The enthalpies of formation and of dehydration of some AFm
phases with singly charged anions
Citation for published version (APA):
Houtepen, C. J. M., & Stein, H. N. (1976). The enthalpies of formation and of dehydration of some AFm phases with singly charged anions. Cement and Concrete Research, 6(5), 651-658. https://doi.org/10.1016/0008-8846(76)90029-6
DOI:
10.1016/0008-8846(76)90029-6
Document status and date: Published: 01/01/1976
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CEMENT and CONCRETE RESEARCH. Vol. 6, pp. 651-658, 1976. Pergamon Press, Inc
Printed in the United States.
THE ENTHALPIES OF FORMATION AND OF D E H Y D R A T I O N OF SOME A F m PHASES WITH SINGLY CHARGED ANIONS 2.
C.J.M. Houtepen and H.N. Stein L a b o r a t o r y of General C h e m i s t r y
T e c h n o l o g i c a l University, Eindhoven, The N e t h e r l a n d s
(Communicated by H. F. W. Taylor)
(Received June 23, 1976)
A B S T R A C T
The enthalpies of formation of some A F m phases of the type C a 2 A I ( O H ) 6 X . x H 2 0 (X = CI-, Br-, I-, NO~, CIO~, CIO3, BrO~ and IOn) were d e t e r m i n e d by m e a s u r i n g £he he~ts of solution in h y d r o c h l o r i c or perchloric acid. F r o m the d i f f e r e n c e s between the heats of solution of the compounds and their d e h y d r a t i o n products, the enthalpies of
d e h y d r a t i o n were established. They increase w i t h increasing enthalpies of h y d r a t i o n of the individual anions X in aqueous solution. However, this r e l a t i o n s h i p is not the same for halide ions as for oxy-anions.
Die B i l d u n g s e n t h a l p i e n einiger A F m Phasen Ca2AI(OH) 6X.xH20 (X = CI-, Br-, J-, NO~, CIO~, CIO~, BrO~ und IOn) wurden b e s t i m m t durch M e s s u n g der L ~ s u n g s w M r m e dieser
V e r b i n d u n g e n in HCI oder HCIO 4. M i t t e l s der D i f f e r e n z e n zwischen den L ~ s u n g w ~ r m e n d e r - h y d r a t i s i e r t e n und
d e h y d r a t i s i e r t e n V e r b i n d u n g e n wurden die D e h y d r a t a t i o n s - e n t h a l p i e n festgestellt. Diese nehmen zu mit steigender H y d r a t a t i o n s e n t h a l p i e der i n d i v i d u e l l e n Anionen X in w ~ s s e r i g e r L6sung; aber die R e l a t i o n ist v e r s c h i e d e n
fur H a l o g e n i o n e n und f~r Oxy-anionen.
2 P a r t of the Ph.D. Thesis by C.J.M. Houtepen, E i n d h o v e n 1975, and p r e s e n t e d as a s u p p l e m e n t a r y paper at the 6~h International Congress on the C h e m i s t r y of Cement,
M o s c o w 1974.
652 Vol. 6, No. 5 C. J. M. Houtepen, H. N. Stein I n t r o d u c t i o n A n u m b e r of d a t a c o n c e r n i n g the e n t h a l p i e s of f o r m a t i o n of c a l c i u m a l u m i n a t e s and s i l i c a t e s h a v e b e e n p u b l i s h e d (I-13). T h e s e v a l u e s are b a s e d p a r t l y on c a l c u l a t e d , p a r t l y on e x p e r i m e n t a l l y d e t e r m i n e d q u a n t i t i e s . T h u s B a b u s h k i n and M c h e d l o v - P e t r o s y a n (I) c a l c u l a t e d the e n t h a l p y of f o r m a t i o n of C 4 A H 1 3 (-1983 kcal) m a k i n g use of the c o n t r i b u t i o n s to this q u a n t i £ y f r o m Ca(OH) 2, AI(OH) 3 and the lattice water. The c o n t r i b u t i o n s f r o m Ca(OH) 2 and AI(OR) 3 to the e n t h a l p y of f o r m a t i o n w e r e c a l c u l a t e d f r o m t h a t of C 3 A H 6
(-1317.6 kcal), on the s u p p o s i t i o n t h a t t h i s c o m p o u n d c o n s i s t s of 3Ca(OH) 2 . 2 A I ( O H ) 3 , and f r o m the e n t h a l p y of f o r m a t i o n of C 2 A H 5
( 2 C a ( O H ) 2 . 2 A I ( O H ) 3 ) , -1078 k c a l (11). S u c h c a l c u l a t i o n s m a y be j u s t i f i e d in some cases, b u t in o t h e r s t h e y lead to l a r g e errors. T h u s the e n t h a l p y of f o r m a t i o n of C 3 A H 6 c a l c u l a t e d f r o m t h a t of C 3 A (8) u s i n g an a p p r o x i m a t e g e n e r a l r e l a t i o n s h i p for the e n t h a l p y of h y d r a t i o n of a n h y d r o u s c o m p o u n d s (14, 15) is - 1 2 8 3 . 5 kcal, but the e x p e r i m e n t a l v a l u e is -1326 k c a l (8). C l e a r l y t h e r e is a n e e d for e x p e r i m e n t a l data. The A F m c o m p o u n d s of g e n e r a l f o r m u l a C a 2 A I ( O H ) 6 X . x H 2 0 have a l m o s t i d e n t i c a l c r y s t a l s t r u c t u r e s , w h i c h are e s s e n t i a l l y u n a f f e c t e d by r e m o v a l of the i n t e r l a y e r w a t e r (16-18). D e h y d r a t i o n c a u s e s m a i n l y a d e c r e a s e of the l a y e r t h i c k n e s s c and is r e v e r s i b l e as l o n g as the d e h y d r a t i o n c o n d i t i o n s are n o t too severe. The q u e s t i o n is w h e t h e r any r e l a t i o n s h i p e x i s t s b e t w e e n the e n t h a l p i e s of d e h y d r a t i o n of t h e s e c o m p o u n d s and the e n t h a l p i e s of h y d r a t i o n of the i n d i v i d u a l a n i o n s X. For the c l a y m i n e r a l s s u c h as
m o n t m o r i l l o n i t e and v e r m i c u l i t e , the c a t i o n s l o c a t e d in the
i n t e r l a y e r s h a v e an i m p o r t a n t e f f e c t on the a b i l i t y to a c c o m o d a t e i n t e r l a y e r w a t e r (19-21); the e n t h a l p i e s of d e h y d r a t i o n of t h e s e c l a y m i n e r a l s d e p e n d on the e n t h a l p i e s of h y d r a t i o n of the c a t i o n s in a q u e o u s solution. In the A F m p h a s e s a n i o n s m i g h t p l a y a s i m i l a r role. P r e p a r a t i o n and C h a r a c t e r i z a t i o n S a m p l e s w e r e p r e p a r e d by the f o l l o w i n g m e t h o d s : a. To a s u s p e n s i o n of C 3 A (2.7 g) in 2 - p r o p a n o l (50 ml), p r e p a r e d as d e s c r i b e d by De J o n g ~22), was a d d e d a s o l u t i o n of 50 to 100 m m o l e s C a X 2 . z H 2 0 in C 0 2 - f r e e w a t e r (500 ml). The 2 - p r o p a n o l was u s e d to p r e v e n t a too r a p i d h y d r a t i o n of C3A. The r e a c t i o n m i x t u r e w a s m a g n e t i c a l l y s t i r r e d for at l e a s t five d a y s at r o o m t e m p e r a t u r e T h e p r e c i p i t a t e was f i l t e r e d off, w a s h e d w i t h a m e t h a n o l - w a t e r m i x t u r e f o l l o w e d by a m e t h a n o l - a c e t o n e m i x t u r e , and d r i e d o v e r
s a t u r a t e d C a C I 2 . 6 H 2 0 (37% r e l a t i v e humidity) in a v a c u u m d e s i c c a t o r b. C 3 A H 6 w a s ~ r e p a r e d by h e a t i n g C 3 A (2.7 g) in w a t e r (250 ml) at
175oc in an a u t o c l a v e for 120 hours, as d e s c r i b e d by T h o r v a l d s o n (2).
c. "C4AH13" was p r e p a r e d at 0°C by a d d i n g a s o l u t i o n of s o d i u m a l u m i n a t e to a s a t u r a t e d s o l u t i o n of c a l c i u m h y d r o x i d e in s u c h a w a y t h a t the C a / A 1 r a t i o in the s o l u t i o n w a s a l w a y s g r e a t e r t h a n
two. This m e t h o d is a n a l o g o u s to that e m p l o y e d by B u s e r (23). d. S a m p l e s of C a 2 A I ( O H ) 6 N O 3 . x H 2 0 w e r e p r e p a r e d by a m e t h o d
a n a l o g o u s to m e t h o d a, but a b o u t 175°C in an a u t o c l a v e , r e s u l t i n g in l a r g e r c r y s t a l s .
e. The i o d a t e c o m p o u n d was p r e p a r e d as d e s c r i b e d by Le Bel and G r a s l a n d (18).
Vol. 6, No. 5 653 ENTHALPY, FORMATION, HYDRATION, AFM PHASES
In all p r e p a r a t i o n s p r e c a u t i o n s w e r e taken a g a i n s t
c a r b o n a t i o n . The w a t e r u s e d was d i s t i l l e d twice at r e d u c e d p r e s s u r e F i l t r a t i o n was p e r f o r m e d in a C O 2 - f r e e g l o v e box. All s a m p l e s w e r e a n a l y s e d c h e m i c a l l y . Table I gives the c o m p o s i t i o n s of the s a m p l e s and their d e h y d r a t i o n products; d e s p i t e the p r e c a u t i o n s t a k e n some c a r b o n a t i o n u s u a l l y o c c u r r e d . The s a m p l e s and d e h y d r a t i o n p r o d u c t s w e r e also c h a r a c t e r i z e d by X - r a y d i f f r a c t i o n , care b e i n g taken to e x c l u d e CO 2 and H20 by c o v e r i n g the samples w i t h "Mylar" foil. For the h y d r a t e d p h a s e s no d i f f e r e n c e s w e r e found b e t w e e n X - r a y
d i f f r a c t i o n p a t t e r n s m a d e w i t h and w i t h o u t such a foil, e x c e p t for C4AH13. The h e m i c a r b o n a t e p h a s e w i t h layer t h i c k n e s s 8.2 ~ was absent, but the 7.9 ~ r e f l e c t i o n had a s h o u l d e r on the l o w - a n g l e side and c h e m i c a l a n a l y s i s (Table I) s h o w e d that CO 2 was present. In g e n e r a l the layer t h i c k n e s s e s found for the A F m p h a s e s and their d e h y d r a t i o n p r o d u c t s a g r e e d w i t h those in the l i t e r a t u r e (17, 18). R e s u l t s of i n t e r e s t w e r e for the h y d r a t e d c h l o r a t e compound,
9.30 ~; for the d e h y d r a t e d nitrate, 7.28 ~; and for the
d e h y d r a t e d bromate, 8.36 ~. All m o l a r q u a n t i t i e s used in this p a p e r r e l a t e to f o r m u l a e of the type Ca2AI(OH) 6X.xH20.
Heat of S o l u t i o n M e a s u r e m e n t s
The heats of s o l u t i o n of the A F m p h a s e s w e r e d e t e r m i n e d in HC1 (IN or 2N) or in p e r c h l o r i c acid (1N) at 25oc w i t h a p r e c i s i o n c a l o r i m e t e r , LKB 8700-I (24). The heats of s o l u t i o n were
c a l c u l a t e d using the g r a p h i c a l e x t r a p o l a t i o n m e t h o d of D i c k i n s o n (25, 26). The c a l o r i m e t e r was c a l i b r a t e d e l e c t r i c a l l y and
c h e m i c a l l y by m e a s u r i n g the heat of s o l u t i o n of CaO, p r e p a r e d by b u r n i n g CaCO 3 (Merck p.a.) at 1100°C for s e v e r a l hours. AHsoln" was found to be -46.7 + 0.3 kcal m o l e -I w h i c h i m p l i e s ~H~ CaO, c = - 1 5 1 . 9 ~ 0 . 3 kcal mole-T, in a g r e e m e n t w i t h l i t e r a t u r e d a t a (10). For a c c u r a t e d e t e r m i n a t i o n s the heat of d i l u t i o n c a u s e d by the d i s a p p e a r e n c e of the acid and the f o r m a t i o n of w a t e r on d i s s o l u t i o n of the s a m p l e s m u s t be t a k e n into account. It can, however, be p r o v e d that, if the a m o u n t of s a m p l e (0.2 mmole) is small c o m p a r e d w i t h the a m o u n t s of acid (80 mmoles) and of w a t e r (4.5 mole), the h e a t of d i l u t i o n is n e g l i g i b l e . The e n t h a l p y of d e h y d r a t i o n i.e. the c h a n g e of e n t h a l p y i n v o l v e d in the h y p o t h e t i c a l reaction:
C a 2 A I ( O H ) 6 X . x H 2 0 ( c ) + Ca2AI(OH) 6X(c) + xH20(1) (I) was d e t e r m i n e d as the d i f f e r e n c e b e t w e e n the heats of s o l u t i o n of the d e h y d r a t e d and the h y d r a t e d c o m p o u n d s . This p r o c e d u r e is j u s t i f i e d b e c a u s e , if e q u i m o l a r a m o u n t s of the h y d r a t e d c o m p o u n d s are d i s s o l v e d , the final states of the r e s u l t i n g s o l u t i o n s are n e a r l y the same.
E n t h a l p i e s of F o r m a t i o n
The e n t h a l p i e s of f o r m a t i o n of the A F m p h a s e s w e r e d e t e r m i n e d f r o m h e a t s of s o l u t i o n in HCI u s i n g the f o l l o w i n g r e a c t i o n scheme:
Ca2AI(OH) 6. yX. 0 . 5 ( I - y ) C O 3. x H 2 0 + n I H C I + n 2 H 2 0 2Ca2+ + A 1 3 + + yX- + (7-y)Cl- + 0 . 5 ( 1 - y ) C O 2 ( a q) + n 3 ( = n I - 7 + y ) H C l + n 4 ( = n 2 + 6.5 - 0.5y + x ) H 2 0
654 Vol. 6, No. 5 C. J. M. Houtepen, H, N. Stein
The enthalpy of formation is then:
&H~ Ca2AI(OH) 6. yX. 0.5(I-y)CO 3. xH20 : 2 ~H~eaCl2, n 4 / 2 H 2 ~ + &H~[AICI3, n4H20 , n3HCI ] + y & H ~ X, n4/YH20 ] - y A H ~ HCI, n 4 / Y H 2 ~
+ 0.5(1-y)&H~CO2(a q) - n2HI(H20) - nIHI(HCI) + n4H2(H20)
+ n3H2(HCI ) - AHsoln" (2)
&H~[CaCIg, n4/2H?O L is the enthalpy of formation of one mole CaCI~ in'n4/2 ~ole=H20~ HI(H20) and H2(H~O) are the partial molar
enthalpies of water in solutions o5 one mole H20 with nl/n 2 mole HCI or n3/n 4 HCl,respectively. HI(HCI) and H2(HCl) are the partial molar enthalpies of HCl in solutions of one mole HCI with n2/n I
and n4/n 3 mole H20 respectively. The enthalpy of formation of AICI 3 in aqueous solution depends strongly on the water and HCI concentrations (12). This enthalpy was found by interpolation of literature data (12) &H~[AICI~, 200HCI, 11000H20 ] = -245.7 kcal mole-1. The other data~were {aken from literature (8,12,13). In
the calculations, thermochemical data for CO 2 dissolved in water have been used (12). This may be questionable in view of the small solubility of CO 2 in IN HCI. However, the difference between the enthalpies of formation of CO 2 in aqueous solution and in the gas phase is too small to have any noticeable influence on the
calculations.
If the heat of solution in perchloric acid was measured, the enthalpy of formation was established as follows:
AH~ Ca2AI(OH) 6. yX. 0.5(I-y)C03. xH20 = -AHsoln" + 2 A H ~ C a 2+, n 4 / 2 H 2 ~ + AH~ A13+, n4H20 ] + yAH~[X, n4/YH2~ +
0.5(1_y)AH~CO2(aq ) + (n4-n2)AH~[H20, n3/n4HCl04] (3)
AH~[Ca 2+, n4/2H20 ] = AH~[ CaCI2, n4/2H20 ] - 2AH~[ HCI, n4/4H20 ] (4)
A H ~ A13+, n4H20 ] = AH~[AICI3, n4H20 ]- 3AH~[ HCI, n4/3H20 ] (5) Discussion
A. The Heats of Solution and the Enthalpies of Formation
The heats of solution of the Ca2Al(OH) 6X±xH20_com~ound ~ are nearly independent of X, if x = 2 and X = C1 , Br , I , NO , CIO-
of solution or C103(Table I). If x > 2 (X = BROW, IOn) the heats 3 4 decrease with increasing amount of interlayer water. With
increasing carbonation the heat of solution increases, since the heat of solution of Ca AI(OH) 6.0.5CO3.2.39H20 is -63.7 kcal (5) 2 which is relatively large. The heat-of solution of the iodide compound containing 0.03 mole % CO 2 differs about I kcal from that of the almost C02-free compound (Table I). The heat of solution of Ca2AI(OH) 6.0.92OH, 0.04CO3.3H20 (-76.9 kcal) differs
considerably from those of the other AFm phases. The heat of solution of pure Ca2AI(OH) 6OH.3H20 (-78 kcal) was established by
Vol. 6, No. 5 655
ENTHALPY, FORMATION, HYDRATION, AFM PHASES
e x t r a p o l a t i o n , a s s u m i n g a l i n e a r r e l a t i o n b e t w e e n the heats of s o l u t i o n of C a 2 A I ( O H ) 6 . 0.920H. 0 . 0 4 C O 3. 3H20 and of
Ca2AI(OH) 6. 0 5CO 3 2-39 H 0 The h e a t s of s o l u t i o n of the
• • • 2 •
d e h y d r a t i o n p r o d u c t s can be c o r r e l a t e d w i t h the i n t e r a c t i o n e n e r g i e s b e t w e e n the a n i o n s X and the Ca2Al(OH) 6 - 1 a y e r s (27). We can c o n c l u d e from e q u a t i o n 2 that, if_x = 2, the e n t h a l p i e s of Zormation of C a ~ A I ( O H ) 6 . X . x H ~ O (X = C1 , Br , J , NO3, CIO 4 and CIO 3) can be ~ x p r e s s e d by The equation:
&H~ C a 2 A I ( O H ) 6 X . 2 H 2 0 = - 8 7 4 ( + I ) + ~Hf[X, I 0 . 0 0 0 H 2 0 ] (6) This m e a n s that the l a t t i c e e n t h a l p i e s of these A F m p h a s e s d e p e n d l i n e a r l y on the e n t h a l p i e s of h y d r a t i o n of the a n i o n s X concerned. For: A H l a t t i c e = ~H~[ Ca2AI(OH) +6, g] + AHf~ X, g] + 2 & H ~ [ H 2 0 , g ]
- AH~ Ca2AI(OH) 6 x . 2 H 2 0 (7)
The C a ~ A l ( O H ) ~ - i o n in the gas p h a s e is an h y p o t h e t i c a l ion. F r o m e q u a t i o n 6 and 7 follows:
= C I + ~H~(X-, g) - &H~(X-, aq) = C I - &Hhydr. (X- , g) AHlat.
(8)
since ~Hhydr.(X-, g) = AH~(X-, aq) - & H ~ (X-, g) (9) The e n t h a l p y of f o r m a t i o n of pure C4AH13 , c a l c u l a t e d f r o m the h e a t of solution, -156 kcal m o l e -I and the e n t h a l p i e s of f o r m a t i o n of the r e a c t a n t s is -1988 kcal. B a b u s h k i n c a l c u l a t e d -1983 kcal; the a g r e e m e n t is in this case very s a t i s f a c t o r y • But if we
c a l c u l a t e the e n t h a l p y of f o r m a t i o n of e.g. C a 2 A I ( O H ) 6 C I . 2 H 2 0 by the m e t h o d of B a b u s h k i n , s u p p o s i n g that the h y d r a t e c o n s i s t s of 1.5 Ca(OH)2, A I ( O H ) ~ and 0.5 C a C I 2 . 4 H 2 0 we get -899 kcal. The e x p e r i m e n t a l v a l u e zs -914 kcal. This d i f f e r e n c e o c c u r s b e c a u s e the s u r r o u n d i n g s of the ions in the A F m p h a s e s d i f f e r from those in Ca(OH)2, AI(OH) 3 and C a C l 2 . 4 H 2 0 , to e x t e n t s that a p p a r e n t l y c a n n o t be n e g l e c t e d .
B. The E n t h a l p i e s of D e h y d r a t i o n
In Fig.1 the e n t h a l p i e s of d e h y d r a t i o n of the A F m phases are g i v e n t o g e t h e r w i t h the a b s o l u t e e n t h a l p i e s of h y d r a t i o n of the anions X. For the h a l i d e ions, the h y d r a t i o n e n t h a l p i e s w e r e c a l c u l a t e d from the e n t h a l p i e s of f o r m a t i o n in the gas state and the "absolute" e n t h a l p i e s of f o r m a t i o n in a q u e o u s solution. The "absolute" e n t h a l p i e s of f o r m a t i o n are r e l a t e d to the c o n v e n t i o n a l e n t h a l p i e s of f o r m a t i o n (tabulated in the literature) by the
e q u a t i o n :
AH~(X, aq)abs. = AH~(X, aq)conv. - ~H~(H ~, aq)abs. (10) °(H+, aq)abs = 95.6 kcal m o l e - 1 ( r e f . 30). w h e r e &Hf
The e n t h a l p i e s of d e h y d r a t i o n of the A F m p h a s e s are l i n e a r l y r e l a t e d to the e n t h a l p i e s of h y d r a t i o n of the a n i o n s X (Fig.l).
For the o x y - a n i o n s , the h y d r a t i o n e n t h a l p i e s are not k n o w n p r e c i s e l y , b e c a u s e the e n t h a l p i e s of f o r m a t i o n of the g a s e o u s
656 Vol. 6, No. 5 C. J. M. Houtepen, H. N. Stein
TABLE I
Analytical and thermochemical results
Chemical composition prep.
method Ca2AI(OH) 0.920H. 0 04CO.. 3.0H~O c Ca~AI(OH) 6[ 0.96N03. G.02C~ 3. 2.0~20 d
Ca~AI(OH)~. 0.96N03. 0.02C03. f
Ca~AI(OH) 6. 0.98CI04. 0.01C03. 2.1H20 a Ca2AI(OH) e.. 0.98CIO 4. 0.01C03 f Ca2A1 (OH)
6
6 [ I. 00el. 2. I H20
a
Ca2AI (OH) I. 00CI f
Ca2AI(OH) 6. 0.96Br. 0.02C03. 2.1H20 a Ca2AI(OH) 6. 0.96Br. 0.02CO 3 f Ca2AI(OH) 6[6 0 . 9 4 I . 0.03C03. 2.1H20 a Ca2AI ( O H ) 0.941. 0.03CO 3 f Ca2AI(OH) ~. 1 . 0 0 I . 2.0H20 a Ca2AI(OH)~. 1.00I f
Ca2AI(OH) 6. 0.98CLO 3. 0.01CO 3. 2.1H20 a Ca2AI(OH) 6. 0 98CIO 3 0.01CO 3 f Ca2AI(OH) 6- 0.98BrO 3. 0.01C03. 2.8H20 a Ca2AI(OH) 6- 0.98BrO 3. 0.01CO 3 f
Ca2AI(OH) 6- 1.00IO 3. 4.1H20 e
Ca2AI(OH) 6" 1.00IO 3 f
C3AH 6 b
mmoles 7&HHoln. -AH~
Kcaz kcal 0.3924 76.9 993 0.4238 61.0 992 0.3982 67.7 0.4416 60.9 910 0.3841 66.2 0.4092 62.0 917 0.4171 72.0 0.4074 60.9 913 0.4084 70.5 0.3585 61.6 894 0.3801 69.2 0.4038 60.5 887 0.4131 68.31 0.4139
60.91
904
0.4113 68.71 0.391658.61
950
0.3909
70.01
0.3395
53.41
1074
0.3389
72.3
0.2311 139.1 1326 XFor m e a s u r i n g the heats of solution of A F m phases with oxy-anionsincorporated, HCIO 4 (IN) was used. f = d e h y d r a t i o n product.
I I I ! I 20
is
cc
BFo...po
20
Z /
s[
o % 0 g/UctoZ
, = = i i i / 60 70 80 90 100-AHhydr" (keaL mole -1)
FIG. 1 TABLE II X AHhydr" -AHhydr" IO 3 1 8.9 97.6 BrO 3 11.4 79.3 CI-- 10.0 76.7 CIO 3 7.8 72.9 Br- 9.6 68.7 NO 3 _ 6.7 67.1 C104 5.3 65.9 I- 7.8 61.8
The enthalpies of dehydration of some compounds of general
formule C a 2 A I ( O H ) 6 X . x H 2 0 plotted against the "absolute" hydration enthalpiesQf the individual anions X in aqueous solution.
Vol. 6, No. 5 657
ENTHALPY, FORMATION, HYDRATION, AFM PHASES
ions cannot be determined experimentally. The most reliable way of e s t i m a t i n g them seems to be that first p r o p o s e d by Morris (28), w h i c h is based on the hypotheses that the s a l t i n g - o u t of iyophilic colloids by e l e c t r o l y t e s is d e t e r m i n e d by the h y d r a t i o n enthalpies of the ions present (29). The salting-out properties of an anion, expressed as a "lyotropic number", can be m e a s u r e d for both
halides and oxy-anions; assuming that the same relation between enthalpy of hydration and lyotropic number applies for halides and oxy-anions, the hydration enthalpies of the oxy-anions are known. Hydration enthalpies thus obtained are shown in Fig.1. As for the halides a linear r e l a t i o n s h i p is found, but the points for the oxy-anions do not lie in the same curve as those for the halides. This is ascribed to d i f f e r e n c e s in the sizes and
chemical c h a r a c t e r i s t i c s of the two kinds of ion. A c k n o w l e d g e
The authors wish to thank Mr. C.L.M. Holten and Mr. F.E.A.M.B. L e m m e r l i n g for their contributions to this investigation.
References
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