Acid-Base Reactions in Fused Salts

9 Acid-Base Reactions in Fused Salts FREDERICK R. DUKE

Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: January 1, 1965 | doi: 10.1021/ba-1965-0049.ch009

Texas

A&M

University,

College Station,

Tex.

A n o x i d e ion a c c e p t o r o r a c i d w i l l r e a c t w i t h c e r t a i n o x y a n i o n s in f u s e d salts t o p r o d u c e a n e w a c i d . The e q u i l i b r i a i n v o l v e d w i t h p y r o -sulfate and dichromate have been studied q u a n t i t a t i v e l y in f u s e d n i t r a t e s a n d , s i n c e n i t r o n i u m i o n is a v e r y s t r o n g a c i d , o t h e r o x y i o n s such a s b r o m a t e a n d i o d a t e m a y b e s t u d i e d i n f u s e d n i t r a t e s o l u t i o n s . The self - d i s s o c i a t i o n of n i t r a t e occurs t o a s l i g h t e x t e n t a n d has b e e n studied q u a n t i t a t i v e l y using an oxygen-oxide ion electrode. By mak ing potentiometrlc measurements on both the basic a n d acidic n i t r a t e s o l u t i o n s , o n e can calculate the self-dissociation constant. S t r o n g l y basic o x y a n i o n s , such as c a r b o n a t e , can t h e n be studied directly in r e g a r d to the o x i d e ion in equilibrium with the o x y a n i o n in fused nitrates.

fused s a l t c o n s i s t i n g , g e n e r a l l y , of a n a l k a l i c a t i o n a n d a n o x y a n i o n is a b l e self-ionize t h e a n i o n t o o x i d e i o n a n d a n a c i d i c s u b s t a n c e . m a y dissociate slightly to form S 0 a n d 0 3

phate to form P 0 ~ a n d 0~ . 2

3

- 2

to

F o r example, sulfate

, nitrate to form Ν(>2 a n d 0 " , a n d phos +

2

N i t r a t e is the o n l y o x y a n i o n studied t o a n y extent

t h u s f a r , o w i n g p r i n c i p a l l y t o t h e c o n v e n i e n t m e l t i n g p o i n t s of i t s a l k a l i s a l t s a n d t h e large t e m p e r a t u r e range of l i q u i d s t a b i l i t y . T h e a c i d - b a s e p r o p e r t i e s of fused a l k a l i n i t r a t e s were first n o t e d w h e n d i c h r o m a t e w a s a d d e d t o fused s o d i u m - p o t a s s i u m n i t r a t e e u t e c t i c ( i ) .

Gaseous nitrogen

d i o x i d e a n d o x y g e n were s l o w l y g i v e n off w i t h t h e c o n v e r s i o n of t h e d i c h r o m a t e t o chromate.

It was postulated that N 0 2 Cr0 7

2

+ NO3-

+

w a s f o r m e d as i n t e r m e d i a t e : N0 + + 2

2CrOr

2

T h e effect ( u p o n t h e o v e r a l l r a t e of c o n v e r s i o n of d i c h r o m a t e t o c h r o m a t e ) of c h a n g ing the chromate i o n concentration was studied.

T h e rate was inversely propor

t i o n a l t o t h e s q u a r e of t h e c h r o m a t e c o n c e n t r a t i o n , as w e l l as p r o p o r t i o n a l t o t h e dichromate concentration. rate, the n i t r y l i o n , N 0 2

+

S i n c e o x y g e n a n d n i t r o g e n d i o x i d e h a d n o effect o n t h e w a s p o s t u l a t e d as i n t e r m e d i a t e .

However, the equilib

r i u m c o n s t a n t for t h e r e a c t i o n c o u l d n o t be d e t e r m i n e d because t o o l i t t l e N 0 2 formed. 220

In Mechanisms of Inorganic Reactions; Kleinberg, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1965.

+

was

9.

DUKE

Acid-Base

Reactions

221

W h e n pyrosulfate was substituted for the diehromate, the e q u i l i b r i u m reaction became ( J ) : S2O7- + N O 3 - t=; N 0 + + 2

2SO4-

2

2

P y r o s u l f a t e is a s u f f i c i e n t l y s t r o n g a c i d so t h a t a reasonable percentage a c i d i t y a p p e a r s i n t h e f o r m of N 0

2

+

of

the

, a n d t h e o v e r a l l r e a c t i o n r a t e is n o l o n g e r i n

v e r s e l y p r o p o r t i o n a l t o t h e s q u a r e of t h e s u l f a t e i o n c o n c e n t r a t i o n .

It was assumed

t h a t i n b o t h t h e d i e h r o m a t e a n d p y r o s u l f a t e r e a c t i o n s , t h e final r e a c t i o n t o p r o d u c e nitrogen dioxide a n d oxygen was: N 0 + + N O 3 - -> 2 N 0


2

+

2

Ο

T h e oxygen atoms, i n solvated or other form, r a p i d l y combine to form molecular oxygen. T h e m e t h o d used for f o l l o w i n g t h e c o n v e r s i o n of d i e h r o m a t e t o c h r o m a t e o r p y r o s u l f a t e t o s u l f a t e i n v o l v e d m e a s u r i n g t h e t o t a l a c i d i t y , AT, time.

as a f u n c t i o n of

I n t h e p y r o s u l f a t e case, —άΑτ A

= [N0 +] + [S 0 - ], and — — at

T

2

2

7

[N0 +][SOr ] 2

2

T h e equilibrium equation, Κ

—

|p u J 2

e q u a t i o n s t o g i v e (J) : —άΑτ

dt

Z[N0

=

2

2

+

].

,

2

, was combined w i t h the two above

7

UKAT

(K + [sor ]) 2

kK

S i n c e s u l f a t e w a s a l w a y s i n large excess of AT, t h e e x p r e s s i o n ,

.

XV. ~r

_ , , occupies 2

L^5v^4

2

J

t h e p o s i t i o n of a first o r d e r r a t e c o n s t a n t i n a n y g i v e n r u n a n d is d e t e r m i n e d as a first o r d e r c o n s t a n t .

T h u s , n u m e r i c a l v a l u e s were o b t a i n e d for k , the p s e u d o c o n f

1 — „ , as a f u n c t i o n of [S04~ ]. P l o t s of 77 vs. [SO4 J Κ + [0U4 J k 1 gave a n o r d i n a t e i n t e r c e p t of 7 a n d a n a b s c i s s a i n t e r c e p t of —K. T h u s , the equilibk kK

stant, where k

=

2

9

r i u m c o n s t a n t for t h e r e a c t i o n b e t w e e n p y r o s u l f a t e a n d n i t r a t e w a s d e t e r m i n e d . A p p a r e n t l y , t h e s a m e r a t e d e t e r m i n i n g step is i n v o l v e d i n t h e d i e h r o m a t e r e a c t i o n , a n d t h e e q u i l i b r i u m c o n s t a n t c a n be c a l c u l a t e d for t h e d i e h r o m a t e case a l s o . e q u i l i b r i u m c o n s t a n t for t h e p y r o s u l f a t e r e a c t i o n is 50.8 21.8 X 10~ a t 2 7 5 ° C . (5). 3

a t 2 5 0 ° C . a n d 3.8 X 1 0 ~

12

X

The

10~~ a t 3 0 0 ° C , a n d 3

T h e c o n s t a n t for t h e d i e h r o m a t e r e a c t i o n is 8.5 X 1 0 ~

14

at 300°C.

T h e existence of t h e n i t r y l i o n i n t h e presence of a c i d i c substances t h a t n i t r a t e i o n i n fused a l k a l i n i t r a t e s m i g h t d i s s o c i a t e i n t o N 0

2

+

suggested

a n d O " i o n s (5). 2

T o d e t e r m i n e t h e e x t e n t of t h e d i s s o c i a t i o n , i t w a s necessary t o d e v e l o p a n elec t r o d e p o t e n t i o m e t r i c a l l y responsive t o e i t h e r N 0 for a r e v e r s i b l e N 0 over platinum.

2

+

o r O" .

+

2

2

T h e only possibility

electrode t h a t c a m e t o m i n d w a s n i t r o g e n d i o x i d e gas b u b b l i n g

T h i s electrode d i d r e s p o n d t o N 0

2

+

i n a c i d i c s o l u t i o n s , b u t as e x -


222

M E C H A N I S M S O F I N O R G A N I C REACTIONS

p e c t e d , t h e f o l l o w i n g n o n - e l e c t r o c h e m i c a l r e a c t i o n o c c u r r e d i n t h e presence of o x i d e ion: N0

2

+ O"

-s. N 0 - +

2

NO3-

2

T h u s , t h e u t i l i t y of the o x y g e n electrode w a s e x p l o r e d . T h e p o t e n t i a l of o x y g e n o v e r p l a t i n u m r e s p o n d e d n i c e l y t o changes i n o x y g e n pressure a c c o r d i n g t o t h e N e r n s t e x p r e s s i o n (5).

T h e reference electrode w a s s i l v e r

i m m e r s e d i n 0 1 i f s i l v e r n i t r a t e i n t h e fused a l k a l i n i t r a t e s , t h e m i x t u r e b e i n g c o n t a i n e d i n a t h i n glass e n v e l o p e .

T o test t h e effect of changes i n o x i d e i o n c o n c e n

t r a t i o n o n the p o t e n t i a l , a source of p u r e a l k a l i o x i d e w a s n e e d e d .

I t is a l s o u l t i -


KNO3

KNO3

x

m a t e l y necessary t o k n o w E° for t h e c e l l O2» P t :

} 0~

2

NaN0 0.1 Ai ; A g , i n o r d e r t o c a l c u l a t e t h e s e l f - d i s s o c i a t i o n c o n s t a n t s . 3

(

g

l

a

S

s

)

N a N 0

A

A

3

g

'

Since a l k a l i oxides

are extremely difficult to prepare i n the pure state a n d to handle, i t was decided to produce the oxide i o n coulometrically.

C o n s e q u e n t l y , a k n o w n c u r r e n t of a p p r o x i

m a t e l y 8 m i c r o a m p . w a s r u n t h r o u g h t h e c e l l , t h e o x y g e n e l e c t r o d e a c t i n g as t h e cathode.

T h e current was d r a w n from a n electronically controlled constant cur

r e n t d e v i c e a n d t i m e d so t h a t t h e n u m b e r of e q u i v a l e n t s of charge p a s s i n g c o u l d be calculated.

T h i s w a s a s s u m e d t o be t h e n u m b e r of o x i d e i o n s p r o d u c e d .

The

v a r i a t i o n i n potential w i t h oxide ion followed the N e r n s t expression very precisely. T o d e t e r m i n e t h e p o t e n t i a l of t h e electrode o n the a c i d side, p y r o s u l f a t e of k n o w n c o n c e n t r a t i o n w a s p l a c e d i n t h e fused s a l t .

K n o w i n g the e q u i l i b r i u m c o n

s t a n t t o p r o d u c e N 0 2 , i t w a s possible t o c a l c u l a t e the N C V " c o n c e n t r a t i o n c o r r e s +

p o n d i n g to the p o t e n t i a l measured.

T h e n the self-dissociation constant was c a l

c u l a t e d as f o l l o w s : ^

RT

P

m

02

i n c l u d i n g t h e reference e l e c t r o d e p o t e n t i a l i n t h e £ ° . O n t h e a c i d side, since KD = [ N 0 ] [ 0 ~ ] , t h e e q u a t i o n b e c o m e s 2

+

2

Po "*[N0 ]

RT

2

2

T h u s , KD m a y be c a l c u l a t e d as d e s c r i b e d a b o v e . 0.27 X 1 0 ~ a t 250° C . a n d 5.66 ± 0.1 X 1 0 ~ 26

24

+

KD w a s f o u n d t o be 2.74

a t 300° C .

±

T h e e n t h a l p y of d i s s o c i a

t i o n is 64 k c a l . p e r m o l e . T h e b a s i c i t i e s of B r 0 ~ , C10 "~ a n d I 0 ~ (6) were s t u d i e d n e x t . 3

determined that B r O e

-

3

3

I t was

first

a n d I O e " were c o n s i d e r a b l y m o r e b a s i c t h a n n i t r a t e i o n .

T h u s , these h a l a t e s were s t u d i e d i n fused a l k a l i n i t r a t e s o l u t i o n s .

T h e reactions

studied were: 2Ba+

2

+ XO3- + C r 0 2

7

2

P r o d u c t s ( B r , 0 ) 2

2


9.

DUKE

Acid-Base

Reactions

223

C 1 0 + + C I " ~* P r o d u c t s ( C l 2

+ C10

2

2

+

0 ) 2

I0 + + B r ~ -* Products (Br , IBr) 2

2

T h e b r o m y l i o n decomposes spontaneously.

C h l o r y l a n d i o d y l ions

studied through their a t t a c k on chloride a n d bromide ions respectively.

were

B y de

t e r m i n i n g the r a t e of d i s a p p e a r a n c e of d i e h r o m a t e a n d u s i n g a n a n a l y s i s s i m i l a r t o t h e p y r o s u l f a t e - n i t r a t e case, t h e f o l l o w i n g e q u i l i b r i u m c o n s t a n t s were f o u n d f o r t h e d i c h r o m a t e - h a l a t e r e a c t i o n s w h i c h p r o d u c e h a l y l a n d c h r o m a t e i o n s (not c a l c u l a t e d as B a C r 0 , b u t as [ C r 0 4

4

- 2

] i n solution) it r0 -

= 3.5 X 1 0 ~

8

^cio -

= 4.0 Χ Ι Ο "

11

Kio -

= 2.2 Χ Ι Ο "

9

Knot-

-


B

3

3

z

4.5 X 1 0 ~

N i t r a t e is included for comparison. since C 1 0

2

+

(2). M "

1

M~

l

M~

l

M~

l

u

T h e c h l o r a t e w a s r u n w i t h c h l o r a t e as s o l v e n t

h a s a b o u t t h e s a m e a c i d i t y as N 0

2

+

.

A l s o , note t h a t B r C ^ " is the

strongest base of t h e t h r e e i o n s . K u s t (4) h a s p o t e n t i o m e t r i c a l l y d e t e r m i n e d t h e e q u i l i b r i u m c o n s t a n t f o r c a r b o n a t e d i s s o c i a t i o n i n fused s o d i u m - p o t a s s i u m n i t r a t e e u t e c t i c : C0 - 2 a n d NOf.

T h e r e is some e v i

+

d e n c e t h a t s o m e of t h i s does e x i s t i n s o l u t i o n as m o l e c u l a r N2O5, a n d t h e n d e c o m poses t o 2NO2 a n d o x y g e n .

I t is f i r s t - o r d e r i n NO2 ", g i v i n g a n o x y g e n a t o m a t t h e 4

e n d , p r o b a b l y NO3, o r o x y g e n s o l v a t e d w i t h t h e n i t r a t e i o n o r s o m e t h i n g else.


a n y rate i t ends u p as oxygen molecules e v e n t u a l l y . tion anyway.

At

W e decided to study this reac

I t t u r n s o u t t h a t , e v e n i n t h e presence of a g o o d p r e c i p i t a n t for t h e

c h r o m a t e i o n , t h e r e a c t i o n w i l l n o t go f r o m left t o r i g h t s u f f i c i e n t l y t o s e p a r a t e t h e rate constant from the e q u i l i b r i u m constant. W e t h o u g h t of u s i n g n i t r y l p e r c h l o r a t e i n fused n i t r a t e s a n d m e a s u r i n g t h e a b s o l u t e r a t e a t w h i c h NO2 " d i d d e c o m p o s e i n c o m b i n a t i o n w i t h n i t r a t e i o n .

But

4

there are too m a n y problems connected getting and keeping it d r y .

w i t h the using n i t r y l

perchlorate—e.g.

W e d e c i d e d i t w o u l d be easier a n d m o r e

convenient

s i m p l y to a d d a stronger acid at this point to displace the e q u i l i b r i u m far enough to separate the e q u i l i b r i u m constant from the rate constant. o n l y s l i g h t l y f r o m left t o r i g h t .

T h e e q u i l i b r i u m goes

B y t h e n the subsequent reaction is a l w a y s s t r i c t l y

inverse second-order i n chromate.

If t h i s o c c u r s a p p r e c i a b l y t o w a r d t h e m i d d l e ,

s a y 10 o r 2 0 % of t h e t o t a l a c i d a p p e a r i n g i n t h e f o r m of N 0

2

+

, then the inverse

o r d e r i n t h i s i o n b e g i n s t o decrease, a n d c a n go a s f a r a s zero o r d e r i f t h e e q u i l i b r i u m goes f a r e n o u g h f r o m left t o r i g h t .

F r o m t h a t decrease i n a n i n v e r s e o r d e r i t i s

possible—just like M i c h a e l i s a n d M e n t o n d i d w i t h e n z y m e s — t o separate the rate and

equilibrium constants.

We

did

this simply

by

substituting sulfur

for

c h r o m i u m , a n d r a t h e r t h a n a d d i n g a p r e c i p i t a n t for t h e s a l t w e h a d t o a d d excess sulfate on the right to keep the reaction from going too far to the right a n d the d e c o m p o s i t i o n of t h e n i t r y l i o n f r o m g o i n g t o o f a s t . I t i s i n t e r e s t i n g t h a t i n b o t h t h e d i c h r o m a t e a n d t h e p y r o s u l f a t e case t h e d e c o m p o s i t i o n of t h e n i t r y l i o n i s i n d e p e n d e n t of t h e a c i d a d d e d t o create i t .

There

fore, h a v i n g t h e p r o d u c t of t h e r a t e a n d e q u i l i b r i u m c o n s t a n t s i n t h e case of t h e dichromate a n d the rate constants separated out from the pyrosulfate work, the r a t e c o n s t a n t w a s d i v i d e d i n t o t h e p r o d u c t t o g i v e t h e e q u i l i b r i u m c o n s t a n t for t h e dichromate.

T h i s c a n be d o n e for a w h o l e series of a c i d s r e g a r d l e s s of

their

s t r e n g t h s , w h e t h e r t h e y a r e s t r o n g e n o u g h so t h a t i t c a n be d o n e i n d e p e n d e n t l y o r not. T h e r e i s a g r o u p of a c i d s w h i c h a r e m e t a l i o n s — z i n c i o n p l u s n i t r a t e i o n , f o r example.

T h i s goes t o i n s o l u b l e z i n c o x i d e p l u s NO2" ". 1

One reason t h a t I d i d n ' t

m e n t i o n these a c i d s i s t h a t we are n o t sure t h a t i t i s t h e d e c o m p o s i t i o n of N 0 2 t h a t l e a d s t o NO2 a n d o x y g e n .

+

I t c o u l d be a d i r e c t d e c o m p o s i t i o n

of t h e z i n c

n i t r a t e c o m p l e x i o n , a n d w e f o u n d t h a t t h i s w a s t h e case w i t h b r o m a t e .

If bromate

i s a d d e d t o fused n i t r a t e s a l o n g w i t h z i n c i o n , i t i s n o t a n e q u i l i b r i u m i n v o l v i n g z i n c oxide a n d B r 0

2

+

w h i c h occurs, b u t Z n B r 0 3 is formed.

t o t h e p r o d u c t s , one of w h i c h i s z i n c o x i d e .

+

T h i s decomposes directly

H e n c e we really don't k n o w whether

these b e l o n g t o t h e same c a t e g o r y as d i c h r o m a t e i n t h e d e t a i l e d w a y t h a t one h a s t o a s s u m e t o get t h e e q u i l i b r i u m c o n s t a n t s e p a r a t e d f r o m t h e r a t e c o n s t a n t .



226

T h i s c o n s i d e r a t i o n l e d us t o d e t e r m i n e t h e e q u i l i b r i u m c o n s t a n t for t h i s r e a c t i o n ΝΟΓ

^

N0 + 2

+

Ο"

2

w h i c h i s a s e l f - d i s s o c i a t i o n of n i t r a t e i o n a n d l o o k s v e r y m u c h l i k e t h e s e l f - d i s s o c i a tion

of

water.

In

to measure either N 0

order 2

to

o r 0~~ .

+

2

determine

t h i s one

needs a

sensitive

method

Analagous to the w a y i t was done i n aqueous s o l u

t i o n we looked for a n electrode t h a t w o u l d respond t o either N 0 O n e c a n get a g o o d electrode for N 0

2

simply by bubbling N 0

+

2

2

+

ions or oxide ions.

over p l a t i n u m , but

i t c a n ' t be u s e d i n t h e presence of o x i d e i o n because there is a d i r e c t r e a c t i o n of N 0 w i t h oxide i o n to m a k e nitrate a n d nitrite.

T h a t excluded this method.

2

W e used


i t o n l y as a p o i n t of i n t e r e s t t o see i f i t w o u l d w o r k o n t h e a c i d s i d e , a n d i t does. N e x t we c o n s i d e r e d t h e o x i d e i o n e l e c t r o d e , a n d we were a l i t t l e f e a r f u l of t h i s one.

T h e r e h a d been s o m e w o r k d o n e i n s u l f a t e s , a n d s e v e r a l h u n d r e d degrees

w a r m e r t h a n t h i s , i n w h i c h t h e r e w a s s o m e q u e s t i o n as t o w h e t h e r o r n o t t h e o x y g e n electrode

was really acting reversibly on p l a t i n u m .

F u r t h e r m o r e , studies

by

Y e a g e r a n d o t h e r s h a v e s h o w n t h a t w h e n e v e r one uses o x y g e n i n a q u e o u s s o l u t i o n a t electrodes t h e r e i s a t e n d e n c y t o e q u i l i b r a t e w i t h p e r o x i d e r a t h e r t h a n w i t h o x i d e or hydroxide.

W e were a f r a i d t h a t w e m i g h t e n d u p w i t h s o m e o n e - e l e c t r o n t r a n s

fer r e a c t i o n s a n d get p e r o x i d e i o n s a n d t h e l i k e ; b u t we h a d t o t r y i t because t h e other electrode just w o u l d n ' t w o r k .

A s it turned out, i t worked very nicely.

W e h a d t o d e t e r m i n e £ ° f o r these cells i n o r d e r t o s w i t c h t o t h e a c i d side a n d i n v o l v e t h e e q u i l i b r i u m c o n s t a n t i n s u c h a w a y t h a t i t c o u l d be d e t e r m i n e d .

This

m e a n t a d d i n g a c a r e f u l l y k n o w n c o n c e n t r a t i o n of o x i d e i o n w h i c h i s b y n o m e a n s simple when working w i t h sodium or potassium. oxide.

I t is possible t o m a k e p u r e s o d i u m

I t m a y be possible t o m a k e p u r e p o t a s s i u m o x i d e .

B u t one a l w a y s l o o k s

for easier w a y s t o d o t h i n g s because these are d i f f i c u l t t h i n g s t o h a n d l e ; a n d we were w o r k i n g w i t h v e r y l o w - c o n c e n t r a t i o n s of o x i d e i o n .

O n e does n o t l i k e t o m e a s u r e

p H u p a r o u n d 14 o r 15, b u t r a t h e r a r o u n d 8, 9, o r 10, a n d we w a n t e d t o m e a s u r e a n oxide i o n electrochemically. o n t h e o r d e r of 1 0 i k f .

It was rather small i n molar

concentration—

F i r s t we s h o w e d t h a t as we c h a n g e d t h e pressure of o x y g e n

- 6

o v e r a buffered s o l u t i o n (buffered a t a n u n k n o w n o x i d e i o n v a l u e b y p u t t i n g i n o r t h o s i l i c a t e , s o d i u m o r t h o s i l i c a t e ) we d i d i n d e e d get t h e N e r n s t slope.

T h e n we

coulometrically added the oxide i o n from the oxygen electrode w i t h a constant c u r r e n t g e n e r a t o r a n d e a s i l y p u t i n s m a l l c o n c e n t r a t i o n s of o x i d e i o n . T h i s w o r k has been c r i t i c i z e d r a t h e r s e v e r e l y b y s a y i n g t h a t w e are j u s t d u m p i n g e l e c t r o n s i n t o t h e r e a c t i o n , a n d there h a d been a l o t of pressure o n us t o m a k e s o d i u m o x i d e a n d see i f i t r e a l l y gives t h e s a m e a n s w e r .

I d o t h i n k i t w o u l d be a

v e r y nice t h i n g to have done, b u t I really d o n ' t w a n t to do i t .

I have lost m y

e n t h u s i a s m for t h a t p o r t i o n of t h e p r o b l e m . T h e o t h e r p r o b l e m c o n n e c t e d w i t h t h i s e q u i l i b r i u m is t h e f a c t t h a t N 0

2

+

does

n o t r e m a i n as s u c h i n t h e r e a c t i o n m i x t u r e w h i l e one m a k e s m e a s u r e m e n t s ; i t decomposes fairly rapidly.

H e n c e , i t was necessary to k n o w the e q u i l i b r i u m c o n

s t a n t for t h e p y r o s u l f a t e p r e c i s e l y a n d t h e r a t e a t w h i c h i t d e c o m p o s e d p r e c i s e l y i n o r d e r t o k n o w , a t a n y p a r t i c u l a r t i m e a f t e r w e a d d p y r o s u l f a t e t o t h e fused n i t r a t e , exactly how much N 0

2

+

was there.

I t t u r n s o u t t h a t one c a n e l e c t r o c h e m i c a l l y

f o l l o w t h e r a t e of d e c o m p o s i t i o n of N 0 persists.

A s the N 0

2

+

2

+

p o t e n t i o m e t r i c a l l y since this e q u i l i b r i u m

d i s a p p e a r s one c a n f o l l o w i t a n d o b t a i n m u c h b e t t e r d a t a

t h a n w e were a b l e t o get w i t h c h e m i c a l a n a l y s e s .

T h e r e f o r e , t h e w h o l e j o b of d e -


9.

DUKE

227

Discussion

t e r m i n i n g t h i s e q u i l i b r i u m c o n s t a n t i s a n i n t e r n a l one.

W e redetermined the equili

b r i u m a n d r a t e c o n s t a n t s for t h i s r e a c t i o n s i m u l t a n e o u s l y . T h e e q u i l i b r i u m c o n s t a n t is a b o u t 2.74 X 1 0 ~ 10~~ a t 300°C. w i t h a large AH.

26

a t 250°C. a n d a b o u t 5.66

X

T i t r a t i o n s w i t h oxide ion w i t h strong acids i n this

24

s o l v e n t g i v e a large p o t e n t i a l c h a n g e , s o m e 26 p H u n i t s .

S i n c e w e were w o r k i n g

a t a h i g h e r t e m p e r a t u r e , RT/2F

I n some w o r k done i n

becomes a l i t t l e l a r g e r .

E g y p t , p e r o x i d e r a t h e r t h a n oxide i o n has b e e n u s e d t o d o t h e t i t r a t i o n s , a n d i t i s r e p o r t e d t h a t t h e p e r o x i d e i o n , as i t h i t s t h e a c i d , d e c o m p o s e s i n t o o x i d e i o n a n d oxygen.

W e h a v e l o o k e d a t t h i s a l i t t l e a n d are n o t sure t h a t one does get c o m


p l e t e d e c o m p o s i t i o n of t h e p e r o x i d e i o n .

Some decomposition occurs, c e r t a i n l y .

H a v i n g d e t e r m i n e d t h e e q u i l i b r i u m c o n s t a n t one s h o u l d be a b l e t o d i s s o l v e a n y p o s s i b l e source of o x i d e i o n — b r o m a t e , c h l o r a t e , s i l i c a t e , p h o s p h a t e , z i n c o x i d e — m a k e measurements, a n d tell how basic or acidic the solution is.

F o r e x a m p l e , we

b e l i e v e t h a t t r a n s f e r of o x i d e i o n f r o m b r o m a t e o c c u r s i n t h e s a m e w a y as n i t r a t e a n d one gets B r 0 oxidant.

+

2

.

This B r 0

2

+

, i f i n d e e d t h a t i s w h a t we d o get, is a n e x c e l l e n t

It reacts w i t h iodide, bromide a n d a n y t h i n g t h a t bromate n o r m a l l y

oxidizes i n acid solution. s u c h a case a s b r o m a t e .

I t m a k e s us b e l i e v e t h a t i t is u l t i m a t e l y t h e o x i d a n t i n U s i n g H B r 0 3 + , one h a s t o d i s p l a c e w a t e r f r o m i t w i t h a 2

r e d u c i n g a g e n t , for e x a m p l e b r o m i d e , for t h e r e a c t i o n t o p r o c e e d . T h e o r g a n i c c h e m i s t s h a v e been c l a i m i n g for y e a r s t h a t t h e y h a v e e x c e l l e n t proof that N 0

2

i s i n d e e d t h e e l e c t r o p h i l i c d i s p l a c e m e n t a g e n t t h a t causes n i t r a t i o n ,

+

particularly on aromatic compounds.

T o t e s t t h i s we a d d e d p y r o s u l f a t e t o fused

n i t r a t e a n d b u b b l e d s o m e benzene v a p o r s t h r o u g h i t o n n i t r o g e n o r a r g o n .

Nothing

b u t benzene c a m e o u t , i n s p i t e of t h e f a c t t h a t t h e r e a c t i o n m i x t u r e w a s h o t , 2 5 0 ° C , and contained N 0 d i d have a n y N 0

2

2

+

+

ions.

N o w t h e o r g a n i c c h e m i s t s are g o i n g t o s a y t h a t we n e v e r

there.

T o f o r e s t a l l t h a t a r g u m e n t we t h e n a d d e d w a t e r t o t h e

b e n z e n e a n d b u b b l e d t h e n i t r o g e n t h r o u g h b o t h t h e w a t e r a n d t h e benzene as i t w e n t i n t o t h e fused s a l t c o n t a i n i n g t h e N 0

2

+

.

A l o t of n i t r o b e n z e n e c a m e o v e r .

I

d o n ' t k n o w w h e t h e r t h i s m e a n s t h a t t h e n i t r a t i n g a g e n t is r e a l l y Η Ν Ο ^ o r w h e t h e r 2

one needs as g o o d a p r o t o n a c c e p t o r as w a t e r i n o r d e r t o d i s p l a c e t h e h y d r o g e n — whether it is catalytic, or stoichiometric, or just what.

W e d e c i d e d t o find o u t

u s i n g a h o m o g e n o u s m i x t u r e r a t h e r t h a n a t w o - p h a s e (or m o r e ) s y s t e m .

W e took

s o m e s o d i u m b e n z e n e s u l f o n a t e , w h i c h i s a g o o d s a l t a n d d i s s o l v e s i n fused n i t r a t e s , a n d a d d e d i t t o fused n i t r a t e s . the acid, the N 0

2

+

W e were a b o u t t o a d d some p y r o s u l f a t e t o generate 7

, when a metathetical reaction occurred, call i t a n acid-base inter

change, t o give nitrobenzene a n d sulfate ions. tion.

T h i s a p p e a r s t o be a r e v e r s i b l e r e a c

W e h a v e s t u d i e d t h e k i n e t i c s of t h i s r e a c t i o n a n d k n o w t h e o r d e r a n d s o m e

a c t i v a t i o n energies.

W e d o n ' t k n o w h o w v a l i d o u r figures are since we h a v e n e v e r

been a b l e t o get m o r e t h a n a 5 0 % y i e l d i n n i t r o b e n z e n e ; a n d we t h i n k t h a t e i t h e r the nitrobenzene oxidizes the unreacted sulfonic a c i d or itself.

T h i s seems t o r e a c t

a l i t t l e f a s t e r t h a n j u s t a d d i n g n i t r o b e n z e n e t o t h e fused n i t r a t e — i t s t a r t s t u r n i n g black after a while.

S i n c e w e u s e d a c o l o r o m e t r i c m e t h o d for f o l l o w i n g t h e r e a c

t i o n s , we are s k e p t i c a l of t h e d a t a .

W e t r i e d large a m o u n t s t o see i f we c o u l d m a k e

a p o u n d of n i t r o b e n z e n e t h i s w a y , b u t t h e y i e l d s decrease g r e a t l y w h e n w e t r y t o m a k e the sulfonic acid more concentrated. b e t t e r t h e y i e l d of n i t r o b e n z e n e . groups on the heterocycles.

B u t the more dilute we make i t , the

P e r h a p s t h i s w o u l d be a g o o d w a y t o p u t n i t r o

B u t , t h a t i s a l i t t l e t o o o r g a n i c for m o s t of u s .


228

M E C H A N I S M S O F I N O R G A N I C REACTIONS W e h a v e d o n e a few o x i d a t i o n - r e d u c t i o n r e a c t i o n s i n fused s a l t s , b u t t h e y

aren't terribly interesting.

T h e o n l y r e a c t i o n t h a t m i g h t be i n t e r e s t i n g i s a d d i n g

i o d i d e i o n t o c h l o r a t e i n a n e u t r a l t o a l k a l i n e s o l u t i o n of f u s e d n i t r a t e s t o give i o d a t e plus chloride.

T h i s r e a c t i o n i s n o t u n u s u a l ; i t h a s been u s e d for y e a r s i n a q u e o u s

solution to make iodate.

B u t we d i d s o m e i n t e r e s t i n g t h i n g s w i t h t h i s r e a c t i o n .

W e m a d e t h e h y p o i o d i t e a n d c h l o r i d e i n t e r m e d i a t e s t h a t w o u l d o c c u r i f one t r a n s ferred one o r t w o o x y g e n s a t a t i m e a n d n o t a l l t h r e e .

These intermediates react

w i t h n i t r a t e i o n t o give p r o d u c t s t h a t are r e a d i l y i d e n t i f i a b l e .

W e d i d n ' t get a n y 7

of these p r o d u c t s ; so we feel t h a t a W a l d e n i n v e r s i o n o p e r a t e s here.

T h e iodide

comes up a n d a l l the oxygens s w i t c h over to the iodine at the same time from the


chloride.

W e d o n ' t h a v e r e a l p r o o f of t h i s , because t h e r e a r e o t h e r w a y s t o e x p l a i n

the observations.

T h e s e i n t e r m e d i a t e s m a y be f a r m o r e r e a c t i v e w i t h t h e r e a g e n t s

t h a n w i t h t h e n i t r a t e , b u t a t least t h e r e is a l i t t l e e v i d e n c e t h a t a l l three o x y g e n s transfer at once. Dieter G r u e n :

P r o f . D u k e h a s g i v e n a n a d m i r a b l e d i s c u s s i o n of

properties in molten nitrates.

acid-base

I w o u l d l i k e t o suggest a m o d e l for t h e t r a n s i t i o n

state c o m p l e x i n t h e r e a c t i o n Y 0f

+

2

2

^

XO3-

X0 +

+

2

2YO4-

studied b y Prof. D u k e in molten alkali metal nitrates. C 1 0 ~ , B r 0 - o r IO3-; Y2O73

3

= Cr 0 -

2

2

7

2

Here X 0 "

=

3

NO3-,

or S 0 " .

2

2

7

2

T h e Y 0 f ~ ions c a n be r e p r e s e n t e d b y t w o o x y g e n t e t r a h e d r a s h a r i n g a c o r n e r 2

2

w i t h t h e c h r o m i u m o r s u l f u r a t o m s a t t h e centers of t h e t e t r a h e d r a . complex [ Y 0 Z X 0 ] 2

7

3

- 2

The activated

t h e n c o n s i s t s of a n a l k a l i m e t a l i o n , Z , i n t h e c e n t e r of a n

o c t a h e d r o n of o x y g e n s : three f r o m t h e Y 0 2

7

g r o u p a n d three f r o m t h e X O 3 -

- 2

group. T h i s m o d e l w a s suggested b y s p e c t r o s c o p i c m o l t e n a l u m i n u m c h l o r i d e (7).

s t u d i e s of d i p o s i t i v e 3d i o n s i n

T h e a b s o r p t i o n s p e c t r a of d i p o s i t i v e T i , V , C r ,

M n , F e , C o , N i a n d C u i n m o l t e n AICI3 c a n be i n t e r p r e t e d o n t h e basis of o c t a h e d r a l c o n f i g u r a t i o n s of c h l o r i d e s a b o u t t h e c e n t r a l t r a n s i t i o n m e t a l i o n s .

A n explanation

of t h i s f a c t i s m a d e p l a u s i b l e b y t h e f o l l o w i n g c o n s i d e r a t i o n s . I n t h e l i q u i d s t a t e , x - r a y d i f f r a c t i o n m e a s u r e m e n t s (3) h a v e s h o w n a l u m i n u m c h l o r i d e t o c o n s i s t of A I C l e d i m e r s .

T h e r e is c o n s i d e r a b l e e v i d e n c e t h a t a d d i t i o n

2

of C l ~ t o A l C l e is a s t e p w i s e p r o c e s s c h a r a c t e r i z e d b y t h e t w o e q u i l i b r i a 2

Al Cl 2

e

Al Clr 2

+

CI"

=

+

CI"

=

A1 C1 2

7

2AICI4-

I t i s l i k e l y therefore t h a t t h e 3d m e t a l d i c h l o r i d e s d i s s o l v e i n m o l t e n A l C l e a c c o r d 2

ing to the equation MC1

2

+

2A1 C1 2

6

=

A m o d e l for t h i s c o m p l e x is s h o w n i n F i g u r e A .

M(A1 C1 ) 2

7

2

I n this model, the M

+

2

i o n is o c t a -

hedrally surrounded b y six chlorides belonging to t w o A 1 C 1 " groups, the A 1 C 1 " 2

7

2

7

g r o u p i n t u r n h a v i n g a s t r u c t u r e c o m p o s e d of t w o AICI4 g r o u p s s h a r i n g a c o r n e r . T h e a n a l o g y of t h e M ( A 1 C 1 ) 2

7

2

complex

w i t h the proposed

[Y 0 ZX0 ] 2

7

3

- 2

a c t i v a t e d state c o m p l e x resides i n t h e f a c t t h a t b o t h c o m p l e x e s p r o v i d e a n o c t a h e d r a l site for t h e m e t a l i o n .

S p e c t r o s c o p i c d a t a o n 3d m e t a l i o n s i n m o l t e n n i t r a t e s



9.

DUKE

229

Discussion

Figure A.

Model of M(AhClT)2

complex

c a n be m o s t e a s i l y i n t e r p r e t e d i n t e r m s of o c t a h e d r a l c o o r d i n a t i o n b y

oxygens.

A l t h o u g h s u c h m e a s u r e m e n t s c a n n o t be p e r f o r m e d o n a l k a l i m e t a l i o n s since t h e y d o n o t possess u n p a i r e d ^-electrons, i t i s n o t u n r e a s o n a b l e t o p o s t u l a t e s i x f o l d oxygen

coordination

[Y2O7ZXO3]-

2

complex

for

these

ions

as

well.

An

a t t r a c t i v e feature

of

the

is t h a t i t b r i n g s t h e t w o r e a c t a n t s i n t o close p r o x i m i t y

w i t h o u t r e q u i r i n g a n excessive a c c u m u l a t i o n of n e g a t i v e c h a r g e a t t h e r e a c t i o n s i t e . Joseph J . Jordan:

P r o f . D u k e s a i d t h a t one of h i s m o t i v a t i o n s for e x p l o r i n g

o x i d a t i o n - r e d u c t i o n i n m o l t e n s a l t s w a s h i s desire t o s t u d y s e p a r a t e l y t h e e l e c t r o n t r a n s f e r process p r o p e r , w h i c h i n a q u e o u s s o l u t i o n s i s i n v a r i a b l y c o m p l i c a t e d a n d encumbered b y overlapping proton transfer. T o m e , t h e m o s t i n t e r e s t i n g feature of t h e L u x - F l o o d a c i d - b a s e c h e m i s t r y i n m o l t e n n i t r a t e s o l v e n t s i s t h e f a c t t h a t one does e n c o u n t e r t h e s a m e t y p e of i n t e r p l a y between acid-base a n d oxidation-reduction chemistry.

However, i n this i n

s t a n c e t h e a c i d - b a s e r e a c t i o n h a p p e n s t o be, n o t a p r o t o n t r a n s f e r , b u t a n o x i d e transfer.

I s h o u l d h o p e , as P r o f . D u k e s a i d , t h a t m o r e p e o p l e w o u l d b e c o m e

a c t i v e i n t h e field of a c i d - b a s e a n d e l e c t r o n t r a n s f e r c h e m i s t r y i n m o l t e n s a l t s .

This

is s t i l l a n a r e a w h e r e i t m i g h t be p o s s i b l e t o d r a w c o n c l u s i o n s f r o m r e l a t i v e l y s i m p l e , almost qualitative, observations.

I f one r e v i e w s i n r e t r o s p e c t t h e i n g e n i o u s a n d

s o p h i s t i c a t e d m a t e r i a l p r e s e n t e d here, t h e i m p r e s s i o n is t h a t a l m o s t a n y c o n c l u s i o n t h a t r e m a i n s t o be d r a w n i n a q u e o u s i n o r g a n i c m e c h a n i s m s c h e m i s t r y h a s t o be based on rather complicated quantitative arguments.

B e c a u s e of t h e m a n y a p

p r o x i m a t i o n s t h a t m u s t be r e l i e d o n i n these s i t u a t i o n s , one c a n n o t h e l p f e e l i n g t h a t he m i g h t lose c o n t a c t w i t h t h e t a n g i b l e r e a l i t y of t h e c h e m i s t r y i n v o l v e d .

One

of t h e v i r t u e s of m o l t e n s a l t s is t h a t a great d e a l of s t r a i g h t f o r w a r d c h e m i s t r y r e m a i n s t o be e l u c i d a t e d , p e r h a p s because t h i s field i s s t i l l m a n y d e c a d e s b e h i n d t h e p r e s e n t d e v e l o p m e n t of a q u e o u s i n o r g a n i c c h e m i s t r y . L u x - F l o o d base d i s s o c i a t i o n e q u i l i b r i a c a n be d i v i d e d i n t o t w o b r o a d c a t e g o r i e s : (a) T h o s e i n w h i c h t h e c o n j u g a t e a c i d is n o t a n e l e c t r o n a c c e p t o r ; (b) T h o s e w h i c h y i e l d a n o x i d i z i n g a g e n t as c o n j u g a t e

acid.

C a r b o n a t e i o n i n a n a p p r o p r i a t e m o l t e n s a l t s o l v e n t i s a n e x a m p l e of (a), t h e c o n -




230

j u g a t e a c i d ' s b e i n g CO2, w h i c h is n o t a n e l e c t r o n a c c e p t o r .

P r o f . D u k e focused o n

(b), a t y p i c a l e x a m p l e of w h i c h is t h e n i t r a t e i o n w h o s e c o n j u g a t e a c i d , t h e n i t r o n i u m i o n NC>2 , is a s t r o n g o x i d i z i n g a g e n t . +

I should like t o present some tangible

e v i d e n c e t h a t o x i d e d i s s o c i a t i o n is i n d e e d a p h e n o m e n o n w h i c h is a n a l o g o u s w i t h t h e d i s s o c i a t i o n of h y d r o g e n i o n s f r o m B r 0 n s t e d a c i d s i n c o n v e n t i o n a l s o l v e n t s . R e l e v a n t experimental d a t a are presented i n F i g u r e B , w h i c h is based on the w o r k of K a r l R o m b e r g e r (8) a n d r e p r e s e n t s a f a m i l y of p o l a r o g r a m s o b t a i n e d a t a r o t a t e d p l a t i n u m d i s k electrode i n a n a l k a l i nitrate solvent melt.

I n the residual current

c u r v e (the t o p c u r v e , o b t a i n e d i n t h e p u r e s o l v e n t ) as t h e p o t e n t i a l is m a d e m o r e a n o d i c , one n o t i c e s a c u r r e n t w h i c h reflects t h e o x i d a t i o n of o x i d e i o n r e s u l t i n g f r o m t h e d i s s o c i a t i o n of n i t r a t e . in

the

massive

base C O 3 - . 2

nitrate melt,

T h e o t h e r p l o t s i n Β were o b t a i n e d b y d i s s o l v i n g , various small concentrations

of

C a r b o n a t e is a s t r o n g e r L u x - F l o o d base t h a n n i t r a t e .

the L u x - F l o o d Curves A , B ,

C , D , a n d Ε c o r r e s p o n d t o t h e e l e c t r o - o x i d a t i o n of t h e o x i d e i o n s d i s s o c i a t e d f r o m the carbonate.

T h e y e v e n t u a l l y l e v e l off i n a l i m i t i n g c u r r e n t d o m a i n , because t h e

p r o c e s s i s m a s s t r a n s f e r c o n t r o l l e d ( b y d i f f u s i o n a n d forced c o r r e s p o n d i n g r a n g e of p o t e n t i a l s .

convection)

i n the

T h e difference i n p o t e n t i a l s a t w h i c h 0~~ i s 2

e l e c t r o - o x i d i z e d i n t h e presence of c a r b o n a t e is s t r i k i n g c o m p a r e d t o w h a t h a p p e n s i n the pure nitrate melt.

N e g l e c t i n g i r r e v e r s i b i l i t y , t h i s difference r e s u l t s f r o m t h e

fact t h a t carbonate is a m u c h more strongly dissociated oxide donor t h a n nitrate ; t h e e q u i l i b r i u m c o n c e n t r a t i o n of o x i d e i o n i n t h e presence of c a r b o n a t e i s a b o u t 20 o r d e r s of m a g n i t u d e l a r g e r t h a n i n p u r e n i t r a t e .

A s a r e s u l t , e l e c t r o - o x i d a t i o n of

0~~ c a n o c c u r a t m u c h less a n o d i c p o t e n t i a l s i n t h e presence t h a n i n t h e absence of 2

co - . 3

2

H y d r o g e n r e d u c t i o n w a v e s are w e l l k n o w n i n a q u e o u s p o l a r o g r a p h y (6) ; t h i s completely analogous phenomenon brings out convincingly a n d tangibly the similar


9.

DUKE

Discussion

231

nature between oxide transfer c h e m i s t r y i n m o l t e n salts a n d p r o t o n transfer c h e m i s t r y i n conventional solvents. A c t u a l l y , t h e m o r e i n t e r e s t i n g s i t u a t i o n i s (b) w h e r e t h e c o n j u g a t e a c i d is a n o x i d i z i n g a g e n t (12).

A s t r i k i n g d e m o n s t r a t i o n of t h e i n t e r d e p e n d e n c e of e l e c t r o n

transfer a n d oxide transfer is inherent i n the following observations.

I t is p o s s i b l e


t o p r e p a r e a s o l u t i o n of p o t a s s i u m i o d i d e i n a v e r y p u r e n i t r a t e m e l t a n d m a i n t a i n


232


s t a b i l i t y i n d e f i n i t e l y , i n t h e sense t h a t t h e r e is n o o x i d a t i v e loss of t h e i o d i d e b y conversion to iodine.

If, h o w e v e r , t h e n i t r a t e m e l t c o n t a i n s a t r a c e of a h e a v y

m e t a l s a l t , s u c h as l e a d o r a l u m i n u m , one sees a n i n s t a n t a n e o u s e v o l u t i o n of i o d i n e . T h e r e a s o n f o r t h i s is t h a t l e a d a n d a l u m i n u m i o n s a r e L u x - F l o o d a c i d s w h i c h a b stract the oxide i o n from the nitrate dissociation e q u i l i b r i u m , thus increasing the a c t i v i t y of t h e c o n j u g a t e a c i d , N 0 2 .

T h e n i t r o n i u m i o n functions as a n electron

+

acceptor w h i c h oxidizes iodide to iodine. I n o r d e r t o i l l u s t r a t e t h e n a t u r e of i n t e r p r e t i v e p r o b l e m s w h i c h p r e v a i l i n c o n t e m p o r a r y m o l t e n s a l t c h e m i s t r y , I s h o u l d l i k e t o describe a n i n t e r e s t i n g c a t a l y t i c r e a c t i o n , v i z . , t h e r e d u c t i o n of n i t r a t e t o n i t r i t e i n d u c e d b y t h e e l e c t r o - r e d u c t i o n


of t r a c e s of w a t e r .

I believe t h a t t h i s m a y e x e m p l i f y t h e m e c h a n i s t i c p r o b l e m s

encountered t y p i c a l l y i n molten salt chemistry a n d hope that this w i l l

engender

s o m e d i s c u s s i o n a n d r e s e a r c h b y t h i s g r o u p w h i c h h a s so a b l y h a n d l e d m u c h m o r e complex problems i n aqueous chemistry.

If one r e c o r d s a d i r e c t c u r r e n t v o l t a g e

c u r v e i n a n i t r a t e a l k a l i n i t r a t e m e l t w h i c h c o n t a i n s t r a c e s of w a t e r , one o b t a i n s t h e p o l a r o g r a m s (2) s h o w n i n F i g u r e C .

T h e curves correspond to a cathodic reduc

t i o n process, a n d t h e " w a v e h e i g h t s " increase w i t h the a m o u n t of m o i s t u r e p r e s e n t . S i m i l a r findings h a v e been r e p o r t e d p r e v i o u s l y f r o m t h e U n i v e r s i t y of I l l i n o i s {11). O n e of m y g r a d u a t e s t u d e n t s , T . E . G e c k l e , b e c a m e i n t e r e s t e d i n t h i s m a t t e r , b e cause t h e q u e s t i o n w h e t h e r a w a t e r m o l e c u l e c a n be e l e c t r o - r e d u c e d d i r e c t l y a s s u c h ( r a t h e r t h a n v i a p r i o r d i s s o c i a t i o n t o a h y d r o g e n ion) is a m a t t e r of c o n s i d e r a b l e , fundamental importance.

M r . Geckle found that, while the limiting currents ob

t a i n e d i n t h e n i t r a t e m e l t were p r o p o r t i o n a l t o t h e c o n c e n t r a t i o n of w a t e r , t h e o n l y p r o d u c t s of t h e e l e c t r o d e r e a c t i o n were n i t r i t e i o n a n d o x i d e i o n s (one h a l f m o l e of e a c h p e r f a r a d a y of e l e c t r i c i t y ) . produced i n a n y w a y or form.

T h e s u r p r i s i n g t h i n g w a s t h a t no h y d r o g e n w a s

D u r i n g e l e c t r o l y s i s one c o u l d o b s e r v e i n t h e m e l t t h e

y e l l o w c o l o r of N O 2 w h i c h f a d e d as s o o n a s t h e e l e c t r o l y s i s w a s s t o p p e d . M a t h e m a t i c a l a n a l y s i s of t h e p o l a r o g r a m s i n d i c a t e d t h a t t h e o v e r a l l electrode r e a c t i o n i n v o l v e d one m o l e of r e a c t a n t , t w o e l e c t r o n s , a n d t h r e e m o l e s of p r o d u c t . T h e m e c h a n i s m w h i c h a c c o u n t s for t h e e x p e r i m e n t a l r e s u l t s i s i l l u s t r a t e d i n F i g u r e D. NET

RESULT

NO3-

+

2e

(from ->

coulometry)

N0 2

+

O"

2

MECHANISM rate controlling

H 0°°

H 0° 2

2

H 0°

+

2

2H°

2e

+

CT

2

fast

NO3-

+

Figure D.

2H°

2H

2 H[00

H 0°° 2

0 0

-h

NOr

Mechanism of the electrode-reduction

W a t e r f r o m t h e b u l k of t h e m e l t i s t r a n s p o r t e d , b y d i f f u s i o n a n d forced c o n v e c t i o n , i n a r a t e - c o n t r o l l i n g s t e p t o t h e electrode surface (this a c c o u n t s for t h e p r o p o r t i o n a l i t y of t h e l i m i t i n g c u r r e n t t o w a t e r c o n c e n t r a t i o n ) ; a t t h e electrode i n t e r f a c e w a t e r


9.

DUKE

Discussion

233

is r e d u c e d t o a t o m i c h y d r o g e n , w h i c h diffuses r a p i d l y t o the b u l k of t h e s o l u t i o n ; there i t reacts w i t h n i t r a t e , p r o d u c i n g n i t r i t e a n d r e g e n e r a t i n g w a t e r ; t h e w a t e r t h e n " r e t u r n s " ( b y d i f f u s i o n a n d forced c o n v e c t i o n ) t o t h e electrode surface a n d c o n t r o l s the c u r r e n t . T h e i n t e r e s t i n g r e a c t i o n t o t h i s sequence is t h e l a s t s t e p : i t m u s t necessarily p r o c e e d v i a a m e c h a n i s m w h i c h i s c o n s i s t e n t w i t h t h e t r a n s i e n t a p p e a r a n c e of t h e c o l o r of N O 2 i n t h e b u l k of t h e s o l u t i o n . c a n a c c o u n t for t h i s o b s e r v a t i o n .

Figure Ε contains two alternatives which

T h e r e a c t i o n sequence o n t h e left i s b a s i c a l l y a n

o x i d e t r a n s f e r m e c h a n i s m a n d is c o n s i s t e n t w i t h s o m e of P r o f . D u k e ' s i d e a s ; h y d r o gen is a s s u m e d t o r e d u c e n i t r a t e t o N O 2 ; N 0

acts as a " m i x e d a c c e p t o r " for oxide

2


y i e l d i n g n i t r a t e p l u s n i t r i t e ; a n d h y d r o x y l i o n s are r e c o n v e r t e d t o o x i d e a n d w a t e r via a known reaction. T h e second sequence i n F i g u r e Ε i n v o l v e s h y d r i d e as a r e a c t i o n i n t e r m e d i a t e . NO3-

+

2H

+

2

N O r

OXIDE

TRANSFER

2NO3-

+

2H

->

2N0

2

+

20H-

2N0

+

CT

->

NO3-

+

N0 "

-»

Ο"

2

2

20HHYDRIDE

2NO320-2 2N0 + 2N0

+ +

2

2

+

Figure E. Gilbert Haight:

2

H 0

+

2

2

TRANSFER

->

2N0

2

+

+

4H

-+

2H-

—

2N0

2

+

2H

Ο"

+

NO3-

+

N0 -

->

Ο"

2

2

+

20-2

2H-

20H-

mechanism.

H 0

-+

+

20H-

2

H 0 2

Alternative mechanisms

I a m g l a d t h a t P r o f . D u k e has d e m o l i s h e d a s t a i d o l d o r g a n i c

I w o u l d l i k e t o c o m m e n t o n t h i s a p p a r e n t n o n p a r t i c i p a t i o n of

N0

2

+

i n n i t r a t i o n r e a c t i o n s because t h e r e is n o w c o n s i d e r a b l e k i n e t i c e v i d e n c e o n w h a t happens when H N 0

2

is a n o x i d a n t i n s o l u t i o n .

T h e Ingold school, a n d specifically

C . A . B u n t o n a n d G . S t e d m a n i n E n g l a n d , h a v e e l a b o r a t e l y s t u d i e d t h e k i n e t i c s of t h e r e a c t i o n of n i t r o u s a c i d w i t h a z i d e i o n .

T h e y conclude t h a t the active inter

m e d i a t e , o r a n a c t i v e i n t e r m e d i a t e is H N 0

2

2

t h e a n h y d r i d e of H N 0 2

2

+

+

.

T h e y offer e v i d e n c e t h a t N O * ,

, is n o t a c t i v e i n the k i n e t i c s , a n d i n f a c t , o n l y w h e n N O *

is a t t a c h e d t o o t h e r t h i n g s does i t b e c o m e l a b i l e a n d a g o o d o x i d a n t . f u r t h e r e v i d e n c e for t h i s . m e t h o d for NOz~.

T h e r e is

H i d d e n i n t h e a n a l y t i c a l l i t e r a t u r e (0) is a n i n c r e d i b l e

T h e m e t h o d is t o reduce n i t r a t e i n c o n c e n t r a t e d s u l f u r i c a c i d

w i t h ferrous i o n g i v i n g a q u a n t i t a t i v e t w o - e l e c t r o n r e d u c t i o n of t h e n i t r a t e .

To

m e , t h i s m e a n s t h a t N O * i s a t least a l i k e l y p r o d u c t a n d t h a t i t i s i n e r t . R e c e n t l y I h a v e a l s o h e a r d f r o m one of S z a b o ' s c o l l a b o r a t o r s ( B a r t h a ) a t Szeged i n H u n g a r y t h a t o x a l a t e i n c o n c e n t r a t e d s u l f u r i c a c i d reduces n i t r a t e b y t w o e q u i v a l e n t s , a g a i n i n d i c a t i n g t h a t N O * is i n e r t .


234

MECHANISMS OF INORGANIC

REACTIONS

Simply by inference, I suggested to one of m y organic friends recently that maybe N 0 2 is inert. +

It is isoelectronic with CO2, has a higher charge on the cen

tral atom, and ought to be unreactive.

B y analogy with H N O 2 kinetics (/, 10),

H2NO3* " may be the kinetically active species of nitrate. 1

I think Prof. Duke has

offered a strong indication that that is so. Dr. Duke:

I would like to warn D r . Haight that organic mechanisms are not

that easily destroyed.

B u t I will let him go ahead and destroy if if he really

chooses to do so. Harry Gray:

I would like to ask D r . Gruen about the nitrate melts.


T h e nitrate ion itself has its first spin-allowed but orbitally-forbidden transi tion at about 3000 A . in aqueous solution, e = 7.

Nitrate ion is planar in the ground

state and in this Dzh symmetry, the band at 3000 Α . , it is orbitally forbidden. If the oxygens in nitrate are bent to form a pyramidal ion, one observes, not a substantial movement in energy of this band, but a substantial increase in intensity because the band becomes orbitally allowed if the planar symmetry is destroyed. I think the intensity is probably a fairly sensitive function of distortion, and I wonder if you have looked in this region to get additional information from the spectra on the structure of the nitrate ion in these melts. I would also like you to speculate on whether the nitrate is bidentate or monodentate. Dr. Gruen: have.

W e have not looked at this particular band, but other people

Pedro Smith at O a k Ridge, for example, has made an extensive study of

this particular band for pure alkali nitrates and finds substantial variations in i n tensity, depending on whether he is looking at lithium, sodium, or potassium nitrate. H e interprets these effects, if I recall correctly, as owing to polarization effects re sulting in slight distortions of the nitrate group.

A s far as our work is concerned

we have not studied these charge transfer bands, and in fact we would not be able to pick up intensity changes because our transition metal ion concentrations are 10~ M. 2

T h e effect of the transition metal ion on the nitrate absorption would be

very difficult to measure.

Y o u r point certainly raises a very interesting problem.

One may learn something about the structure of the nitrate group in the melt b y detailed consideration of the first band. Dr. Gray:

I have observed that in some cases the € of that band goes well

over 1,000, in one case to 5,000. Dr. Duke:

A l l of these equilibrium constants involving oxide ion, such as a

carbonate going to oxide ion in the nitrates, are much too large.

I believe that we

are getting orthonitrate here and would like to have some comments on that. Leonard K a t z i n : tion spectrum.

M y first point refers to the question of the nitrate absorp

If one goes back a decade or more in the literature, there are ob

servations on this peak of a nitrate in nonaqueous solutions of inorganic salts, and I don't believe that the wave length stays

fixed.

It shifts toward shorter wave

lengths—not a great deal, but perceptibly (5). I have doubts about the nonplanarity.

Our infrared studies (4) on some nitrate

salts in nonaqueous systems indicate that we are taking one of these nitrates, tying it down, and shifting the vibrational pattern.

B u t whether or not a nonplanarity

occurs with this, I can't say.


9.

DUKE

Discussion

235

I a m i n t r i g u e d b y one a s p e c t of P r o f . D u k e ' s w o r k , n a m e l y , t h a t a c o o r d i n a t i o n n u m b e r a n d c o n f i g u r a t i o n c h a n g e of t h e c a t i o n o c c u r s . to metal ion situations.

T h i s has certain analogies

I t raises i n t e r e s t i n g a n d p r o v o k i n g q u e s t i o n s .

I w o u l d l i k e t o a d d r e s s m y s e l f t o t h e s e c o n d p a r t of D r . G r a y ' s

Dr. Gruen: question.

W e have considered how m a n y nitrate groups are a r o u n d a m e t a l i o n .

T o get

s i x - f o l d c o o r d i n a t i o n one m a y h a v e s i x n i t r a t e g r o u p s o r m o n o d e n t a t e , o r f o u r w i t h t w o b i d e n t a t e a n d t w o m o n o d e n t a t e , o r t h r e e w i t h t h r e e of t h e m a c t i n g b i d e n t a t e , o r t w o w i t h t w o of t h e m a c t i n g t r i d e n t a t e . T o d i s t i n g u i s h a m o n g these a l t e r n a t i v e s o n the basis of m e l t s p e c t r a is i m p o s


sible, I t h i n k .

I h a v e been i n t e r e s t e d i n f o l l o w i n g t h e w o r k of P i p e r I b e l i e v e , w h o

h a s i s o l a t e d d o u b l e s a l t s of t h i s t y p e , o r s i m i l a r t h i n g s , i n w h i c h I b e l i e v e t h i s s i t u a tion is found. A s f a r a s I k n o w , c r y s t a l s t r u c t u r e s t u d i e s h a v e n o t been p u b l i s h e d o n t h i s k i n d of c o m p o u n d .

B u t I t h i n k t h e y w o u l d be r e v e a l i n g a n d b y a n a l o g y p e r h a p s t e l l

us s o m e t h i n g a b o u t t h e m e l t s i t u a t i o n .

If one c a n d e t e r m i n e — a n d I a m sure i t i s

possible b y x - r a y a n a l y s i s of these c o m p o u n d s — w h a t

the nitrate coordination is

a r o u n d t h e m e t a l i o n , one m i g h t be a b l e t o i n f e r s o m e t h i n g a b o u t t h e m e l t s i t u a t i o n . Dr. Jordan:

I should like to reorient the discussion to the question

D u k e addressed to the audience.

Prof.

If I understood this question correctly i t referred

t o t h e d e t a i l e d n a t u r e , t h e f o r m , i n w h i c h t h e o x i d e i o n m a y be p r e s e n t i n m o l t e n nitrates.

H e suggested t h a t t h i s m a y w e l l be t h e i o n N O 4 " .

T h i s is a n i t r a t o

3

s o l v a t e of t h e i o n Or —i.e.,

0 ~ · NO3".

2

2

I p r e s u m e t h a t one c a n v i s u a l i z e i n a m a s

s i v e c a r b o n a t e m e l t a s i m i l a r aggregate, v i z . , Ο · CO3 » o r C O 4 - . - 2

-2

4

T h i s is related

t o t h e n a t u r e of t h e species a c t u a l l y p r e s e n t i n c o n v e n t i o n a l s o l v e n t s , w h i c h a r e ignored i n chemical formula-writing shorthand.

T h e solvated hydrogen is quite

different i n a q u e o u s s o l u t i o n f r o m w h a t i t is i n g l a c i a l a c e t i c a c i d .

A r e there a n y

suggestions r e g a r d i n g t h e n a t u r e of t h e " s o l v a t e d o x i d e i o n " i n m o l t e n s a l t s ? I feel t h a t o r t h o n i t r a t e i s v e r y r e a s o n a b l e .

F o r carbonate I would postulate

the analog. Dr. Y a l m a n : Dr. Jordan: Dr. Yalman:

Y e s , t h e p r o t o n a n d t h e o x i d e i o n are n o t a n a l o g o u s . I c e r t a i n l y agree t h a t t h e y are n o t a n a l o g o u s i n a l l respects. T h e o x i d e d o e s n o t h a v e t o be s o l v a t e d i n t h e s a m e w a y t h a t t h e

proton or electron is solvated. Dr. Jordan:

D o y o u feel t h a t t h e o x i d e i o n i s u n s o l v a t e d i n t h e m e l t s ?

I did

not i n t e n d to i m p l y a complete analogy between oxide ions a n d protons. Dr. Yalman:

I t h i n k t h e a n a l o g y b e t w e e n t h e o x i d e a n d t h e p r o t o n is a b a d

one, a n d I d o n ' t t h i n k t h a t t h e o x i d e i o n h a s t o be n e c e s s a r i l y s o l v a t e d i n t h e s a m e w a y t h a t w e a c c e p t t h e s o l v a t i o n of a p r o t o n o r a n e l e c t r o n . Arthur Adamson:

A c t u a l l y , D r . H a r r i s is the better m a n to make this p a r

ticular r e m a r k , I suspect.

I n t h e case of o x a l a t e c o m p l e x e s i t seems n e c e s s a r y t o

a s s u m e a n o r t h o o r h y d r a t e d f o r m u l a t i o n of one e n d of a n o x a l a t e as i t d e t a c h e s f r o m t h e c o o r d i n a t i o n sphere i n o r d e r t o e x p l a i n t h e 0

1 8

exchange.

T h i s is not

e x a c t l y w h a t y o u are t a l k i n g a b o u t , b u t i t is a n i l l u s t r a t i o n of one i n s t a n c e w h e r e o r t h o a c i d f o r m a t i o n seems d e s i r a b l e . Michael E . Mirhej:

I p r o p o s e t h a t t h i s m i g h t be a p o l y m e r of a s t r u c t u r e

similar to either tungstates or

phosphates.


236 Literature

MECHANISMS OF INORGANIC REACTIONS Cited


(1) (2) (3) (4) (5) (6)

B u n t o n , C. Α., S t e d m a n , G., J. Chem. Soc. 1959, 3466. G e c k l e , T. E., Thesis, P e n n s y l v a n i a State U n i v e r s i t y , 1964. H a r r i s , R. L., W o o d , R. E., R i t t e r , H. L., J. Am. Chem. Soc. 73, 3151 (1951). K a t z i n , L. I., J. Inorg. Nucl. Chem. 24, 245 (1962). K a t z i n , L. I., J. Chem. Phys. 18, 789 (1950). Holthoff, I. M., L i n g a n e , J. J., " P o l a r o g r a p h y , " V o l . I, P. 409, Interscience, N e w Y o r k , 1952. (7) O y e , H., G r u e n , D. M., Inorg. Chem. 3, 836 (1964). (8) Romberger, Κ. Α., P e n n s y l v a n i a State U n i v e r s i t y , unpublished results. (9) Scott, W i l f r e d W., ed., " S t a n d a r d M e t h o d s of C h e m i c a l A n a l y s i s , " 6th e d i t i o n , V a n N o s t r a n d , P r i n c e t o n , 1962. (10) S t e d m a n , G., J. Chem. Soc. 1959, 2943, 2946. (11) Swofford, H. S., L a i t i n e n , Η. Α., J. Electrochem. Soc. 110, 814 (1963). (12) V a n N o r m a n , J. D., Osteryoung, R. Α., Anal. Chem. 32, 398 (1960).


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