- Email: [email protected]
Thermal studies on oxalate complexes. II. Complexes of iron (III) and chromium (III)
Thermochimica Acre, 36 (1980) 79--84 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
79
T H E R M A L S T U D I E S O N O X A L A T E C O M P L E X E S . II. C O M P L E X E S O F IRON(III) AND CHROMIUM(III) *
J.E. HOUSE, JR. and THOMAS G. BLUMTHAL Department o f Chemistry, Illinois State University, Normal, IL 61761 (U.S.A.)
(Received 12 July 1979)
ABSTRACT The decomposition of solid K3[Fe(C204)3[ - 2 I-I20 and Ka[Cr(C204)3] - 3 H 2 0 has been studied using TGA and DSC. After dehydration, the chromium c o m p o u n d was found to decompose by the loss of CO in two steps, the loss of CO2 and additional CO, and finally the loss of CO2. The final product appears to be either K3CrOa or the mixed oxides of chromium and potassium. Kinetic parameters and enthalpy data arc presented for these reactions. In the case of K a [Fe(C204)a ] " 2 H20, dehydration is followed by the loss of CO2 and CO, CO2 alone, and finally CO. The final product appears to be a basic carbonate of the type K3 [Fe(O)2(CO3 )]. Kinetic and thermal data are presented for most of these decomposition reactions. INTRODUCTION T h e f i r s t s t e p in t h e d e c o m p o s i t i o n o f s o l i d o x a l a t e s is t h e l o s s o f c a r b o n monoxide to produce a carbonate, and simple oxalates have been the subject of numerous studies by a variety of thermal techniques [1--3]. Studies on o x a l a t e c o m p l e x e s have also r e c e i v e d s o m e a t t e n t i o n [ 4 - - 7 ] . T h e p o t a s s i u m salts o f c o m p l e x a n i o n s o f o x a l a t e w i t h C r ( I I I ) , M n ( I I I ) , C o ( I I I ) , C o ( I I ) , N i ( I I ) , a n d C u ( I I ) h a v e b e e n s t u d i e d [ 8 ] . F r o m t h e s e s t u d i e s , it a p p e a r s t h a t w h e n t h e t r a n s i t i o n m e t a l is e a s i l y r e d u c e d , a v a r i e t y o f r e d o x r e a c t i o n s takes place during decomposition. This occurs for complexes of Mn(III), Co(III), Ni(II), and Cu(II). No reduction of the transition metal occurs when t h e t r a n s i t i o n m e t a l is F e ( I I I ) o r C r ( I I I ) . In a previous report, w e d e s c r i b e d t h e b e h a v i o r o f cis- a n d t r a n s K[Cr(C204)2(H20)2] and the products obtained after dehydration of the starting m a t e r i a l [ 9 ] . W h i l e b o t h t h e cis a n d t r a n s d i a q u o c o m p o u n d s l o s e w a t e r in t h e i n i t i a l s t a g e s o f d e c o m p o s i t i o n , t h e r e a r e s i g n i f i c a n t d i f f e r e n c e s in t h e d e c o m p o s i t i o n taking place a f t e r d e h y d r a t i o n . In b o t h cases, t h e final p r o d u c t is K C r O 2 . I t is a p p a r e n t f r o m t h e s t u d i e s o n o x a l a t e c o m p l e x e s t h a t t h e s e c o m pounds undergo several interesting reactions during decomposition. We have consequently continued our study of oxalate complexes, and this report
* For Part I, see ref. 9.
S0 presents
results
obtained
for
the
decomposition
of
K3[Cr(C204)3]
-3
H.O
a n d Ka[Fe(C~_O4)a] • 2 H~_O.
EXPERIMENTAL
T h e K3[Cr(C,_O4)3] • 3 H 2 0 a n d K 3 [ F e ( C = O 4 ) a ] • 2 H~.O w e r e o b t a i n e d f r o m A p a c h e C h e m i c a l Co.. a n d w e r e u s e d w i t h o u t f u r t h e r t r e a t m e n t . T h e d e g r e e of hydration of these compounds was established by both TG and isothermal m a s s loss s t u d i e s . T G s t u d i e s w e r e c a r r i e d o u t u s i n g a P e r k i n - - E l m e r t h e r m o g T a v i m e t r i c syst e m . T G S - 2 . T h e c o m p o u n d s w e r e h e a t e d in a n i t r o g e n a t m o s p h e r e a n d t h e procedures employed were similar to those described previously [ 10]. I s o t h e r m a l k,:netic s t u d i e s o n t h e d e h y d r a t i o n r e a c t i o n s w e r e c a r r i e d o u t as d e s c r i b e d p r e v i o u s l y [ 1 1 , 1 2 ] . RESULTS
AND
DISCUSSION
As e x p e c t e d ; t h e T G c u r v e s s h o w t h a t d e h y d r a t i o n o c c u r s p r i o r t o d e c o m p o s i t i o n o f t h e c o m p o u n d s . I n a d d i t i o n t o t h e D S C a n d T G s t u d i e s t o be described later, the d e h y d r a t i o n processes were studied isothermally. T h e fi-action d e h y d r a t e d , ~. w a s f o u n d t o o b e y t h e e q u a t i o n -ln(1
-- a)
=
kt
(I)
w h i c h is c h a r a c t e r i s t i c o f f i r s t - o r d e r p r o c e s s e s . T h e l i n e a r p l o t s o f - - I n ( 1 - - a ' ) vs. t i m e w e r e u s e d t o c a l c u l a t e t h e r a t e c o n s t r u c t s a t v a r i o u s t e m p e r a t u r e s b y means of a linear regression analysis. Table I shows the values of the rate c o n s t a n t s f o r t h e d e h y d r a t i o n o f K3[Cr(C._Oa)3] " 8 H 2 0 a n d K3[Fe(C2Oa).~] " 2 H,_O. F o r b o t h K3[Fe(C~_O4)~] " 2 H 2 0 a n d K3[Cr(C_-Oah] • 3 H,_O t h e d e h y d r a tion processes are a d e q u a t e l y d e s c r i b e d by a first-order rate law. Analysis of t h e vm'iation in r a t e c o n s t a n t w i t h t e m p e r a t u r e a c c o r d i n g t o t h e A n - h e n i u s e q u a t i o n l e a d s t o a n a c t i v a t i o n e n e r g y o f 8 6 . 1 k J m o l e -x a n d a f r e q u e n c y f a c t o r o f 1.1 × 109 s e c - ' in t h e c a s e o f K 3 [ F e ( C 2 0 4 ) j ] " 2 H~_O. F o r K.~[ C r ( C z O ~ ) 3 ] - 3 H~O, t h e s e p a r a m e t e r s h a v e v a l u e s o f 6 4 . 1 k J m o l e - ' a n d
TABLE
1
Rate constants
for the dehydration
Temp.
K3[Fe(C~Oa)3
of oxalate
] " 21-120
complexes K3[Cr(C~_O4)3]
• 3 H20
{'-C) k (sec -t ) 75 85 90 100 110
1.11X 4.32 X 4_68 X 8.55 X --
10 I0 10 10
-4 -~ -4 -4
Corr. coeff,
k (sec -I )
Corr. coeff.
0.994 0.980 0_974 0.986 --
--1.20 X 10 -4 2.60 X 10 -4 3.64 X 10 -4
--0.991 0.992 0.998
S1
2.1 X l 0 s sec-*, r e s p e c t i v e l y . T h e a c t i v a t i o n e n e r g i e s a r e e s t i m a t e d t o be a c c u r a t e t o a b o u t -+5%. T h e T G , D T G , a n d D S C c u r v e s f o r K3[Cr(C,_O4)3]" 3 H , O a n d K3[Fe(C,_O4)3] " 2 H : O a r e s h o w n in Figs. 1 a n d 2, r e s p e c t i v e l y . I n t h e case o f K 3 [ F e ( C : O , h ] - 2 H , O , t h e d e h y d r a t i o n in t h e T G r e s u l t s in a mass-loss curve having the appearance of a two-step dehydration. Consequently, a k i n e t i c a n a l y s i s o f t h i s p r o c e s s w a s n o t p o s s i b l e . H o w e v e r , f o r K.~[ C r ( C : O j ) 3 ] " 3 H_,O it w a s p o s s i b l e t o s t u d y t h e d e h y d r a t i o n f r o m t h e T G curves using the Coats and Redfern equation for a first-order process [13] 1
In In
AR 2 In T = l n - -
1 -- ~
E
/3E
12)
RT
w h e r e ~ is t h e f r a c t i o n d e h y d r a t e d , E is t h e a c t i v a t i o n e n e r g y . T is t h e absol u t e t e m p e r a t u r e , R is t h e gas c o n s t a n t , / 3 is t h e h e a t i n g r a t e , a n d A is t h e frequency factor. For four separate samples, the results for the activation energy and the correlation coefficients calculated from the linear regression a n a l y s i s f i t t i n g t h e d a t a t o e q n . (2) are as f o l l o w s : 4 2 . 6 k J m o l e -!, 0 . 9 9 7 ; 4 5 . 3 k J m o l e -~, 0 . 9 9 7 ; 4 3 . 2 k J m o l e -~, 0 . 9 9 5 ; 4 6 . 5 k3 m o l e -~, 0 . 9 9 4 . T h e s e r e s u l t s y i e l d a m e a n v a l u e o f 4 4 . 4 k J m o l e -~ f o r t h e a c t i v a t i o n e n e r g y , w i t h a s t a n d a r d d e v i a t i o n o f 1.6 k J m o l e -~. A f i r s t - o r d e r e q u a t i o n fits t h e d a t a well, a n d r e p r o d u c i b l e v a l u e s f o r t h e a c t i v a t i o n e n e r g y m'e o b t a i n e d . H o w e v e r . tile a c t i v a t i o n energ%, f r o m i s o t h e r m a l s t u d i e s is a b o u t 6 4 . 1 k J m o l e -~. N o e x p l a n a t i o n f o r t h i s d i f f e r e n c e is r e a d i l y available. T h e DSC curve f o r K3[Cr(CzO4)s] - 8 H : O shows t h a t the d e h y d r a t i o n
I00
.
-, ..\
.
_
DTG
,-!
90 F IR m
i
80~
-
:
:
:
<
7o~1 i
60
.2
,\
: ',
iL
so) 1 ! i I
/•ii
[ I I
I
I00
/DSC
200 300 400 500 600 TEMPERATURE. ° C
Fig. 1. TG, DTG, and DSC curves for the decomposition of K3[Cr(C204)3 ] " 3 H~_O.
82 DTG
I00 m 90 U) Cn
t
|
i
la L1 ,J
70
jl
: It
i i
i
i
i
i
',:
60
T
5 MCAL
I
100
200
300
400
TEMPERATURE,
500
600
740
°C
F i g . 2. T G , D T G , a n d D S C c u r v e s f o r t h e d e c o m p o s i t i o n o f K3[Fe(C~_O4)3 ] " 2 H : O .
d o e s n o t t a k e pl a c e c o m p l e t e l y in o n e step. T h e t o t a l A H o f 1 2 5 . 5 kJ m o l e -1 c o r r e s p o n d s to 41.8 k J m o l e -1 o f w a t e r r e m o v e d . T h e s e results m'e in g o o d a g r e e m e n t w i t h t h o s e p r e v i o u s l y p u b l i s h e d f o r this c o m p l e x [4]. F o r K3[Fe(C:O4)3] " 2 H : O , a w e l l - d e f i n e d e n d o t h e r m i c p e a k is seen in t h e d e h y d r a t i o n r e g i o n . A A H value o f 6 0 . 8 k J m o l e -1 is o b t a i n e d , w h i c h is equival e n t t o 3 0 . 4 k J m o l e -I of w a t e r r e m o v e d . A n a l y s i s o f t h e TG curves s h o w s t h a t t h e d e c o m p o s i t i o n of K3[Fe(C20~)3] • 2 H 2 0 t a k e s place a c c o r d i n g t o eqns. (3)--(6). K3[Fe(C204)3] " 2 H~O(s) -* K3[Fe(C:O4)3](s) + 2 H 2 0 ( g )
(3)
K3[Fe(C204)3](s) -~ K3[Fe(C~_O4)(CO3)(O)](s) + 2 CO(g) + CO:(g)
(4)
K3[Fe(C=O4)(CO3)(O)](s) -~ K ~ [ F e ( C : O 4 ) ( O ) 2 ] ( s ) + CO2(g)
(5)
K 3 [ F e ( C 2 0 4 ) ( O ) 2 ] ( s ) -~ K3[Fe(CO3)(O)~_](s) + CO(g)
(6)
T h e final p r o d u c t stable a t 7 5 0 ° C is w r i t t e n as a basic c a r b o n a t e , b u t it is also possible t h a t t h e p r o d u c t is an e q u i m o l a r m i x t u r e o f K:CO3 a n d KFeO~. F o l l o w i n g t h e d e h y d r a t i o n o f K3[Fe(C204)3] • 2 H 2 0 in t h e DSC, an exot h e r m is o b s e r v e d f r o m 2 8 0 - - 2 9 5 ° C. This p e a k d o e s n o t a p p e a r t o be associa t e d w i t h a n y o f t h e processes involving mass loss, e x c e p t p e r h a p s in t h e initial stages, a n d m a y r e p r e s e n t a s t r u c t u r a l r e a r r a n g e m e n t in t h e d e h y d r a t e d m a t e r i a l . A t 3 1 5 - - 3 3 0 ° C , a s e c o n d e x o t h e r m is seen, w h i c h corres p o n d s t o t h e r e a c t i o n r e p r e s e n t e d b y e q n . (41. B e c a u s e a n e n d o t h e r m follows i m m e d i a t e l y , it is n o t possible t o resolve t h e p e a k s a n d o b t a i n A H v a l u e s f o r t h e s e p a r a t e steps in t h e d e c o m p o s i t i o n . T a b l e 2 s h o w s t h e therm a l p a r a m e t e r s f o r t h e d e c o m p o s i t i o n o f K3[Fe(C204)3] • 2 H 2 0 . T h e activa-
83
TABLE 2 Thermal parameters for the d e c o m p o s i t i o n o f K3 [Fe(C204)3 ] " 2 H 2 0 Eqn.
Temp.
range
% Mass loss
(°C) 3
100--145 285--385
4 5 6
Calc.
Obs.
7.6 21.2
7.5 20.5
9.2
6.0 44.0
405--525
Total
625--725 100--725
~
Ea
(kJ mole -1 )
(kd mole -I )
60.8 a
173.6
10.2
---
24.8 38.3
4.5 42.7
---
94.6 --
a 30.4 kJ mole -1 o f H 2 0 released.
tion energies were obtained by fitting the TG data to eqn, (2), and a good linear fit was obtained in each case. For K3[Cr(C204h] - 3 H20, the analysis of the TG curves shows that the d e c o m p o s i t i o n t a k e s p l a c e in f i v e s t e p s a c c o r d i n g t o e q n s . ( 7 ) - - ( 1 1 ) . K 3 [ C r ( C : O 4 ) a ] " 3 H~_O(s)-~ K a [ C r ( C - . O 4 ) 3 ] ( s ) + 3 H 2 0 ( g )
(7)
K~[Cr(C,_O4)~](s) -~ K a [ C r ( C - _ O 4 ) s , 2 ( C O a ) a / 2 ] ( s ) + 3 / 2 C O ( g )
(8)
Ka[ C r ( C 2 0 4 ) a / 2 ( C O a ) a i 2 ]
(S) "+ K 3 [ C r ( C 2 0 4 ) l / 2 ( C O a ) s / 2 ] (s) +
CO(g)
(9)
K a [ C r ( C : O 4 ) u : ( C O a ) s , 2 ] ( s ) -~ K a [ C r ( C O a ) ( O ) 2 ] ( s ) + 1 / 2 C O ( g ) + 2 CO2(g)
(10)
K 3 [ C r ( C O 3 ) ( O ) : ] ( s ) -~ K a C r O 3 ( s ) + C O : ( g )
(11)
T h e f i n a l p r o d u c t is w r i t t e n as p o t a s s i u m c h r o m i t e , a l t h o u g h i t c o u l d a l s o b e c o n s i d e r e d as a m i x t u r e o f t h e m e t a l o x i d e s . T h e s t e p s i n t h e d e c o m p o s i t i o n were sufficiently separated that kinetic and thermal parameters could be determined from the TG and DSC curves. In each case, the first-order Coats m~d R e d f e r n e q u a t i o n p r o v i d e d t h e b e s t f i t t o t h e T G A d a t a . I n g e n e r a l , c o r r e l a t i o n c o e f f i c i e n t s in t h e r a n g e 0 . 9 9 2 - - 0 . 9 9 9 were obtained from the
TABLE 3 Thermal parameters for the decomposition of K3 [Cr(C204)3 Eqn.
Temp.
% Mass loss
range
7 8 9 10
11 Total
(°C)
Calc.
Found
100--165 185--265 285--310 345--485 48:---545 100---545
11.1 8.6 5.7 20.9 9.0 55.4
10.5 9.5
a 4 1 . 8 k J m o l e -1 o f H 2 0
released.
] -
3 H20
A/-/
Ea
( k J m o l e -1 )
(kJ mole -l )
125.5 a 22.3
44.4 174.9
5.0
20.9
--
20.9 9.6 54.6
89.7 ---
163.1 --
84
curves for several runs. Table 3 s h o w s the t h e r m a l parameters for t h e d e c o m p o s i t i o n o f K3[Cr(C=O4)3] " 3 H 2 0 . T h e results o f this w o r k s h o w that d e c o m p o s i t i o n o f K3[Fe(C204)3] and K3[Cr(C204)3] takes place in s o m e w h a t d i f f e r e n t ways. For t h e c h r o m i u m c o m p o u n d , o n l y CO is lost in the first t w o steps as o x a l a t e is c o n v e r t e d to carbonate. For t h e iron c o m p o u n d , this is n o t t h e case, CO and CO_~ being lost in the initial step. However, equivalent products, K3[Fe(CO3}(O)2] and K3[Cr(CO3)(O)2], axe o b t a i n e d during t h e processes [eqns. (6) and ( 1 0 ) ] . It has n o t b e e n possible to d e t e r m i n e t h e k i n e t i c s o f all t h e steps, b u t the d e c o m p o s i t i o n patterns appear to be r e p r o d u c i b l e and reliable.
REFERENCES 1 D . A . Y o u n g , D e c o m p o s i t i o n o f Solids, P e r g a m o n , O x f o r d , 1 9 6 6 , p p . 7 7 - - 7 9 , 1 5 2 - -
160. 2 T. P a l a n i s a m y , J. G o p a l a k r i s h n a n , B. V i s w a n a t h a n , V. S t r i n i v a s a n a n d V.V.C. Sastin, T h e r m o c h i m . A c t a , 2 (1971) 2 6 5 . 3 D. D o l l i m o r e a n d D . L . G r i f f i t h s , J. T h e r m . Anal., 2 ( 1 9 7 0 ) 229. 4 W.W. W e n d l a n d t a n d E . L . S i m m o n s , T h e r m o c h i m . A c t a , 2 ( 1 9 7 1 ) 2 1 7 . 5 E . L . S i m m o n s a n d W.W. W e n d l a n d t , J. I n o r g . N u c l . C h e m . , 27 ( 1 9 6 5 ) 2 3 2 5 . G W.W. W e n d l a n d t a n d E . L . S i m m o n s , J. I n o r g . N u c l . C h e m . , 27 ( 1 9 6 5 ) 2 3 1 7 . 7 N. T a n a k a a n d K. Nagase, Bull. C h e m . Soc. J p n . , 41 ( 1 9 6 8 ) 1 1 4 3 . 8 D. B r o a d b e n t , D. D o l l i m o r e a n d J. D o l l i m o r e , in R . F . S c h w e n d e r , Jr. a n d P_D. G a r n (Eds.), T h e r m a l Analysis, V o l . 2, A c a d e m i c Press, N e w Y o r k , 1 9 6 9 , p p . 7 3 9 - - 7 6 0 . 9 J.E. H o u s e , J r a n d A.M. L e a r n a r d , T h e r m o c h i m . A c t a , 18 ( 1 9 7 7 } 2 9 5 . 10 J.E. H o u s e , Jr., G . L . J e p s e n a n d J.C. Bailar, Jr., I n o r g . C h e m . , 18 ( 1 9 7 9 ) 1 3 9 7 . 11 A. A k h a v e i n a n d J.E. H o u s e , Jr., J. I n o r g . Nucl. C h e m . , 32 ( 1 9 7 0 ) 1 4 7 9 . 12 J.E. H o u s e , Jr. a n d J.C. Bailar, Jr., J. A m . C h e m . Soc., 91 ( 1 9 6 9 ) 67. 13 A.W. C o a t s a n d J.P. R e d f e r n , N a t u r e ( L o n d o n ) , 2 0 1 ( 1 9 6 4 ) 68.
79
T H E R M A L S T U D I E S O N O X A L A T E C O M P L E X E S . II. C O M P L E X E S O F IRON(III) AND CHROMIUM(III) *
J.E. HOUSE, JR. and THOMAS G. BLUMTHAL Department o f Chemistry, Illinois State University, Normal, IL 61761 (U.S.A.)
(Received 12 July 1979)
ABSTRACT The decomposition of solid K3[Fe(C204)3[ - 2 I-I20 and Ka[Cr(C204)3] - 3 H 2 0 has been studied using TGA and DSC. After dehydration, the chromium c o m p o u n d was found to decompose by the loss of CO in two steps, the loss of CO2 and additional CO, and finally the loss of CO2. The final product appears to be either K3CrOa or the mixed oxides of chromium and potassium. Kinetic parameters and enthalpy data arc presented for these reactions. In the case of K a [Fe(C204)a ] " 2 H20, dehydration is followed by the loss of CO2 and CO, CO2 alone, and finally CO. The final product appears to be a basic carbonate of the type K3 [Fe(O)2(CO3 )]. Kinetic and thermal data are presented for most of these decomposition reactions. INTRODUCTION T h e f i r s t s t e p in t h e d e c o m p o s i t i o n o f s o l i d o x a l a t e s is t h e l o s s o f c a r b o n monoxide to produce a carbonate, and simple oxalates have been the subject of numerous studies by a variety of thermal techniques [1--3]. Studies on o x a l a t e c o m p l e x e s have also r e c e i v e d s o m e a t t e n t i o n [ 4 - - 7 ] . T h e p o t a s s i u m salts o f c o m p l e x a n i o n s o f o x a l a t e w i t h C r ( I I I ) , M n ( I I I ) , C o ( I I I ) , C o ( I I ) , N i ( I I ) , a n d C u ( I I ) h a v e b e e n s t u d i e d [ 8 ] . F r o m t h e s e s t u d i e s , it a p p e a r s t h a t w h e n t h e t r a n s i t i o n m e t a l is e a s i l y r e d u c e d , a v a r i e t y o f r e d o x r e a c t i o n s takes place during decomposition. This occurs for complexes of Mn(III), Co(III), Ni(II), and Cu(II). No reduction of the transition metal occurs when t h e t r a n s i t i o n m e t a l is F e ( I I I ) o r C r ( I I I ) . In a previous report, w e d e s c r i b e d t h e b e h a v i o r o f cis- a n d t r a n s K[Cr(C204)2(H20)2] and the products obtained after dehydration of the starting m a t e r i a l [ 9 ] . W h i l e b o t h t h e cis a n d t r a n s d i a q u o c o m p o u n d s l o s e w a t e r in t h e i n i t i a l s t a g e s o f d e c o m p o s i t i o n , t h e r e a r e s i g n i f i c a n t d i f f e r e n c e s in t h e d e c o m p o s i t i o n taking place a f t e r d e h y d r a t i o n . In b o t h cases, t h e final p r o d u c t is K C r O 2 . I t is a p p a r e n t f r o m t h e s t u d i e s o n o x a l a t e c o m p l e x e s t h a t t h e s e c o m pounds undergo several interesting reactions during decomposition. We have consequently continued our study of oxalate complexes, and this report
* For Part I, see ref. 9.
S0 presents
results
obtained
for
the
decomposition
of
K3[Cr(C204)3]
-3
H.O
a n d Ka[Fe(C~_O4)a] • 2 H~_O.
EXPERIMENTAL
T h e K3[Cr(C,_O4)3] • 3 H 2 0 a n d K 3 [ F e ( C = O 4 ) a ] • 2 H~.O w e r e o b t a i n e d f r o m A p a c h e C h e m i c a l Co.. a n d w e r e u s e d w i t h o u t f u r t h e r t r e a t m e n t . T h e d e g r e e of hydration of these compounds was established by both TG and isothermal m a s s loss s t u d i e s . T G s t u d i e s w e r e c a r r i e d o u t u s i n g a P e r k i n - - E l m e r t h e r m o g T a v i m e t r i c syst e m . T G S - 2 . T h e c o m p o u n d s w e r e h e a t e d in a n i t r o g e n a t m o s p h e r e a n d t h e procedures employed were similar to those described previously [ 10]. I s o t h e r m a l k,:netic s t u d i e s o n t h e d e h y d r a t i o n r e a c t i o n s w e r e c a r r i e d o u t as d e s c r i b e d p r e v i o u s l y [ 1 1 , 1 2 ] . RESULTS
AND
DISCUSSION
As e x p e c t e d ; t h e T G c u r v e s s h o w t h a t d e h y d r a t i o n o c c u r s p r i o r t o d e c o m p o s i t i o n o f t h e c o m p o u n d s . I n a d d i t i o n t o t h e D S C a n d T G s t u d i e s t o be described later, the d e h y d r a t i o n processes were studied isothermally. T h e fi-action d e h y d r a t e d , ~. w a s f o u n d t o o b e y t h e e q u a t i o n -ln(1
-- a)
=
kt
(I)
w h i c h is c h a r a c t e r i s t i c o f f i r s t - o r d e r p r o c e s s e s . T h e l i n e a r p l o t s o f - - I n ( 1 - - a ' ) vs. t i m e w e r e u s e d t o c a l c u l a t e t h e r a t e c o n s t r u c t s a t v a r i o u s t e m p e r a t u r e s b y means of a linear regression analysis. Table I shows the values of the rate c o n s t a n t s f o r t h e d e h y d r a t i o n o f K3[Cr(C._Oa)3] " 8 H 2 0 a n d K3[Fe(C2Oa).~] " 2 H,_O. F o r b o t h K3[Fe(C~_O4)~] " 2 H 2 0 a n d K3[Cr(C_-Oah] • 3 H,_O t h e d e h y d r a tion processes are a d e q u a t e l y d e s c r i b e d by a first-order rate law. Analysis of t h e vm'iation in r a t e c o n s t a n t w i t h t e m p e r a t u r e a c c o r d i n g t o t h e A n - h e n i u s e q u a t i o n l e a d s t o a n a c t i v a t i o n e n e r g y o f 8 6 . 1 k J m o l e -x a n d a f r e q u e n c y f a c t o r o f 1.1 × 109 s e c - ' in t h e c a s e o f K 3 [ F e ( C 2 0 4 ) j ] " 2 H~_O. F o r K.~[ C r ( C z O ~ ) 3 ] - 3 H~O, t h e s e p a r a m e t e r s h a v e v a l u e s o f 6 4 . 1 k J m o l e - ' a n d
TABLE
1
Rate constants
for the dehydration
Temp.
K3[Fe(C~Oa)3
of oxalate
] " 21-120
complexes K3[Cr(C~_O4)3]
• 3 H20
{'-C) k (sec -t ) 75 85 90 100 110
1.11X 4.32 X 4_68 X 8.55 X --
10 I0 10 10
-4 -~ -4 -4
Corr. coeff,
k (sec -I )
Corr. coeff.
0.994 0.980 0_974 0.986 --
--1.20 X 10 -4 2.60 X 10 -4 3.64 X 10 -4
--0.991 0.992 0.998
S1
2.1 X l 0 s sec-*, r e s p e c t i v e l y . T h e a c t i v a t i o n e n e r g i e s a r e e s t i m a t e d t o be a c c u r a t e t o a b o u t -+5%. T h e T G , D T G , a n d D S C c u r v e s f o r K3[Cr(C,_O4)3]" 3 H , O a n d K3[Fe(C,_O4)3] " 2 H : O a r e s h o w n in Figs. 1 a n d 2, r e s p e c t i v e l y . I n t h e case o f K 3 [ F e ( C : O , h ] - 2 H , O , t h e d e h y d r a t i o n in t h e T G r e s u l t s in a mass-loss curve having the appearance of a two-step dehydration. Consequently, a k i n e t i c a n a l y s i s o f t h i s p r o c e s s w a s n o t p o s s i b l e . H o w e v e r , f o r K.~[ C r ( C : O j ) 3 ] " 3 H_,O it w a s p o s s i b l e t o s t u d y t h e d e h y d r a t i o n f r o m t h e T G curves using the Coats and Redfern equation for a first-order process [13] 1
In In
AR 2 In T = l n - -
1 -- ~
E
/3E
12)
RT
w h e r e ~ is t h e f r a c t i o n d e h y d r a t e d , E is t h e a c t i v a t i o n e n e r g y . T is t h e absol u t e t e m p e r a t u r e , R is t h e gas c o n s t a n t , / 3 is t h e h e a t i n g r a t e , a n d A is t h e frequency factor. For four separate samples, the results for the activation energy and the correlation coefficients calculated from the linear regression a n a l y s i s f i t t i n g t h e d a t a t o e q n . (2) are as f o l l o w s : 4 2 . 6 k J m o l e -!, 0 . 9 9 7 ; 4 5 . 3 k J m o l e -~, 0 . 9 9 7 ; 4 3 . 2 k J m o l e -~, 0 . 9 9 5 ; 4 6 . 5 k3 m o l e -~, 0 . 9 9 4 . T h e s e r e s u l t s y i e l d a m e a n v a l u e o f 4 4 . 4 k J m o l e -~ f o r t h e a c t i v a t i o n e n e r g y , w i t h a s t a n d a r d d e v i a t i o n o f 1.6 k J m o l e -~. A f i r s t - o r d e r e q u a t i o n fits t h e d a t a well, a n d r e p r o d u c i b l e v a l u e s f o r t h e a c t i v a t i o n e n e r g y m'e o b t a i n e d . H o w e v e r . tile a c t i v a t i o n energ%, f r o m i s o t h e r m a l s t u d i e s is a b o u t 6 4 . 1 k J m o l e -~. N o e x p l a n a t i o n f o r t h i s d i f f e r e n c e is r e a d i l y available. T h e DSC curve f o r K3[Cr(CzO4)s] - 8 H : O shows t h a t the d e h y d r a t i o n
I00
.
-, ..\
.
_
DTG
,-!
90 F IR m
i
80~
-
:
:
:
<
7o~1 i
60
.2
,\
: ',
iL
so) 1 ! i I
/•ii
[ I I
I
I00
/DSC
200 300 400 500 600 TEMPERATURE. ° C
Fig. 1. TG, DTG, and DSC curves for the decomposition of K3[Cr(C204)3 ] " 3 H~_O.
82 DTG
I00 m 90 U) Cn
t
|
i
la L1 ,J
70
jl
: It
i i
i
i
i
i
',:
60
T
5 MCAL
I
100
200
300
400
TEMPERATURE,
500
600
740
°C
F i g . 2. T G , D T G , a n d D S C c u r v e s f o r t h e d e c o m p o s i t i o n o f K3[Fe(C~_O4)3 ] " 2 H : O .
d o e s n o t t a k e pl a c e c o m p l e t e l y in o n e step. T h e t o t a l A H o f 1 2 5 . 5 kJ m o l e -1 c o r r e s p o n d s to 41.8 k J m o l e -1 o f w a t e r r e m o v e d . T h e s e results m'e in g o o d a g r e e m e n t w i t h t h o s e p r e v i o u s l y p u b l i s h e d f o r this c o m p l e x [4]. F o r K3[Fe(C:O4)3] " 2 H : O , a w e l l - d e f i n e d e n d o t h e r m i c p e a k is seen in t h e d e h y d r a t i o n r e g i o n . A A H value o f 6 0 . 8 k J m o l e -1 is o b t a i n e d , w h i c h is equival e n t t o 3 0 . 4 k J m o l e -I of w a t e r r e m o v e d . A n a l y s i s o f t h e TG curves s h o w s t h a t t h e d e c o m p o s i t i o n of K3[Fe(C20~)3] • 2 H 2 0 t a k e s place a c c o r d i n g t o eqns. (3)--(6). K3[Fe(C204)3] " 2 H~O(s) -* K3[Fe(C:O4)3](s) + 2 H 2 0 ( g )
(3)
K3[Fe(C204)3](s) -~ K3[Fe(C~_O4)(CO3)(O)](s) + 2 CO(g) + CO:(g)
(4)
K3[Fe(C=O4)(CO3)(O)](s) -~ K ~ [ F e ( C : O 4 ) ( O ) 2 ] ( s ) + CO2(g)
(5)
K 3 [ F e ( C 2 0 4 ) ( O ) 2 ] ( s ) -~ K3[Fe(CO3)(O)~_](s) + CO(g)
(6)
T h e final p r o d u c t stable a t 7 5 0 ° C is w r i t t e n as a basic c a r b o n a t e , b u t it is also possible t h a t t h e p r o d u c t is an e q u i m o l a r m i x t u r e o f K:CO3 a n d KFeO~. F o l l o w i n g t h e d e h y d r a t i o n o f K3[Fe(C204)3] • 2 H 2 0 in t h e DSC, an exot h e r m is o b s e r v e d f r o m 2 8 0 - - 2 9 5 ° C. This p e a k d o e s n o t a p p e a r t o be associa t e d w i t h a n y o f t h e processes involving mass loss, e x c e p t p e r h a p s in t h e initial stages, a n d m a y r e p r e s e n t a s t r u c t u r a l r e a r r a n g e m e n t in t h e d e h y d r a t e d m a t e r i a l . A t 3 1 5 - - 3 3 0 ° C , a s e c o n d e x o t h e r m is seen, w h i c h corres p o n d s t o t h e r e a c t i o n r e p r e s e n t e d b y e q n . (41. B e c a u s e a n e n d o t h e r m follows i m m e d i a t e l y , it is n o t possible t o resolve t h e p e a k s a n d o b t a i n A H v a l u e s f o r t h e s e p a r a t e steps in t h e d e c o m p o s i t i o n . T a b l e 2 s h o w s t h e therm a l p a r a m e t e r s f o r t h e d e c o m p o s i t i o n o f K3[Fe(C204)3] • 2 H 2 0 . T h e activa-
83
TABLE 2 Thermal parameters for the d e c o m p o s i t i o n o f K3 [Fe(C204)3 ] " 2 H 2 0 Eqn.
Temp.
range
% Mass loss
(°C) 3
100--145 285--385
4 5 6
Calc.
Obs.
7.6 21.2
7.5 20.5
9.2
6.0 44.0
405--525
Total
625--725 100--725
~
Ea
(kJ mole -1 )
(kd mole -I )
60.8 a
173.6
10.2
---
24.8 38.3
4.5 42.7
---
94.6 --
a 30.4 kJ mole -1 o f H 2 0 released.
tion energies were obtained by fitting the TG data to eqn, (2), and a good linear fit was obtained in each case. For K3[Cr(C204h] - 3 H20, the analysis of the TG curves shows that the d e c o m p o s i t i o n t a k e s p l a c e in f i v e s t e p s a c c o r d i n g t o e q n s . ( 7 ) - - ( 1 1 ) . K 3 [ C r ( C : O 4 ) a ] " 3 H~_O(s)-~ K a [ C r ( C - . O 4 ) 3 ] ( s ) + 3 H 2 0 ( g )
(7)
K~[Cr(C,_O4)~](s) -~ K a [ C r ( C - _ O 4 ) s , 2 ( C O a ) a / 2 ] ( s ) + 3 / 2 C O ( g )
(8)
Ka[ C r ( C 2 0 4 ) a / 2 ( C O a ) a i 2 ]
(S) "+ K 3 [ C r ( C 2 0 4 ) l / 2 ( C O a ) s / 2 ] (s) +
CO(g)
(9)
K a [ C r ( C : O 4 ) u : ( C O a ) s , 2 ] ( s ) -~ K a [ C r ( C O a ) ( O ) 2 ] ( s ) + 1 / 2 C O ( g ) + 2 CO2(g)
(10)
K 3 [ C r ( C O 3 ) ( O ) : ] ( s ) -~ K a C r O 3 ( s ) + C O : ( g )
(11)
T h e f i n a l p r o d u c t is w r i t t e n as p o t a s s i u m c h r o m i t e , a l t h o u g h i t c o u l d a l s o b e c o n s i d e r e d as a m i x t u r e o f t h e m e t a l o x i d e s . T h e s t e p s i n t h e d e c o m p o s i t i o n were sufficiently separated that kinetic and thermal parameters could be determined from the TG and DSC curves. In each case, the first-order Coats m~d R e d f e r n e q u a t i o n p r o v i d e d t h e b e s t f i t t o t h e T G A d a t a . I n g e n e r a l , c o r r e l a t i o n c o e f f i c i e n t s in t h e r a n g e 0 . 9 9 2 - - 0 . 9 9 9 were obtained from the
TABLE 3 Thermal parameters for the decomposition of K3 [Cr(C204)3 Eqn.
Temp.
% Mass loss
range
7 8 9 10
11 Total
(°C)
Calc.
Found
100--165 185--265 285--310 345--485 48:---545 100---545
11.1 8.6 5.7 20.9 9.0 55.4
10.5 9.5
a 4 1 . 8 k J m o l e -1 o f H 2 0
released.
] -
3 H20
A/-/
Ea
( k J m o l e -1 )
(kJ mole -l )
125.5 a 22.3
44.4 174.9
5.0
20.9
--
20.9 9.6 54.6
89.7 ---
163.1 --
84
curves for several runs. Table 3 s h o w s the t h e r m a l parameters for t h e d e c o m p o s i t i o n o f K3[Cr(C=O4)3] " 3 H 2 0 . T h e results o f this w o r k s h o w that d e c o m p o s i t i o n o f K3[Fe(C204)3] and K3[Cr(C204)3] takes place in s o m e w h a t d i f f e r e n t ways. For t h e c h r o m i u m c o m p o u n d , o n l y CO is lost in the first t w o steps as o x a l a t e is c o n v e r t e d to carbonate. For t h e iron c o m p o u n d , this is n o t t h e case, CO and CO_~ being lost in the initial step. However, equivalent products, K3[Fe(CO3}(O)2] and K3[Cr(CO3)(O)2], axe o b t a i n e d during t h e processes [eqns. (6) and ( 1 0 ) ] . It has n o t b e e n possible to d e t e r m i n e t h e k i n e t i c s o f all t h e steps, b u t the d e c o m p o s i t i o n patterns appear to be r e p r o d u c i b l e and reliable.
REFERENCES 1 D . A . Y o u n g , D e c o m p o s i t i o n o f Solids, P e r g a m o n , O x f o r d , 1 9 6 6 , p p . 7 7 - - 7 9 , 1 5 2 - -
160. 2 T. P a l a n i s a m y , J. G o p a l a k r i s h n a n , B. V i s w a n a t h a n , V. S t r i n i v a s a n a n d V.V.C. Sastin, T h e r m o c h i m . A c t a , 2 (1971) 2 6 5 . 3 D. D o l l i m o r e a n d D . L . G r i f f i t h s , J. T h e r m . Anal., 2 ( 1 9 7 0 ) 229. 4 W.W. W e n d l a n d t a n d E . L . S i m m o n s , T h e r m o c h i m . A c t a , 2 ( 1 9 7 1 ) 2 1 7 . 5 E . L . S i m m o n s a n d W.W. W e n d l a n d t , J. I n o r g . N u c l . C h e m . , 27 ( 1 9 6 5 ) 2 3 2 5 . G W.W. W e n d l a n d t a n d E . L . S i m m o n s , J. I n o r g . N u c l . C h e m . , 27 ( 1 9 6 5 ) 2 3 1 7 . 7 N. T a n a k a a n d K. Nagase, Bull. C h e m . Soc. J p n . , 41 ( 1 9 6 8 ) 1 1 4 3 . 8 D. B r o a d b e n t , D. D o l l i m o r e a n d J. D o l l i m o r e , in R . F . S c h w e n d e r , Jr. a n d P_D. G a r n (Eds.), T h e r m a l Analysis, V o l . 2, A c a d e m i c Press, N e w Y o r k , 1 9 6 9 , p p . 7 3 9 - - 7 6 0 . 9 J.E. H o u s e , J r a n d A.M. L e a r n a r d , T h e r m o c h i m . A c t a , 18 ( 1 9 7 7 } 2 9 5 . 10 J.E. H o u s e , Jr., G . L . J e p s e n a n d J.C. Bailar, Jr., I n o r g . C h e m . , 18 ( 1 9 7 9 ) 1 3 9 7 . 11 A. A k h a v e i n a n d J.E. H o u s e , Jr., J. I n o r g . Nucl. C h e m . , 32 ( 1 9 7 0 ) 1 4 7 9 . 12 J.E. H o u s e , Jr. a n d J.C. Bailar, Jr., J. A m . C h e m . Soc., 91 ( 1 9 6 9 ) 67. 13 A.W. C o a t s a n d J.P. R e d f e r n , N a t u r e ( L o n d o n ) , 2 0 1 ( 1 9 6 4 ) 68.