Adsorption of Hydrolyzed Hafnium Ions on Glass - Advances in

5 Adsorption of Hydrolyzed Hafnium Ions on Glass

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LYNDEN

J . S T R Y K E R and

EGON

MATIJEVIĆ

Department of Chemistry and Institute of C o l l o i d and Surface Science, Clarkson College of Technology, Potsdam, N. Y .

The

adsorption

increase

with

of hafnium

species

the solution

pH

The effects on the adsorption

on glass was found

and hafnium

of the solution

age were studied and the equilibration tion process was determined. was determined

vapor.

The results are discussed

hafnium(IV)

species.

erage was obtained hafnium

is in the

by the B.E.T.

>

solution

soluble

sorption

layer the cross-sectional is nearly

the

using

species.

glass water

hydrolyzed

nearly monolayer Under

these

in its entirety

neutral, which

Hf(OH)4

4.5.

and adsorp-

area of the

method

in terms of the

At equilibrium, at pH

preparation

time for the

The surface

sample

in the

form

In the close packed water

on silica

cov-

conditions

area of this species is 24

same as for

to

concentration.

of adA.

2

surfaces.

T ^ T u m e r o u s experiments d e a l i n g w i t h the a d s o r p t i o n of m e t a l ions o n glass f r o m aqueous solutions h a v e b e e n r e p o r t e d i n the literature. T h e s e studies dealt, for example, w i t h s o d i u m (6,11,18), c e s i u m (11, c i u m (6),

18),

z i r c o n i u m (29,

m e t h i u m (4), thorium

t h a l l i u m ( I ) a n d t h a l l i u m ( I I I ) (32),

(24),

34, 36),

r u t h e n i u m (4, 31),

g o l d ( 2 6 ) , b i s m u t h (17), r a d i u m (22),

p l u t o n i u m (7, 8, 9, 10,13,

14).

l e a d (17),

p r o t a c t i n i u m (35),

potassium silver ( I I ) , c e r i u m (4),

(6), cal pro-

p o l o n i u m (17, 28,

33),

u r a n i u m (30),

and

I n the m a j o r i t y of the experiments r a d i o

active isotopes w e r e u s e d i n tracer q u a n t i t i e s . It w a s established w i t h o u t e x c e p t i o n t h a t the p H w a s the most i m p o r t a n t p a r a m e t e r affecting the a d s o r p t i o n . I n the case of s i m p l e n o n - h y d r o l y z a b l e ions the p H effects w e r e i n t e r p r e t e d i n terms of the h y d r o l y s i s of the glass surface. It is also g e n e r a l l y suggested that the m e c h a n i s m of a d s o r p t i o n of n o n - h y d r o l y z e d ions is essentially an i o n exchange cations i n the glass (3),

of the adsorbate

species w i t h

the

a l t h o u g h there are n u m e r o u s observations w h i c h 44

In Adsorption From Aqueous Solution; Weber, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

5.

STRYKER

A N D

Hydrolyzed

M A T I J E V I C

Hafnium

45

Ions

i n d i c a t e that the a s s u m p t i o n of the ion-exchange as the o n l y cause for a d s o r p t i o n is a gross o v e r s i m p l i f i c a t i o n

(18).

T h e a d s o r p t i o n of p o l y v a l e n t m e t a l ions o n glass is a c o n s i d e r a b l y m o r e c o m p l e x process.

S u c h ions easily h y d r o l y z e to give a v a r i e t y of

soluble c o m p l e x species a n d , f r e q u e n t l y , i n s o l u b l e h y d r o x i d e s at rather l o w p H values. T h u s , a n increase i n p H not o n l y affects the surface of the glass b u t also changes t h e entire c o m p o s i t i o n of the adsorbate i n solution.

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

increases

d r a m a t i c a l l y a b o v e a c e r t a i n p H . I n some cases, the a d s o r b e d Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0079.ch005

rises c o n t i n u o u s l y w i t h a n increase i n p H (32). nounced

adsorption m a x i m a were observed

amount

I n other cases,

pro

at some i n t e r m e d i a t e p H

values. T h e p o s i t i o n of the m a x i m u m v a r i e d w i t h the element (7, 8, 24, 26, 30, 31, 35, 36).

W h i l e there w a s evidence t h a t this m a x i m u m w a s

i n d e p e n d e n t of the adsorbent (4),

a b i g difference w a s f o u n d w h e n glass

a n d q u a r t z w e r e c o m p a r e d u s i n g the same c o u n t e r i o n (7,8).

It is u s u a l l y

a s s u m e d that at v e r y l o w p H , w h e r e h y d r o l y s i s is n e g l i g i b l e , the p o l y v a l e n t m e t a l ions to b e e x c h a n g e d for cations i n the glass are i n c o m p e t i t i o n w i t h the h i g h excess of h y d r o g e n ions. T h e increase i n a d s o r p t i o n w i t h p H has b e e n i n t e r p r e t e d i n v a r i o u s w a y s . I n general, the h y d r o l y s i s of the a d s o r b i n g m e t a l ions has b e e n a s s u m e d responsible f o r the e n h a n c e d a d s o r p t i o n , a l t h o u g h the use a n d the m e a n i n g of the t e r m " h y d r o l y s i s " is not a l w a y s consistent.

I n some instances this refers to the

f o r m a t i o n of soluble, w h i l e i n other cases to i n s o l u b l e p r o d u c t s .

The

greater a d s o r p t i v i t y of soluble c o m p l e x species w a s p r o p o s e d b y several investigators (7,8,13,14,24, to m a k e

30, 34,36).

H o w e v e r , no one ever a t t e m p t e d

a quantitative correlation between

the

composition

of

the

adsorbate s o l u t i o n a n d the a d s o r b e d q u a n t i t i e s . I n g e n e r a l i t is b e l i e v e d that the a d s o r p t i o n depends on the charge of the i o n i c complexes that l o w e r c h a r g e d or n e u t r a l sspecies adsorb less s t r o n g l y (34,

and

36).

M o r e f r e q u e n t l y the d e p e n d e n c e of a d s o r p t i o n u p o n p H w a s r e l a t e d to the f o r m a t i o n of c o l l o i d a l m e t a l h y d r o x i d e s .

A g a i n , some authors

expressed the o p i n i o n that the f o r m a t i o n of c o l l o i d s p r o m o t e d the a d s o r p t i o n (31,32)

w h i l e some others c l a i m e d the opposite (7, 8, 24, 36).

How

ever, it w a s g e n e r a l l y agreed that w h e n a n a d s o r p t i o n m a x i m u m w a s o b s e r v e d as a f u n c t i o n of the p H , the r e d u c e d a d s o r p t i o n at h i g h e r p H values w a s e x p l a i n e d b y the electrostatic r e p u l s i o n b e t w e e n the c o l l o i d particles a n d glass surface b e a r i n g the same charge. T h e entire p i c t u r e is s t i l l m o r e c o n f u s i n g because of the fact t h a t several different types of c o l l o i d s are d i s t i n g u i s h e d — i . e . , " r a d i o c o l l o i d s , " "pseudo-colloids"

(7, 8, 28, 33),

a n d " t r u e c o l l o i d s . " R a d i o - c o l l o i d s refer

to systems of radiotracers w h i c h a p p e a r to b e i n c o l l o i d a l f o r m a l t h o u g h t h e y are i n concentrations w e l l b e l o w t h e i r i o n i c s o l u b i l i t y (25, 26).

The

t e r m p s e u d o - c o l l o i d is u s e d to describe the f o r m a t i o n of a c o l l o i d system


46

ADSORPTION

F R O M

AQUEOUS

SOLUTION

b y the a d s o r p t i o n of a r a d i o e l e m e n t o n s o l i d i m p u r i t i e s c o n t a i n e d i n the s o l u t i o n (7, 8, 29). v e r s i a l (25, 27)

W h i l e the n a t u r e of " r a d i o c o l l o i d s " is s t i l l c o n t r o

the m e a n i n g of " p s e u d o - c o l l o i d s " is c o m p l e t e l y

obscure.

O n e gets the f e e l i n g that the latter c o n c e p t w a s i n t r o d u c e d for l a c k of u n d e r s t a n d i n g of the c o m p l e x process of a d s o r p t i o n of h y d r o l y z e d species f r o m aqueous solutions. F r o m the p r e c e d i n g s u r v e y it is a p p a r e n t that f u r t h e r studies are n e e d e d to e l u c i d a t e the m e c h a n i s m of a d s o r p t i o n of h y d r o l y z a b l e ions o n v a r i o u s adsorbents, p a r t i c u l a r l y o n glass.

O f s p e c i a l interest is t h e


q u e s t i o n w h e t h e r a n e n h a n c e d a d s o r p t i o n at h i g h e r p H is c a u s e d

by

soluble h y d r o l y z e d species or b y the f o r m a t i o n of c o l l o i d a l h y d r o x i d e s . I f the s o l u b l e complexes are responsible for the greater a d s o r p t i v i t y t h e r e l a t i o n s h i p of the latter to the charge, size, shape, configuration, a n d l i g a n d c o m p o s i t i o n of the adsorbate species becomes p e r t i n e n t . T h e a n s w e r to these p r o b l e m s is essential for the u n d e r s t a n d i n g of v a r i o u s surface p h e n o m e n a i n the presence of h y d r o l y z a b l e ions s u c h as sol s t a b i l i t y , flota t i o n , c o p r e c i p i t a t i o n , a d h e s i o n , p a p e r s i z i n g , etc. S e v e r a l attempts h a v e b e e n m a d e to correlate the a d s o r p t i v i t y of h y d r o l y z a b l e cations to the c o m p o s i t i o n of the species i n aqueous s o l u t i o n (1, 2, 20).

I n p a r t i c u l a r , the a d s o r p t i o n of t h o r i u m o n s i l v e r h a l i d e s

i n d i c a t e d a v e r y close r e l a t i o n s h i p b e t w e e n the change i n the a m o u n t of t h o r i u m a d s o r b e d a n d the c o n c e n t r a t i o n of the h y d r o l y z e d species i n s o l u t i o n (19).

soluble

T h e major difficulty i n this t y p e of w o r k is the

l a c k of q u a n t i t a t i v e d a t a o n the h y d r o l y s i s of v a r i o u s m e t a l ions.

The

o t h e r u n c e r t a i n t y is w i t h r e g a r d to the k n o w l e d g e of the true surface area of the adsorbent i n aqueous solution. T h i s latter i n f o r m a t i o n is n e e d e d if surface coverages are to b e e v a l u a t e d . A t least some of these difficulties h a v e b e e n o v e r c o m e i n t h e w o r k to b e r e p o r t e d i n this s t u d y , w h i c h deals w i t h the a d s o r p t i o n of h a f n i u m h y d r o l y z e d species o n p o w d e r e d glass as a f u n c t i o n of the a c i d i t y of the m e d i u m . T h e a d s o r p t i o n of h a f n i u m species f r o m aqueous s o l u t i o n has a p p a r e n t l y never b e e n i n v e s t i g a t e d , yet this i o n lends itself c o n v e n i e n t l y to studies of the p r o b l e m s discussed above. T h e c h e m i s t r y of the h a f n i u m i o n i n w a t e r is f a i r l y w e l l u n d e r s t o o d

(23)

a v a i l a b l e for a d s o r p t i o n studies.

W h a t makes h a f n i u m a p a r t i c u l a r l y

a n d a s u i t a b l e isotope,

1 8 1

H f , is

interesting system is the fact that i t forms the entire series of h y d r o l y z e d species: H f ( O H )

n

( 4

"

w ) +

w h e r e n ^ 4. A t i n t e r m e d i a t e acidities ( p H >

the solutions of l o w concentrations species H f ( O H ) . 4

4)

c o n t a i n o n l y the n e u t r a l , s o l u b l e

It s h o u l d be e m p h a s i z e d t h a t there is a p H a n d a

c o n c e n t r a t i o n range over w h i c h this species is present w i t h o u t s i m u l t a n e ous f o r m a t i o n of h a f n i u m h y d r o x i d e .

T h u s , it is possible to e l u c i d a t e

the effect of the i o n i c charge u p o n the a d s o r p t i o n of h y d r o l y z e d species i n systems v o i d of c o l l o i d a l h y d r o x i d e s .

T h e glass p o w d e r w a s u s e d i n


5.

STRYKER

A N D

M A T I J E V I C

Hydrolyzed

Hafnium

Ions

47

order to h a v e a sufficient surface area of adsorbent w h i c h c a n b e deter m i n e d w i t h reasonable a c c u r a c y a n d w h i c h w o u l d not change a p p r e c i a b l y u p o n dispersion i n water.

T h e p r e l i m i n a r y w o r k o f t h e a d s o r p t i o n of

h a f n i u m o n silver h a l i d e sols (21) c o u l d not b e f u l l y a n a l y z e d because the surface area o f the adsorbent w a s not k n o w n . A n o t h e r a d v a n t a g e of h a f n i u m is that, i f the a d s o r p t i o n o f the n e u t r a l species takes p l a c e , a close p a c k e d m o n o l a y e r s h o u l d e v e n t u a l l y result o w i n g to t h e absence of electrostatic r e p u l s i o n .

K n o w i n g t h e surface

area o f the adsorbent this w o u l d enable one t o evaluate the cross-sectional Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0079.ch005

area of t h e h y d r o l y z e d c o m p l e x

ion.

This information has not been

available. Experimental Materials. H f was e m p l o y e d i n tracer concentrations a n d was o b t a i n e d f r o m O a k R i d g e N a t i o n a l L a b o r a t o r i e s i n the f o r m of the c h l o r i d e i n a p p r o x i m a t e l y I N H C 1 . T h e analysis of t h e g a m m a r a y s p e c t r u m r e v e a l e d that p u r i f i c a t i o n of the isotope w a s unnecessary. T h e solutions w e r e p r e p a r e d i n the f o l l o w i n g m a n n e r . O n e p o r t i o n of the a c i d i f i e d iso tope s o l u t i o n w a s d i l u t e d t o a d e s i r e d v o l u m e w i t h n i t r i c a c i d g i v i n g a final c o n c e n t r a t i o n o f 0 . 4 N H N 0 . A second isotope s o l u t i o n w a s p r e p a r e d i n exactly the same m a n n e r except t h a t the d i l u t i o n w a s m a d e w i t h d o u b l y d i s t i l l e d w a t e r , r e s u l t i n g i n a final s o l u t i o n of w h i c h the p H w a s 3.5. A k n o w n q u a n t i t y of these r a d i o a c t i v e h a f n i u m solutions w a s t h e n a d d e d to a s o l u t i o n o f stable h a f n i u m t e t r a c h l o r i d e to o b t a i n a reasonable c o u n t rate over t h e c o n c e n t r a t i o n r a n g e s t u d i e d . T h e p H o f t h e final stock s o l u t i o n w a s either 2.0 or 3.1 d e p e n d i n g o n w h e t h e r it w a s p r e p a r e d w i t h a c i d o r w a t e r d i l u t e d tracer. A p e r i o d of three days w a s a l l o w e d for e q u i l i b r a t i o n b e t w e e n the s o l u t i o n a n d the container w a l l s . A l l subse q u e n t d i l u t i o n s w e r e p r e p a r e d f r o m these stock solutions. A n e e d f o r the p r o p e r p r o c e d u r e of p r e p a r a t i o n of the l a b e l l e d solutions of h y d r o l y z a b l e m e t a l ions w a s e m p h a s i z e d b y several investigators (25, 26). F r e s h l y p r e p a r e d stock solutions, o b t a i n e d as d e s c r i b e d a b o v e a p p e a r e d to b e h o m o g e n e o u s a n d free o f c o l l o i d a l p r e c i p i t a t e s . H o w e v e r , after p r o l o n g e d storage traces of h a f n i u m h y d r o x i d e w e r e f o u n d . S u c h s o l u tions w e r e not u s e d i n experiments. I n s t e a d fresh stock solutions w e r e p r e p a r e d every f e w weeks. 1 8 1

3

H a f n i u m tetrachloride, nitric acid, and potassium hydroxide solu tions w e r e p r e p a r e d u s i n g d o u b l y - d i s t i l l e d w a t e r f r o m a n a l l b o r o s i l i c a t e glass s t i l l . T h e c h e m i c a l s w e r e of the highest p u r i t y grade c o m m e r c i a l l y available a n d were used without further purification. T h e glass p o w d e r , w h i c h served as the adsorbent, w a s o b t a i n e d f r o m the A r t h u r S. L a p i n e C o m p a n y i n the f o r m of s p h e r i c a l beads a p p r o x i m a t e l y 40 n i n d i a m e t e r . T h e beads w e r e w a s h e d w i t h a l a r g e q u a n t i t y o f d i s t i l l e d w a t e r a n d d r i e d i n a n o v e n at 1 2 0 ° C . b e f o r e use i n t h e a d s o r p t i o n experiments. A s a m p l e of the glass p o w d e r was l e a c h e d b y r e f l u x i n g w i t h 1 5 % H C 1 a t 8 0 ° C . f o r a p p r o x i m a t e l y f o u r days. A f t e r this t h e beads w e r e

A. C. S. Editorial Library In Adsorption From Aqueous Solution; Weber, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

48

ADSORPTION

F R O M

AQUEOUS

SOLUTION

w a s h e d t h o r o u g h l y w i t h d i s t i l l e d w a t e r a n d t h e n h e a t e d i n a n o v e n at 3 8 0 ° C . for several h o u r s . T h e surface area of the adsorbent, c l e a n e d b y the first p r o c e d u r e , w a s m e a s u r e d b y the B . E . T . m e t h o d u s i n g w a t e r v a p o r as the adsorbate. A s s u m i n g the cross-sectional area of w a t e r to b e 12.5 A . ( 5 ) , three deter m i n a t i o n s r e s u l t e d i n a v a l u e of 0.80 db 0.05 m e t e r / g r a m . F o r c o m p a r i son reasons, the geometric surface area w a s also d e t e r m i n e d f r o m a h i s t o g r a m o b t a i n e d b y c o u n t i n g several h u n d r e d glass particles o n m i c r o p h o t o g r a p h s . T h i s surface area w a s o n l y 0.04 m e t e r / g r a m . T h e difference b e t w e e n the t w o procedures e m p l o y e d indicates that the glass beads u s e d e x h i b i t e d significant surface roughness. Method. A l l a d s o r p t i o n samples w e r e p r e p a r e d b y w e i g h i n g the glass p o w d e r o n a n a n a l y t i c a l b a l a n c e a n d b y a d d i n g the a p p r o p r i a t e amounts of r a d i o a c t i v e h a f n i u m , n i t r i c a c i d or p o t a s s i u m h y d r o x i d e , a n d d o u b l y - d i s t i l l e d w a t e r to g i v e a constant final v o l u m e of 10 m l . T h e solutions w e r e a g i t a t e d b y means of a m a g n e t i c stirrer for the d e s i r e d p e r i o d s of t i m e w h e r e u p o n the systems w e r e c e n t r i f u g e d at 3,500 r . p . m . , c o r r e s p o n d i n g to 2.5 X 1 0 g, for 15 m i n u t e s i n a n I E C I n t e r n a t i o n a l C e n t r i f u g e . A n a l i q u o t of the supernatant s o l u t i o n w a s t h e n r e m o v e d for r a d i o a c t i v e analysis. T h e r e m a i n i n g s o l u t i o n w a s u s e d for p H m e a s u r e ments e m p l o y i n g c a l i b r a t e d glass electrodes i n a B e c k m a n M o d e l G p H meter. T h e glass beads f r o m the same s a m p l e w e r e w a s h e d t w i c e w i t h a n i n a c t i v e s o l u t i o n of h a f n i u m of the same c o n c e n t r a t i o n as the m o t h e r l i q u o r . T h e w a s h i n g s w e r e r e m o v e d a n d d i s c a r d e d . T o d e t e r m i n e the a m o u n t of h a f n i u m a d s o r b e d o n the glass, 10 m l . of a 0 . 5 N H N 0 s o l u t i o n w e r e a d d e d to the samples a n d stirred o v e r n i g h t after c o m p l e t i o n o f the w a s h i n g . T h i s w a s necessary i n o r d e r to m a i n t a i n constant geometry c o n d i t i o n s . R a d i o a c t i v e countings w e r e p e r f o r m e d w i t h a T r a c e r l a b G a m m a G u a r d F u l l y A u t o m a t i c W e l l Scintillation Console System. 2

2


2

3

3

C e n t r i f u g a t i o n w a s u s e d to d e t e r m i n e the p r e c i p i t a t i o n l i m i t of the h a f n i u m solutions as a f u n c t i o n of the p H . A series of solutions c o n t a i n i n g a constant a m o u n t of h a f n i u m , to w h i c h s o d i u m h y d r o x i d e w a s a d d e d i n i n c r e a s i n g amounts to v a r y the p H i n s m a l l increments f r o m system to system, w a s a l l o w e d to e q u i l i b r a t e for a c e r t a i n p e r i o d of t i m e . T h e d e s i r e d p H w a s adjusted a u t o m a t i c a l l y u s i n g a R a d i o m e t e r M o d e l P H M - 2 8 p H meter, w i t h glass electrodes. T h e solutions w e r e t h e n c e n t r i f u g e d at 3,500 r . p . m . for 1/2 h o u r . F i v e m i l l i l i t e r a l i q u o t s w e r e d r a w n f r o m the u p p e r p a r t of e a c h of the solutions a n d the a c t i v i t y of this p o r t i o n was c o m p a r e d w i t h the a c t i v i t y of the r e m a i n i n g 5 m l . sample. F r o m this the f r a c t i o n of the c e n t r i f u g e d h a f n i u m w a s d e t e r m i n e d . I n c e r t a i n cases, w h i c h w i l l b e discussed later o n , it w a s necessary to u l t r a c e n t r i f u g e the systems. T h e samples w e r e p r e p a r e d i n a n analogous m a n ner and centrifuged i n a B e c k m a n Preparative Ultracentrifuge M o d e l L - 2 at 25,000 r . p . m . , c o r r e s p o n d i n g to 5.5 X 1 0 g, for one h o u r . 4

I n a l l c a l c u l a t i o n s , corrections w e r e m a d e for the a d s o r p t i o n o n the test tubes. A s a r u l e these corrections w e r e s m a l l d u e to the s m a l l surface area of the test tubes i n c o m p a r i s o n to that of the glass p o w d e r u s e d . All H f d e t e r m i n a t i o n s w e r e m a d e o n constant final v o l u m e s of 10 m l . to insure r e p r o d u c i b l e c o u n t i n g c o n d i t i o n s . S t a n d a r d samples w e r e p r e p a r e d f r o m the w o r k i n g s o l u t i o n of r a d i o a c t i v e h a f n i u m t e t r a c h l o r i d e t a k i n g k n o w n v o l u m e s of this s o l u t i o n 1 8 1


5.

STRYKER

A N D

M A T I J E V I C

Hydrolyzed

Hafnium

49

Ions

a n d d i l u t i n g to a v o l u m e of 10 m l . T h e s e standards w e r e a l w a y s c o u n t e d i m m e d i a t e l y f o l l o w i n g the a d s o r p t i o n samples to e l i m i n a t e c o r r e c t i o n for d e c a y losses. A l i n e a r r e l a t i o n s h i p b e t w e e n the a c t i v i t y a n d the a m o u n t of h a f n i u m d i s s o l v e d w a s f o u n d over a c o n c e n t r a t i o n range of t w o orders of m a g n i t u d e . E x p e r i m e n t a l results s h o w e d that the c o m b i n e d a c t i v i t y o b t a i n e d for the a d s o r b e d h a f n i u m a n d h a f n i u m r e m a i n i n g i n s o l u t i o n w a s w i t h i n ± 3 % of the i n t r o d u c e d a c t i v i t y . T a b l e I gives some t y p i c a l results i n d i c a t i n g this g o o d agreement.


Table I.

HfCl>

Count Rate Introduced Count Rate (c.p.m.) of the Total (c.p.m.) of the Supernatant Count Rate Count Rate (c.p.m.) Adsorbed Amount Solution (c.p.m.)

% Error

1,136,364 1,136,364 568,729 568,729 210,713 210,713

+2.6 -0.5 +2.1 -0.5 +2.9 -0.6

1 X 10" M 1 X 10" M 5 X 10" M 5 X 10" M 2.5 X 1 0 " M 2.5 X 1 0 " M

442,137 495,500 332,049 281,611 213,658 169,867

4

4

5

5

5

5

1,166,119 1,131,786 580,769 565,855 216,860 209,395

723,982 636,286 248,720 284,244 3,204 39,528

Results Precipitation of H a f n i u m Hydroxide. I n o r d e r to i n t e r p r e t the a d s o r p t i o n d a t a it w a s necessary to d e t e r m i n e the c o n d i t i o n s w h i c h l e a d to the p r e c i p i t a t i o n of h a f n i u m h y d r o x i d e . It is not u s u a l l y a d v i s a b l e to d e p e n d o n the s o l u b i l i t y p r o d u c t b e c a u s e the i n f o r m a t i o n o n this q u a n t i t y is often u n r e l i a b l e for h y d r o x i d e s of p o l y v a l e n t m e t a l ions. I n a d d i t i o n , "radiocolloids" may apparently form m u c h below saturation conditions i n r a d i o a c t i v e isotope solutions. I n the specific case of h a f n i u m h y d r o x i d e o n l y t w o measurements of the s o l u b i l i t y seem to h a v e b e e n A c c o r d i n g to L a r s o n a n d G a m m i l l (16)

K

4

only

X

10"

Hf(OH) 10"

55

2

26

2 +

.

(15).

a s s u m i n g the

existence

s

of

T h e second r e p o r t e d v a l u e is K

=

s o

[Hf(OH) one

2

2 +

reported.

] [OH-]

hydrolyzed

— [Hf ] [ O H " ] 4 +

4

2

=

species =

If one uses the s o l u b i l i t y d a t a b y L a r s o n a n d G a m m i l l

3.7

X

(Ref.

16, T a b l e s I a n d I I I ) a n d takes i n t o c o n s i d e r a t i o n a l l m o n o m e l i c h a f n i u m species ( 2 3 ) a K

so

Because

of

v a l u e of 4 X 1 0 " the i n c o n s i s t e n c y

58

is c a l c u l a t e d . of

these results, experiments

were

c a r r i e d out to establish the p r e c i p i t a t i o n b o u n d a r i e s , as d e s c r i b e d earlier. F i g u r e 1 gives as a n e x a m p l e f o u r curves i n w h i c h the f r a c t i o n of h a f n i u m r e m o v e d b y p r e c i p i t a t i o n as h y d r o x i d e is p l o t t e d against the p H for four different concentrations of H f C l . 4

O p e n a n d b l a c k e n e d symbols are for

experiments i n w h i c h systems w e r e e q u i l i b r a t e d before c e n t r i f u g a t i o n 1 h o u r a n d 70 h o u r s , respectively.

I n a l l cases i n s o l u b l e precipitates are



50

ADSORPTION F R O M

10.0

AQUEOUS SOLUTION

12.0

Figure 1. The fraction of hafnium tetrachloride removed from solution by centrifugation at 3,500 r.p.m. for 1 /2 hour as a function of the pH. Open and blackened symbols represent centrifugation 1 hour and 70 hours after mixing the precipitating components, respectively. Squares represent the fraction of hafnium removed by ultracentrifugation at 25,000 r.p.m. for 1 hour and the corresponding dashed line represents the curves which would remit from these studies f o r m e d a b o v e a c e r t a i n p H a n d this l i m i t increases w i t h a decrease i n the salt c o n c e n t r a t i o n .

I n these examples the p H range for the onset of

p r e c i p i t a t i o n varies b e t w e e n 5.7 a n d 6.5. A b o v e this p H the r e m o v a l of h a f n i u m b y p r e c i p i t a t i o n f r o m the s o l u t i o n of h a f n i u m c h l o r i d e is n e a r l y complete.

H o w e v e r , except for the highest c o n c e n t r a t i o n of H f C l

4

the

p r e c i p i t a t i o n r e g i o n is f o l l o w e d at still h i g h e r p H values b y a r e g i o n over w h i c h p r e c i p i t a t i o n a n d s e t t l i n g of h a f n i u m h y d r o x i d e does not take


5.

STRYKER

Hydrolyzed

A N D M A T I J E V I G

Hafnium

p l a c e u n d e r the e x p e r i m e n t a l c o n d i t i o n s e m p l o y e d .

51

Ions

T h i s c o u l d either b e

c a u s e d b y the f o r m a t i o n of s o l u b l e a n i o n i c complexes of h a f n i u m or b y e x t r e m e l y finely d i s p e r s e d h y d r o x i d e .

I n o r d e r to d i s t i n g u i s h b e t w e e n

the t w o possibilities several samples i n the second r e g i o n of l o w fractions r e m o v e d w e r e u l t r a c e n t r i f u g e d as d e s c r i b e d . 10.3 a n d 10.6 for

1 X

10" M and 5 5

X

Results at p H values of

10 M HfCl , _ 6

4

respectively,

s h o w e d that the h a f n i u m is c o m p l e t e l y r e m o v e d f r o m s o l u t i o n i n d i c a t i n g the presence of finely d i s p e r s e d h y d r o x i d e .

T h e s e points are i n d i c a t e d

i n the d i a g r a m as squares. If a l l the systems over the h i g h p H r a n g e w e r e Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0079.ch005

u l t r a c e n t r i f u g e d the curves w o u l d l o o k as i n d i c a t e d b y the d a s h e d lines. T h e o r i g i n a l measurements at h i g h e r p H values o b t a i n e d u s i n g the l o w e r speed centrifuge are r e p o r t e d to s h o w that erroneous conclusions m a y easily result o w i n g to the s t a b i l i t y of the extremely

finely

dispersed

h a f n i u m h y d r o x i d e . N o attempt w a s m a d e at this p o i n t to c h a r a c t e r i z e this h y d r o x i d e sol. S i m i l a r u l t r a c e n t r i f u g a t i o n experiments w e r e c a r r i e d out w i t h sys tems at p H values b e l o w the p r e c i p i t a t i o n r e g i o n . H o w e v e r , the results w e r e i d e n t i c a l to those o b t a i n e d u s i n g the l o w e r speed c e n t r i f u g e as s h o w n i n F i g u r e 1. It is therefore c o n c l u d e d that over the l o w p H r a n g e a n d the concentrations u s e d the solutions of h a f n i u m t e t r a c h l o r i d e are v o i d of c o l l o i d a l p a r t i c l e s .

o

-

UJ CD

O

afro—^Q^.

o 1 CO

s § .8 O ho

Adsorption of Hydrolyzed Hafnium Ions on Glass - Advances in

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