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THE HYDRATION OF THE AMMONIUM ION IN AQUEOUS AMMONIUM SULPHATE SOLUTION AS
STUDIED BY NEUTRON DIFFRACTION
S. Cummings
To cite this version:
S. Cummings. THE HYDRATION OF THE AMMONIUM ION IN AQUEOUS AMMONIUM SUL-
PHATE SOLUTION AS STUDIED BY NEUTRON DIFFRACTION. Journal de Physique Colloques,
1984, 45 (C7), pp.C7-131-C7-135. �10.1051/jphyscol:1984713�. �jpa-00224274�
Colloque C7, s u p p l i m e n t a u n09, Tome B5, s e p t e m b r e 1984 page C7-131
T H E H Y D R A T I O N OF T H E A M M O N I U M ION IN A Q U E O U S A M M O N I U M S U L P H A T E S O L U T I O N A S S T U D I E D B Y N E U T R O N D I F F R A C T I O N
S . Cummings
H . H . W i 22s Physics Laboratory, Royal Fort, TyndalZ Avenue, BristoZ BS8 ITL, U.K.
Rdsumd - L ' h y d r a t a t i o n d e l ' i o n ammonium, d a n s une s o l u t i o n a q u e u s e c o n c e n t r d e ( 4 , 9 1 m o l a l ) d e s u l p h a t e d'ammonium, a d t 6 d t u d i 6 e p a r l a t e c h n i q u e de s u b s t i - t u t i o n i s o t o p i q u e . P l u s s p G c i f i q u e m e n t , l a s e c t i o n e f f i c a c e de d i f f u s i o n de l ' a z o t e d e l ' i o n ammonium e s t v a r i 6 e p a r e n r i c h i s s e m e n t i s o t o p i q u e , e t l e s s p e c t r e s de d i f f r a c t i o n a s s o c i g s s o n t mesurds. En f a i s a n t l a d i f f g r e n c e de deux t e l s s p e c t r e s , on p e u t a v o i r une idEe d e " l ' a t m o s p h ~ r e " a u t o u r d e l'atorne d ' a z o t e e t a i n s i de l ' h y d r a t a t i o n d e l ' i o n ammonium. Les r d s u l t a t s d ' u n e t e l l e e x p 6 r i e n c e s o n t p r 6 s e n t d s e t d i s c u t g s .
A b s t r a c t - The h y d r a t i o n o f t h e ammonium i o n i n a c o n c e n t r a t e d ( 4 . 9 1 m o l a l ) aqueous s o l u t i o n of ammonium s u l p h a t e was s t u d i e d by t h e t e c h n i q u e of i s o t o p i c s u b s t i t u t i o n . S p e c i f i c a l l y , t h e s c a t t e r i n g c r o s s s e c t i o n of n i t r o g e n w i t h i n t h e ammonium i o n is v a r i e d by i s o t o p i c e n r i c h m e n t , and t h e a s s o c i a t e d d i f f r a c t i o n p a t t e r n measured. By t a k i n g t h e d i f f e r e n c e of two s u c h d i f f r a c t i o n p a t t e r n s one i s a b l e t o i n f e r t h e ' a t m o s p h e r e ' around t h e n i t r o g e n atom and hence t h e h y d r a t i o n o f t h e ammonium i o n . The r e s u l t s of s u c h an e x p e r i m e n t a r e p r e s e n t e d and d i s c u s s e d .
HYDRATION OF AMMONIUM IONS IN AMMONIUM SULPHATE SOLUTION I n t r o d u c t i o n
The K-dependent [K
=(2r/X) s i n 8 where 28 i s t h e s c a t t e r i n g a n g l e ] c o h e r e n t s c a t t e r i n g from a s o l u t i o n of (ND4)2
SO4i n D20 i s g i v e n by t h e e x p r e s s i o n
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984713
C7-132 JOURNAL DE PHYSIQUE
Here, a i j ( ~ ) a r e t h e p a r t i a l s t r u c t u r e f a c t o r s of atom-types
' i 'and 'j' , Fi a r e s c a t t e r i n g l e n g t h s o f atom-types ' i ' and ' c ' i s t h e a t o m i c c o n c e n t r a t i o n of S i n t h e s o l u t i o n [I] .
A t f i r s t s i ~ h t , t h e e x p r e s s i o n seems t o b e t o o c o m p l i c a t e d t o be o f u s e . However, by m e a s u r i n g t h e s c a t t e r i n g from a n o t h e r s o l u t i o n i d e n t i c a l i n a l l r e s p e c t s e x c e p t t h e i s o t o p i c s t a t e a n d , h e n c e , t h e s c a t t e r i n q l e n g t h , o f one of t h e components ( s a y N), a major s i m p l i f i c a t i o n may b e made.
The d i f f e r e n c e of t h e s e F ( K ) ' s , t h e s o - c a l l e d f i r s t - o r d e r d i f f e r e n c e , i s g i v e n
I s o t o p e S c a t t e r i n g l e n g t h Enrichment S c a t t e r i n g l e n g t h of N
(10-12cm.
)-12
i n s o l u t i o n (10 cm.)
I 1
n a t
T a b l e 1 : I s o t o p i c composition and s c a t t e r i n g p a r a m e t e r s f o r
(ND4)2S04
-
I s o t o p e S c a t t e r i n g l e n g t h Enrichment S c a t t e r i n g l e n g t h of N
0.30 1
0.645
.644 99.7 %
1
T a b l e 2 : I s o t o p i c composition and s c a t t e r i n g p a r a m e t e r s f o r ( 1 5 ~ ~
)90
- 4 2' 4 1
T h i s d i f f e r e n c e i s g i v e n i n t e r m s of o n l y f o u r p a r t i a l s t r u c t u r e f a c t o r s and t h e w a t e r - w a t e r t e r m s , which p r e v i o u s l y dominated t h e
s c a t t e r i n g p a t t e r n , h a v e dropped o u t of t h e e x p r e s s i o n .
A s t r a i g h t f o r w a r d F o u r i e r t r a n s f o r m o f t h i s d i f f e r e n c e w i l l t h e n y i e l d t h e
' s t r u c t u r e ' around t h e n i t r o g e n atom.
Two 4 . 9 1 m o l a l s o l u t i o n s o f ammonium s u l p h a t e were p r e p a r e d , one u s i n g n a t u r a l ammonium s u l p h a t e , and one i n which t h e n i t r o g e n had been e n r i c h e d ( t o 99.3%) i n 1 5 N .
Employing t h e t e c h n i q u e of i n f r a - r e d a b s o r p t i o n , t h e l i g h t w a t e r c o n t a m i n a t i o n of e a c h s o l u t i o n was d e t e r m i n e d . The l i g h t w a t e r c o n t a m i n a t i o n o f t h e f i r s t s o l u t i o n was found t o b e 0 . 4 % , and t h a t o f t h e s e c o n d s o l u t i o n was found t o b e 0.45%. T h i s s m a l l d i f f e r e n c e i n l i g h t w a t e r c o n t e n t l e a d s b o t h t o an i n c r e a s e i n t h e g e n e r a l l e v e l of i n c o h e r e n t s c a t t e r i n g and t o a c h a r a c t e r i s t i c t r e n d i n t h e d a t a a t h i g h K ( s e e f i g . 1 ) . The d a t a were c o r r e c t e d f o r a b s o r p t i o n and m u l t i p l e s c a t t e r i n g and t h e s t r u c t u r e f a c t o r s f o r t h e two s o l u t i o n s were o b t a i n e d a f t e r n o r m a l i s a t i o n w i t h a vanadium s t a n d a r d [2] .
The a l g e b r a i c d i f f e r e n c e of t h e s e s t r u c t u r e f a c t o r s was t h e n t a k e n T h i s f i r s t o r d e r d i f f e r e n c e , which h a s t h e e x p l i c i t form
& ( ~ 1 = 0. ooisF,,,,(~) I * 0.003 [ao,,, IN\-' )
was, a f t e r a p p l i c a t i o n of an e m p i r i c a l c o r r e c t i o n f o r t h e P l a c z e k e f f e c t ( x ) and f u r t h e r n o r m a l i s a t i o n u s i n g a 'sum-rule' i n t e g r a t i o n t e s t , t r a n s f o r m e d t o g i v e
-
S c a t t . l e n g t h of N S c a t t . l e n g t h of N
i n 2nd. s o l ' n A B
CD
'S
i n 1 s t . s o l ; n
(10-12m.) (10-'~Cm.
)(mbarn) (mbarn) (mbarn) (mbarn)
.006 .930 .645 1.50 3.00 .040 .070
L
Table 1: S c a t t e r i n g parameters f o r a 4.9 molal s o l u t i o n of (ND4)2 i n D20 D i s c u s s i o n
F i r s t of a l l t h e r e a r e two p o i n t s which s h o u l d be n o t e d ( i ) gDN(r) c o n t a i n s b o t h i n t e r - and i n t r a - m o l e c u l a r c o n t r i b u t i o n s , a n d ( i i ) A , B >> C,D ( t a b l e 2 ) s o t h a t t h e f i r s t o r d e r d i f f e r e n c e i s dominated by g,,,(r) and g o p ( r ) .
- GN(r) i s shown i n f i g . 2 . The f i r s t peak a t r = 1 . 0 1 2 0.02 2 was a s s i g n e d t o i n t r a m o l e c u l a r n i t r o g e n - d e u t e r i u m c o r r e l a t i o n s and i n d e e d a c o o r d i n a t i o n number of 4 . 0
-+ 0.5 seems t o c o n f i r m t h i s .
The broad f e a t u r e s c e n t r e d a t r
=3 . 3 4 0.05 2 was a s s i g n e d t o t h e n e a r e s t n e i g h b o u r w a t e r m o l e c u l e s on t h e b a s i s o f ( i i ) above. The G N ( r ) does n o t show any f u r t h e r c o r r e l a t i o n s between t h e i o n and w a t e r m o l e c u l e s l e a d i n g t o t h e
1 " )
S . Cummings, p r i v a t e communication.
C7-134 JOURNAL DE PHYSIQUE
b e l i e f t h a t t h e ND4+ i o n i s n o t w e l l h y d r a t e d t o t h e same e x t e n t a s t h e L i + i o n o r t h e ~ a 2 + i o n 4 i n t h e aqueous s o l u t i o n . 13
The r e s u l t s of t h i s e x p e r i m e n t compare f a v o u r a b l y w i t h t h o s e o f S z a z s and H e i n z i n g e r 1 5 1 o b t a i n e d by m o l e c u l a r dynamics s i m u l a t i o n which show t h a t t h e ammonium i s s u r r o u n d e d by 10-11 w a t e r molecule^. There i s , however, a l a r g e d i s c r e p a n c y a r i s i n g from t h e o v e r a l l s h a p e of GN(r), t h e MD r e s u l t b e i n g more w e l l - d e f i n e d t h a n t h a t d e r i v e d from e x p e r i m e n t . T h i s was t h o u g h t t o be b e c a u s e i n t h e s i m u l a t i o n e i t h e r t h e h y d r a t i n g s t r e n g t h of t h e N D ~ + i o n o r t h e s p h e r i c a l n a t u r e of i t s e f f e c t i v e p o t e n t i a l had been o v e r e s t i m a t e d .
F i g . 1: The f i r s t - o r d e r d i f f e r e n c e , A ~ k ) , f o r a
4 . 9molal s o l u t i o n
-.
