnAnam
in aqueous solution has recently been utilized in this laboratory (3~ for the rapid radiochemical separation of antimony, and we decided therefore to study the exchange reaction in some detail. EXPERIMENTAL 200 mg of freshly-prepared antimony trioxide were agitated in 5 ml of de-ionized water containing l"Sb tracer (0'01 pg of Sb) at a controlled temperature. The sample was then centrifuged for ,-,30 sec, the supernate was removed, and the precipitate transferred to another cone. It was washed twice with water and counted in a scintillation well counter. Precipitates were prepared by dissolving 8 g SbCls in 10 ml cone. HCI and pouring this solution into 450 ml of boiling water. After decantation the precipitate was washed thoroughly with hot de-ionized water, filtered and dried at 60°C for about 1 hr. RESULTS AND DISCUSSION The exchange reactions were studied at temperatures of 15, 20, 26, 30 and 35°C; the temperature exerted marked influence on the rate of exchange, which was greater at high temperatures. Thus at 35°C equilibrium is attained in ,~10 rain, whereas at 15°C it requires about 8 hr. Results were plotted according to the McKay equation (4~ ab k . . . . . aq-b
I In (1 -- F) t
(l)
where k is the rate constant in g atoms/sec; a and b are the g atoms/1, of the reactants in each of the two phases, F is the ratio of the quantity exchanged at time t to that exchanged at equilibrium. Values of log (1 -- F) as a function of agitation time at each temperature are plotted in Fig. 1. The resulting graphs are curves similar to those of independently decaying activities, indicating the non-homogeneous nature of the exchange reaction. In order to analyse the curve the final linear portion was extrapolated back to t = 0, and on subtracting the extrapolated line from the original curve, another straight line is obtained which represents the initial reaction. The over-all exchange is thus seen to consist of a fast initial process followed by a relatively slower process which completes the exchange. It is, however, possible that there may be other contributing reactions which are not resolved by this analysis. The rate constants for both reactions are determined at each temperature from the exchange half-times of the two lines using the equation k=
ab a÷b
I in (1 -- F ' ~ t \ I - - F °]
(2)
where F ' and F ° are the apparent exchange fractions at time t and at zero time respectively (Table 1). The apparent activation energies are obtained by plotting log k vs. 1/T(Fig. 3), giving values of 36.6 keal/mole and 41.4 kcal]mole for two stages. The exchange fractions corresponding to each process have been obtained at three temperatures by subtracting from F the corresponding apparent zero time exchange fractions Fx = Fx' - - Fx° (3) Fa=F2'--Fg. °
(4)
c,~ O. E. Ms'~ns and R. J. PRESTWOOD,Radioactivity Applied to Chemistry (Edited by A. C. WArm and N. A. BONNER)(2nd Ed.) p. 7. J. Wiley, New York (1958)
Isotopic exchange between precipitated antimony oxide and antimony ions
/
[]
2605
>
S
E~
J° m
~>~
c0
/ // I
o
o 0
-"
1
i
(-I-I) ~Ol
7o
03
o=
°~ ..a
o
.io c
_
o
_
I
o
_
_o
o o d~ i
i
(J-I) ~ l
2606
I . H . Q t m ~ m and M. SHASB~. -8.0
--
-IOq
,,t OD o
-I1'(
--
-12'0
--
~'II
-13,o
3.0
I
~.l
I
3.z
I
]
3.~
I
3.s
3,4 T
\I
I
3-s
I
3-?
I ~.a
X I0 3
FIG. 3.--Activation energy, log k vs. 1/T°K [(l/T) x 108]. TABLE
Temp. (°C) 15 20* 26 30 35
Exchange half-time (rain) Process 1 Process 2 5-97 0-569 0.538 0.0994 0.0793
232 15.3 8"7 6"4 2"32
1
kl (g atom/see) 3"27 8.708 3"63 1"96 2"46
× x × × x
10-11 10-11 10-1° 10-s 10-9
kl (g atom/see) 8.41 3.27 2.24 3"05 8"41
× × × X ×
10 -is 10-ll 10 -it 10-It 10-11
Amount of tracer 1"69 × 10-8 g atom/1. * Amount of tracer 4.29 × 10-9 g atom/l.
and the values of the exchange fraction F1 for the first process are plotted against agitation time in Fig. 4a. The exchange rate increases with increasing temperature and equilibrium is rapidly attained at the higher temperatures. At low temperatures the process is slower and the equilibrium values of the exchange fraction are higher. It is concluded that adsorption is the predominant phenomenon in the initial stage of the exchange reaction; adsorption does not stop at the end of the first stage of the reaction, but reaches an equilibrium, and when part of the adsorbed layer is destroyed by seE-diffusion or recrystallization it is restored by adsorption at the interface.
Isotopic exchange between precipitated antimony oxide and antimony ions 0.85
--
__2gg
..xlx----'fg~
0"75
30"C
~x x,,~~x
u.0"65
/
/x x-x xS
7
x/x
!
o
g
x
/
c ~
w
t
0-5!
0.45
0.3501
r 6I 8[ I0I
[
4
Time, FIG.
i
20
30 rain
4a.--Exchange fraction vs. agitation time for process I.
o3 ~-
30°C
°tY 0.2I-
~
/ /
0-1
¢ 0
~ I0
20
30
x/
x
~
X
50 SO m 90 90 Time, rain FIG. 4b.--Exchange fraction vs. agitation time for process 2. 40
2607
2608
I . H . Qug~sm and M. SItAnBIR
Values of the exchange fraction F2 corresponding to the second process are plotted in Fig. 4b and are greater at the higher temperatures. The process which predominates during this stage may be either self-diffusion or recrystallization. The apparent activation energy, 41.4 kcal/mole, is an order of magnitude greater than that for recrystallization (usually 3-8 kcal/moley51 but appears to be of the proper order of magnitude for self-diffusion processes, and it is concluded that self-diffusion is the predominant mechanism in the last stage of the exchange. ~ O. E. MYERSand R. J. PRESTWOOD,Radioactivity Applied to Chemistry (Edited by A. C. WAn1 and N. A. Boa,mR) (2nd Ed.) p. 38. J. Wiley, New York (1958).
ISOTOPIC
EXCHANGE BETWEEN PRECIPITATED MONY OXIDE AND ANTIMONY IONS IN AQUEOUS SOLUTION* I. H.
ANTI-
QURESHIand M. SHABBIR
Atomic Energy Centre, Ferozepur Road, Lahore, Pakistan
(Received 11 January 1966; in revised form 13 April 1966) Al~'act--Heterogeneous isotopic exchange between freshly precipitated antimony oxide and antimony ions in aqueous solution has been studied at different temperatures. The temperature has a marked influence on the exchange reaction, which appears to be composed of two main processes, viz. adsorption and self-diffusion. The rate constants and apparent activation energies for the two processes have been determined.
IN SOLID-LIQUID, heterogeneous isotopic exchange reactions, rapid initial exchange at the interface is followed by other, relatively slower, processes of self-diffusion and recrystallization in which the adsorbed material is incorporated into the solid. The initial exchange is also governed by diffusion through the adsorbed solution layer. By vigorous and uniform shaking diffusion in the liquid phase can be made very rapid and may then be neglected. The initial exchange depends on the surface area of the crystal, while subsequent processes are controlled by particle size, crystal perfection, solubility, the self-diffusion, coefficient and the temperature. The total exchange may thus be regarded as composed of an exchange reaction at interface of the crystal, diffusion into the interior of the crystal and recrystallization, and the over-all rate will depend not only on the rate of exchange of radioactive atoms at the interface but also on the rate at which the adsorbed material is incorporated in the interior of the crystals by means of self-diffusion and/or recrystallization. The mechanism by which the adsorbed material is incorporated into the crystal appears to vary for different systems, cl~ In the systems PbSO4-Pb ~-, PbCrO4-Pb ~+ and AgBr-Br--, KOLTHOFFet aL found that recrystallization was the controlling mechanism, whereas LANGERfound that self-diffusion was the main process in the incorporation of adsorbed radioactive silver ions into silver chloride crystals. IONESCUet aL ~ studied the exchange between zinc oxide and zinc ions in solution, and concluded that the over-all exchange reaction consisted of the superposition of two processes, adsorption and self-diffusion, but anion exchanges studies of the Zn3(PO4)2-HsPO4 system, in which anion diffusion is very slow, recrystallization is the controlling step. Isotopic exchange between freshly precipitated antimony oxide and antimony ions * This work was supported in part by the U.S. National Bureau of Standards under International Research Grant Program, Contract No. NBS(G)-46. ~1~O. E. MYEF,s and R. J. PRESTWOOD,Radioactivity Applied to Chemistry (Edited by A. C. WAHL and N. A. BONNER)(2rid Ed.) p. 37. J. Wiley, New York (1958). ~2~S. IONESCU,I. NEGOESCUand I. GAINAR,Proc. 2nd U.N. Int. Conf. Peaceful Uses Atomie Energy, Geneva Vol. 20, p]1279 Romania, p. 123. Sept. (1958). ~s~I. H. QtmF..sm and M. SHABBIR, Talanta 13, 847 (1966). 2603
2604
I.H. QuREsm and M. SnAnam
in aqueous solution has recently been utilized in this laboratory (3~ for the rapid radiochemical separation of antimony, and we decided therefore to study the exchange reaction in some detail. EXPERIMENTAL 200 mg of freshly-prepared antimony trioxide were agitated in 5 ml of de-ionized water containing l"Sb tracer (0'01 pg of Sb) at a controlled temperature. The sample was then centrifuged for ,-,30 sec, the supernate was removed, and the precipitate transferred to another cone. It was washed twice with water and counted in a scintillation well counter. Precipitates were prepared by dissolving 8 g SbCls in 10 ml cone. HCI and pouring this solution into 450 ml of boiling water. After decantation the precipitate was washed thoroughly with hot de-ionized water, filtered and dried at 60°C for about 1 hr. RESULTS AND DISCUSSION The exchange reactions were studied at temperatures of 15, 20, 26, 30 and 35°C; the temperature exerted marked influence on the rate of exchange, which was greater at high temperatures. Thus at 35°C equilibrium is attained in ,~10 rain, whereas at 15°C it requires about 8 hr. Results were plotted according to the McKay equation (4~ ab k . . . . . aq-b
I In (1 -- F) t
(l)
where k is the rate constant in g atoms/sec; a and b are the g atoms/1, of the reactants in each of the two phases, F is the ratio of the quantity exchanged at time t to that exchanged at equilibrium. Values of log (1 -- F) as a function of agitation time at each temperature are plotted in Fig. 1. The resulting graphs are curves similar to those of independently decaying activities, indicating the non-homogeneous nature of the exchange reaction. In order to analyse the curve the final linear portion was extrapolated back to t = 0, and on subtracting the extrapolated line from the original curve, another straight line is obtained which represents the initial reaction. The over-all exchange is thus seen to consist of a fast initial process followed by a relatively slower process which completes the exchange. It is, however, possible that there may be other contributing reactions which are not resolved by this analysis. The rate constants for both reactions are determined at each temperature from the exchange half-times of the two lines using the equation k=
ab a÷b
I in (1 -- F ' ~ t \ I - - F °]
(2)
where F ' and F ° are the apparent exchange fractions at time t and at zero time respectively (Table 1). The apparent activation energies are obtained by plotting log k vs. 1/T(Fig. 3), giving values of 36.6 keal/mole and 41.4 kcal]mole for two stages. The exchange fractions corresponding to each process have been obtained at three temperatures by subtracting from F the corresponding apparent zero time exchange fractions Fx = Fx' - - Fx° (3) Fa=F2'--Fg. °
(4)
c,~ O. E. Ms'~ns and R. J. PRESTWOOD,Radioactivity Applied to Chemistry (Edited by A. C. WArm and N. A. BONNER)(2nd Ed.) p. 7. J. Wiley, New York (1958)
Isotopic exchange between precipitated antimony oxide and antimony ions
/
[]
2605
>
S
E~
J° m
~>~
c0
/ // I
o
o 0
-"
1
i
(-I-I) ~Ol
7o
03
o=
°~ ..a
o
.io c
_
o
_
I
o
_
_o
o o d~ i
i
(J-I) ~ l
2606
I . H . Q t m ~ m and M. SHASB~. -8.0
--
-IOq
,,t OD o
-I1'(
--
-12'0
--
~'II
-13,o
3.0
I
~.l
I
3.z
I
]
3.~
I
3.s
3,4 T
\I
I
3-s
I
3-?
I ~.a
X I0 3
FIG. 3.--Activation energy, log k vs. 1/T°K [(l/T) x 108]. TABLE
Temp. (°C) 15 20* 26 30 35
Exchange half-time (rain) Process 1 Process 2 5-97 0-569 0.538 0.0994 0.0793
232 15.3 8"7 6"4 2"32
1
kl (g atom/see) 3"27 8.708 3"63 1"96 2"46
× x × × x
10-11 10-11 10-1° 10-s 10-9
kl (g atom/see) 8.41 3.27 2.24 3"05 8"41
× × × X ×
10 -is 10-ll 10 -it 10-It 10-11
Amount of tracer 1"69 × 10-8 g atom/1. * Amount of tracer 4.29 × 10-9 g atom/l.
and the values of the exchange fraction F1 for the first process are plotted against agitation time in Fig. 4a. The exchange rate increases with increasing temperature and equilibrium is rapidly attained at the higher temperatures. At low temperatures the process is slower and the equilibrium values of the exchange fraction are higher. It is concluded that adsorption is the predominant phenomenon in the initial stage of the exchange reaction; adsorption does not stop at the end of the first stage of the reaction, but reaches an equilibrium, and when part of the adsorbed layer is destroyed by seE-diffusion or recrystallization it is restored by adsorption at the interface.
Isotopic exchange between precipitated antimony oxide and antimony ions 0.85
--
__2gg
..xlx----'fg~
0"75
30"C
~x x,,~~x
u.0"65
/
/x x-x xS
7
x/x
!
o
g
x
/
c ~
w
t
0-5!
0.45
0.3501
r 6I 8[ I0I
[
4
Time, FIG.
i
20
30 rain
4a.--Exchange fraction vs. agitation time for process I.
o3 ~-
30°C
°tY 0.2I-
~
/ /
0-1
¢ 0
~ I0
20
30
x/
x
~
X
50 SO m 90 90 Time, rain FIG. 4b.--Exchange fraction vs. agitation time for process 2. 40
2607
2608
I . H . Qug~sm and M. SItAnBIR
Values of the exchange fraction F2 corresponding to the second process are plotted in Fig. 4b and are greater at the higher temperatures. The process which predominates during this stage may be either self-diffusion or recrystallization. The apparent activation energy, 41.4 kcal/mole, is an order of magnitude greater than that for recrystallization (usually 3-8 kcal/moley51 but appears to be of the proper order of magnitude for self-diffusion processes, and it is concluded that self-diffusion is the predominant mechanism in the last stage of the exchange. ~ O. E. MYERSand R. J. PRESTWOOD,Radioactivity Applied to Chemistry (Edited by A. C. WAn1 and N. A. Boa,mR) (2nd Ed.) p. 38. J. Wiley, New York (1958).