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Mechanism of ion exchange in heteropoly salts
J. inorg, nucl. Chem., 1971,Vol. 33. pp. 1933to 1936. PergamonPress. Printedin Great Britain
MECHANISM
OF
DEEPALI
ION
EXCHANGE SALTS
CHOUDHURI
IN
HETEROPOLY
and S. K. M U K H E R J E E
University College of Science. Calcutta-19, India ( R e c e i v e d I 0 A u,~,ust .I 970) A b s t r a c t - T h e kinetics of exchange of pyridinium ion in pyridiniumtungstophosphate by a m m o n i u m ion has been studied and from these kinetic studies it has been possible to demonstrate that the exchange reaction in the heteropoly salts is controlled by particle diffusion process. INTRODUCTION
ALTHOUGH the exchange properties of heteropoly salts are well established [1-5], nothing concerning the kinetics of the exchange processes in these salts has yet been reported. It may be mentioned here that in the cases of resins and similar exchangers, the rate determining step of the exchange proces~ was established to be the diffusion of counter ions, rather than an actual chemical reaction. Considering the exchange reaction as a diffusion phenomenon, two types of diffusion of the counter ions must be taken into account; one within the exchange material (particle diffusion), the other across the interface between the solid exchanger and the thin liquid layer which adheres to the exchanger surface (film diffusion). The slower of these two processes will determine the rate of the exchange reaction. Particle diffusion and film diffusion controlled exchange reactions differ in their rate laws and in the nature of dependency of the rate on the particle size. solution concentration and degree of agitation. It is thus possible to identify the rate controlling mechanism through an analysis of the experimentally observed rate laws and the study of the dependence on the variables. Investigation of the kinetic behavior of pyridinium tungstophosphate (PWP) together with the plausible explanation of the results obtained has been presented in this paper. EXPERIMEN'I
AL
A m o n g the salts of tungstophosphric or molybydophosphoric acids, P W P was found to be the most suitable for exchange study since it is less dispersive and exhibits a high degree of exchangeability. P W P ~,as prepared according to the procedure described earlier[4]. Rates of exchange were measured by batch techniques. PWP was ground well in order to give particles of nearly uniform size. A definite amount (0. Ig) of the material was accurately weighed in different Erlenmeyer flasks. T o each of these a measured volume (20 ml) of a m m o n i u m nitrate solution (0.4N in 0-04N HNO:~) was added and the mixtures were thoroughly agitated. After appropriate intervals, the contents of the flasks were filtered using millipore filter paper (pore size 0.65/~ millipore. 1. ,1. Van R. Smit,.l.J. Jacobs and W. R~bb,J. inor~,,, nu¢'l. C h e m . 12, 95 (1959). 2..I. Van R. Smit and W. R o b b , J . inorg,, hurl. ( ' h e m . 26,509 (1964). 3. ,I. Van R. Smit,J. inor,e, hurl. C h e m . 27, 227 (1965). 4. D. ('houdhuri and S. K. Mukheljee.,l. ittor~, tutti. C h e m . 30, 3091 (1968). 5. D. Choudhuri and S. K. M u k h e r j e e . J . it,or~,,, mlcl. C h e m . 32, 1023 (1970). 1933
1934
D. C H O U D H U R ! and S. K. M U K H E R J E E
grade DA, Millipore Filter Company, Bedford, Mass., U.S.A.). The whole operation was conducted at 20°C. The pyridinium ion, liberated at different intervals of time, was estimated spectrophotometr.ically (Beckman model DU) by measuring the extinction at 256 m~. The experiment was repeated using two other concentrations of ammonium nitrate solution, viz. 0.7N and 1N, both in 0.04N HNO3. Since the number of equivalents of ammonium ion in the solution is far greater than that of the pyridinium ion in the exchanger, the concentration of the former may be assumed to be constant all throughout the experiments. Hence, although the rates were measured in limited bath, the conditions were essentially those of an infinite bath. RESULTS AND DISCUSSION
The extent of the exchange reaction F has been expressed as, F=
The amount exchanged at time t The amount exchanged at infinite time
and is plotted as a function of time (Fig. 1). Experimental results obtained using three different concentrations of ammonium nitrate solution exhibited same type bo
o.9
® 0.7
0.6 e~ 0,5 .4 o,4 ¢o 0.3
0.2
Oq
0
I 4
I 8
I 12
I ~
I 20
I 24
l 20
~2
T/ME / H HOUR
Fig. I. Rate of exchange of PWP with NH4 + ion.
of curve and that obtained with 0.4N solution has been shown. The milliequivalents of pyridinium ion liberated from the exchanger at equilibrium (after 48 hr) was also the same, namely 0.095 for any of the three concentrations of the amino-
Ion exchange in heteropoly salts
1935
nium nitrate solutions used. The rate was also found to be independent of the rate of agitation. For particle diffusion to be the rate determining step, Boyd et al. [6] deduced an expression describing diffusion into spherical particles of radius r given by,
F=I -~6, ~
exp(--n2Bt) 1l 2
77""
Where B = (~r"-Di/~"), D i being the effective diffusion coefficient of ammonium and pyridinium ions within the exchanger, Values of Bt as a function of F has been tabulated by Reichenberg[7]. Provided Di is independent of F, plot of Bt against t should be a straight line passing through the origin. The plot is shown in Fig. 2. The nature of the plot together with the facts that the reaction is very 1.0
1.6
1.4
1.2
1.0
0.6
0.2
tT 0
I 4
I 8
I I2
I I6
I 20
I 2,4
28
T/ME IN HOUR
Fig. 2. Relation o f B t values with the time.
slow, independent of the solution concentration or the rate of agitation proves definitely that the exchange reaction is controlled by particle diffusion process. An interesting point to be noticed is that the plot of B t against t produced a single straight line meaning thereby that all the three exchangeable pyridinium 6. G. E. Boyd, A. W. Adamson and L. S. M y e r e , J r . . J . Am. c h e m . S o c . 69.2836(1947). 7. D. Reichenberg,J. Am. c h e m . S o c . 7 5 , 5 8 9 (1953).
1936
D. CHOUDHURI and S. K. MUKHERJEE
ions in P W P exchange at the same rate with the counter ions. This further implies that all three cations are equivalently placed in the crystal of the heteropoly salt and supports the structure suggested from X-ray crystallographic studies [8] on some of the 12-heteropoly salts. 8. A. F. Wells, Structural Inorganic Chemistry 2nd Edn. p. 351. Oxford University Press, Oxford (1950).
MECHANISM
OF
DEEPALI
ION
EXCHANGE SALTS
CHOUDHURI
IN
HETEROPOLY
and S. K. M U K H E R J E E
University College of Science. Calcutta-19, India ( R e c e i v e d I 0 A u,~,ust .I 970) A b s t r a c t - T h e kinetics of exchange of pyridinium ion in pyridiniumtungstophosphate by a m m o n i u m ion has been studied and from these kinetic studies it has been possible to demonstrate that the exchange reaction in the heteropoly salts is controlled by particle diffusion process. INTRODUCTION
ALTHOUGH the exchange properties of heteropoly salts are well established [1-5], nothing concerning the kinetics of the exchange processes in these salts has yet been reported. It may be mentioned here that in the cases of resins and similar exchangers, the rate determining step of the exchange proces~ was established to be the diffusion of counter ions, rather than an actual chemical reaction. Considering the exchange reaction as a diffusion phenomenon, two types of diffusion of the counter ions must be taken into account; one within the exchange material (particle diffusion), the other across the interface between the solid exchanger and the thin liquid layer which adheres to the exchanger surface (film diffusion). The slower of these two processes will determine the rate of the exchange reaction. Particle diffusion and film diffusion controlled exchange reactions differ in their rate laws and in the nature of dependency of the rate on the particle size. solution concentration and degree of agitation. It is thus possible to identify the rate controlling mechanism through an analysis of the experimentally observed rate laws and the study of the dependence on the variables. Investigation of the kinetic behavior of pyridinium tungstophosphate (PWP) together with the plausible explanation of the results obtained has been presented in this paper. EXPERIMEN'I
AL
A m o n g the salts of tungstophosphric or molybydophosphoric acids, P W P was found to be the most suitable for exchange study since it is less dispersive and exhibits a high degree of exchangeability. P W P ~,as prepared according to the procedure described earlier[4]. Rates of exchange were measured by batch techniques. PWP was ground well in order to give particles of nearly uniform size. A definite amount (0. Ig) of the material was accurately weighed in different Erlenmeyer flasks. T o each of these a measured volume (20 ml) of a m m o n i u m nitrate solution (0.4N in 0-04N HNO:~) was added and the mixtures were thoroughly agitated. After appropriate intervals, the contents of the flasks were filtered using millipore filter paper (pore size 0.65/~ millipore. 1. ,1. Van R. Smit,.l.J. Jacobs and W. R~bb,J. inor~,,, nu¢'l. C h e m . 12, 95 (1959). 2..I. Van R. Smit and W. R o b b , J . inorg,, hurl. ( ' h e m . 26,509 (1964). 3. ,I. Van R. Smit,J. inor,e, hurl. C h e m . 27, 227 (1965). 4. D. ('houdhuri and S. K. Mukheljee.,l. ittor~, tutti. C h e m . 30, 3091 (1968). 5. D. Choudhuri and S. K. M u k h e r j e e . J . it,or~,,, mlcl. C h e m . 32, 1023 (1970). 1933
1934
D. C H O U D H U R ! and S. K. M U K H E R J E E
grade DA, Millipore Filter Company, Bedford, Mass., U.S.A.). The whole operation was conducted at 20°C. The pyridinium ion, liberated at different intervals of time, was estimated spectrophotometr.ically (Beckman model DU) by measuring the extinction at 256 m~. The experiment was repeated using two other concentrations of ammonium nitrate solution, viz. 0.7N and 1N, both in 0.04N HNO3. Since the number of equivalents of ammonium ion in the solution is far greater than that of the pyridinium ion in the exchanger, the concentration of the former may be assumed to be constant all throughout the experiments. Hence, although the rates were measured in limited bath, the conditions were essentially those of an infinite bath. RESULTS AND DISCUSSION
The extent of the exchange reaction F has been expressed as, F=
The amount exchanged at time t The amount exchanged at infinite time
and is plotted as a function of time (Fig. 1). Experimental results obtained using three different concentrations of ammonium nitrate solution exhibited same type bo
o.9
® 0.7
0.6 e~ 0,5 .4 o,4 ¢o 0.3
0.2
Oq
0
I 4
I 8
I 12
I ~
I 20
I 24
l 20
~2
T/ME / H HOUR
Fig. I. Rate of exchange of PWP with NH4 + ion.
of curve and that obtained with 0.4N solution has been shown. The milliequivalents of pyridinium ion liberated from the exchanger at equilibrium (after 48 hr) was also the same, namely 0.095 for any of the three concentrations of the amino-
Ion exchange in heteropoly salts
1935
nium nitrate solutions used. The rate was also found to be independent of the rate of agitation. For particle diffusion to be the rate determining step, Boyd et al. [6] deduced an expression describing diffusion into spherical particles of radius r given by,
F=I -~6, ~
exp(--n2Bt) 1l 2
77""
Where B = (~r"-Di/~"), D i being the effective diffusion coefficient of ammonium and pyridinium ions within the exchanger, Values of Bt as a function of F has been tabulated by Reichenberg[7]. Provided Di is independent of F, plot of Bt against t should be a straight line passing through the origin. The plot is shown in Fig. 2. The nature of the plot together with the facts that the reaction is very 1.0
1.6
1.4
1.2
1.0
0.6
0.2
tT 0
I 4
I 8
I I2
I I6
I 20
I 2,4
28
T/ME IN HOUR
Fig. 2. Relation o f B t values with the time.
slow, independent of the solution concentration or the rate of agitation proves definitely that the exchange reaction is controlled by particle diffusion process. An interesting point to be noticed is that the plot of B t against t produced a single straight line meaning thereby that all the three exchangeable pyridinium 6. G. E. Boyd, A. W. Adamson and L. S. M y e r e , J r . . J . Am. c h e m . S o c . 69.2836(1947). 7. D. Reichenberg,J. Am. c h e m . S o c . 7 5 , 5 8 9 (1953).
1936
D. CHOUDHURI and S. K. MUKHERJEE
ions in P W P exchange at the same rate with the counter ions. This further implies that all three cations are equivalently placed in the crystal of the heteropoly salt and supports the structure suggested from X-ray crystallographic studies [8] on some of the 12-heteropoly salts. 8. A. F. Wells, Structural Inorganic Chemistry 2nd Edn. p. 351. Oxford University Press, Oxford (1950).