Relations Among Equilibrium and Nonequilibrium Aqueous Species of

10 Relations Among Equilibrium and Nonequilibrium Aqueous Species of Downloaded by UNIV OF MISSOURI COLUMBIA on March 15, 2013 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0106.ch010

Aluminum Hydroxy Complexes ROSS W. S M I T H University of Nevada, Reno, N e v . 89503

The form by

of aluminum

preparing

of aluminum composition

compositions colorimetric

of three separate a

b

Al , Al

of small,

n

values)

and Al .

Al

solid

Al(OH) a

of Al

tration,

c

and Al

increased

In all cases, equilibrium

the

of

were

aging

determined estimation

that have been c

material.

was constant,

varying

that allowed

particles.

3

concentration

but with

was composed

was polynuclear

studied

concentration

as a function

types of aluminum a

was

and determining

of the solutions procedure c

Al , b

species.

(r

and pH of the solutions

The

media

constant

ionic strength,

to aluminum

by a timed nated

containing

and constant -

ratios of OH time.

in acid aqueous

solutions

Al

of

b

Al

monomeric

was

For each r

in concentration

composed value,

n

decreased

was only slowly

desig-

in

the

concen-

with aging

time.

achieved.

' " p h e n a t u r e o f a l u m i n u m ( I I I ) i n aqueous e n v i r o n m e n t s has b e e n exp l o r e d i n a n u m b e r o f papers (1-24).

I f t h e p H o f t h e s o l u t i o n is

a b o v e n e u t r a l i t y , i t appears t h a t t h e p r e d o m i n a n t species present is t h e a n i o n A l ( O H ) ( H 0 ) - (8, 9,10,13). 4

2

D e l t o m b e a n d P o u r b a i x (4) w r i t e

2

this species as A 1 0 " a n d / o r H A 1 0 ~ . I f t h e p H is b e l o w a b o u t 4, m o s t 2

2

3

authors agree that t h e h e x a a q u o - A l ( I I I ) i o n A 1 ( H 0 ) 2

6

3 +

dominates. B e -

t w e e n p H 4 a n d 7, there is l i t t l e agreement as to w h a t species are present. Schofield a n d T a y l o r ( 2 1 ) , F r i n k a n d Peech ( 5 ) , a n d R a u p a c h

(17)

b e l i e v e that t h e system c a n b e h a n d l e d s a t i s f a c t o r i l y i n this r e g i o n o n t h e basis o f s i m p l e m o n o m e r i c species.

T h e agreement a m o n g t h e i r deter-

m i n a t i o n s o f K i , t h e e q u i l i b r i u m constant f o r t h e r e a c t i o n

( p K values 4.98, 5.02, a n d 4.97, r e s p e c t i v e l y ) is excellent.

In Nonequilibrium Systems in Natural Water Chemistry; Hem, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

10.

Equilibrium

SMITH

in Aluminum

Hydroxy

251

Complexes

I n spite of this g o o d agreement a n d the f a c t t h a t s i m p l e h y d r o l y s i s reactions represent t h e i r d a t a w e l l , the s i m p l e h y d r o l y t i c m e c h a n i s m has b e e n q u e s t i o n e d b y a n u m b e r of researchers. Brosset ( 2 ) , f r o m measurements of t h e p H of a l u m i n u m p e r c h l o r a t e solutions, p o s t u l a t e d t h a t the p r o d u c t of h y d r o l y s i s of the a l u m i n u m i o n is a n infinite series of p o l y n u c l e a r complexes w i t h t h e g e n e r a l i z e d f o r m u l a Al[(OH) Al] 3

n

3 +

.

Brosset, B i e d e r m a n n , a n d Sillén ( 3 )

later r e c a l c u l a t e d

Brossets data a n d concluded that the major hydrolysis product c o u l d be either a single c o m p l e x s u c h as Α 1 ( Ο Η ) ι Downloaded by UNIV OF MISSOURI COLUMBIA on March 15, 2013 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0106.ch010

6

plexes of the t y p e A l [ ( O H ) A l ] n 5

2

+ 3 + w

3 +

or a n infinite series of c o m

. Aveston, using ultracentrifugation,

f o u n d evidence for either [ A l ( O H ) ] 2

δ

2

or [ A l

4 +

species has b e e n f a v o r e d r e c e n t l y b y Sillén ( 2 5 ) A c c o r d i n g to H s u a n d Bates (11, 12),

1 3

(OH) 2] 3

7 +

.

T h e latter

a n d b y Johansson

(26).

d i s s o l v e d a l u m i n u m b e l o w or

near p H 4 is i n the f o r m of a s i x - m e m b e r r i n g of a p p r o x i m a t e c o m p o s i t i o n Al (OH)i2 6

6 +

.

T h e r i n g s p o l y m e r i z e as p H is i n c r e a s e d a b o v e 4 w i t h a n

average e q u i l i b r i u m p o l y m e r i z a t i o n n u m b e r that increases w i t h p H u n t i l p H 7 is e x c e e d e d . W h e n this h a p p e n s , b a y e r i t e o r g i b b s i t e is p r e c i p i t a t e d . T h u s , i n t h e i r scheme, p o l y n u c l e a r complexes of a size d e t e r m i n e d b y p H exist i n solution—i.e., the h i g h e r the p H u p to 7, the greater t h e size of the complexes.

A l s o , as p H increases, the average c h a r g e p e r a l u m i n u m

a t o m decreases f r o m a b o u t Γ at p H 4 to 0 n e a r p H 7 w h e r e the r a t i o of O H / Α Ι is 3 or greater. U p t o this p o i n t the h y d r o x o - a l u m i n u m p o l y m e r s w o u l d r e p e l one another a n d l i m i t t h e i r g r o w t h , b u t s l i g h t l y a b o v e i t t h e y s h o u l d a n d d o r a p i d l y p r e c i p i t a t e as A l ( O H ) . A t s t i l l h i g h e r p H v a l u e s , 3

m o r e a l u m i n u m goes i n t o s o l u t i o n as i n c r e a s i n g amounts of A l ( O H ) ~ 4

are f o r m e d . H e m and Roberson

(8)

b e l i e v e t h a t p o l y m e r i c species,

probably

s i x - m e m b e r rings a n d c o m b i n a t i o n s of these r i n g s , f o r m r a p i d l y i n f r e s h l y p r e p a r e d s u p e r s a t u r a t e d a l u m i n u m solutions i n w h i c h the i n i t i a l p H is b e t w e e n 4 a n d 7 a n d the r a t i o of O H to a l u m i n u m i n complexes b e t w e e n 0.6 a n d 3.

averages

T h e c o m p l e x species are n o t stable, h o w e v e r ,

and

u l t i m a t e l y g r o w to a size that m u s t be c o n s i d e r e d a s o l i d phase. A l s o , t h e v e r y large p o l y m e r s are o r g a n i z e d a n d a p p e a r to b e c r y s t a l l i n e w i t h t h e structure of g i b b s i t e , yet are s m a l l e n o u g h to pass a 0 . 4 5 - m i l l i p o r e a n d h e n c e s h o u l d be c o n s i d e r e d c o l l o i d a l .

filter

After e q u i l i b r i u m becomes

e s t a b l i s h e d , w h i c h m a y take years, it m a y b e t h a t the o n l y i o n i c a l u m i n u m species present i n significant q u a n t i t i e s are the m o n o m e r i c A l ( O H ) 2

Al(OH)(OH ) 2

5

2 +

6

3

\

, A l ( O H ) ( O H ) \ a n d A l ( O H ) " ions. 2

2

4

4

It is possible t h a t d i s t r i b u t i o n of A l ( I I I ) a m o n g

hydroxo-complexes

is h i g h l y v a r i a b l e a n d is sensitive to i o n i c s t r e n g t h , t o t a l a l u m i n u m c o n c e n t r a t i o n , t o t a l O H a v a i l a b l e , p H , t e m p e r a t u r e , the i d e n t i t y of

other

species present s u c h as N 0 " , C 1 0 " , S 0 " , C I " , etc., a n d , p e r h a p s

most

3

4

4

2


252

NONEQUILIBRIUM SYSTEMS IN N A T U R A L WATERS

i m p o r t a n t , t i m e . T h e latter factor appears p a r t i c u l a r l y i m p o r t a n t , c o n s i d e r i n g the e x p e r i m e n t a l w o r k of H e m a n d R o b e r s o n ( 8 ) a n d t h e statem e n t b y Brosset, B i e d e r m a n n , a n d Sillén (3) ties w e r e

that "Considerable difficul-

m e t w i t h b e c a u s e e q u i l i b r i u m was

attained rather slowly,

e s p e c i a l l y i n the r e g i o n w h e r e p r e c i p i t a t i o n o c c u r r e d .

It is, h o w e v e r ,

t h o u g h t t h a t the values finally g i v e n w e r e not f a r f r o m those at r e a l e q u i librium.

F r o m the present w o r k , i t w o u l d a p p e a r that the d a t a u s e d b y

, ,

these authors w e r e c o n s i d e r a b l y f u r t h e r f r o m e q u i l i b r i u m t h a n t h e y h a d Downloaded by UNIV OF MISSOURI COLUMBIA on March 15, 2013 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0106.ch010

t h o u g h t . A t a n y rate, i t appears that the n a t u r e of A l ( I I I ) i n a q u e o u s m e d i a is n o t w e l l k n o w n , p a r t i c u l a r l y at m i l d l y a c i d p H values. Experimental T h e f o r m of a l u m i n u m i n a c i d a q u e o u s m e d i a w a s s t u d i e d b y p r e p a r i n g a series of solutions c o n t a i n i n g the same t o t a l c o n c e n t r a t i o n

of

a l u m i n u m , b u t w i t h v a r y i n g a m o u n t s of a d d e d base a n d d e t e r m i n i n g the c o m p o s i t i o n a n d p H of the solutions after v a r i o u s p e r i o d s of a g i n g .

For

c o n v e n i e n c e , these solutions w i l l b e d e s i g n a t e d " a g i n g s t u d y s o l u t i o n s . " E l e c t r o n m i c r o s c o p y w a s u s e d to h e l p d e t e r m i n e the n a t u r e of c o l l o i d a l size m a t e r i a l t h a t f o r m e d i n some of the solutions. T h e solutions s t u d i e d c o n t a i n e d 4.54

X

10~ m o l e / l i t e r a l u m i n u m 4

a n d t o t a l i o n i c s t r e n g t h was 10" , the r e m a i n d e r of the t o t a l i o n i c strength 2

b e i n g m a d e u p w i t h s o d i u m a n d p e r c h l o r a t e ions. T h e r a t i o of O H to A l i n the solutions as m a d e u p ( n o m i n a l r v a l u e or r ) n

to 3.01.

v a r i e d f r o m 0.55

I n p r e p a r i n g these a g i n g s t u d y solutions, three " s t o c k " solutions

w e r e p r e p a r e d i n i t i a l l y a n d m i x e d together a c h i e v e the d e s i r e d r

n

value.

T h e procedure

described by H e m and Roberson (8).

i n correct p r o p o r t i o n s

to

for d o i n g this has b e e n

I n a l l cases, the s o l u t i o n c o n t a i n -

i n g base, b u t n o a l u m i n u m , w a s a d d e d

last i n s o l u t i o n p r e p a r a t i o n .

R e a g e n t g r a d e c h e m i c a l s w e r e used. Analytical

Procedure

T h e c o m p o s i t i o n s of solutions w e r e d e t e r m i n e d b y a t i m e d spectrop h o t o m e t r i c m e t h o d . T h e t e c h n i q u e w a s a m o d i f i c a t i o n of a s t a n d a r d f e r r o n - o r t h o p h e n a n t h r o l i n e m e t h o d f o r a l u m i n u m (27, 28) a n d is i d e n t i c a l i n p r i n c i p l e to a m e t h o d d e v e l o p e d b y T u r n e r (22). T h e method was m o d i f i e d i n the f o l l o w i n g m a n n e r . Standard

Modification

Procedure

1 ) P i p e t a v o l u m e of s a m p l e c o n t a i n i n g not m o r e t h a n 0.075 m g (25 m l max.) into a 50-ml beaker a n d adjust the v o l u m e to 25.0 m l .

1)

S a m e as s t a n d a r d p r o c e d u r e .


10.

SMITH

Equilibrium

in Aluminum

2 ) P r e p a r e a 2 5 - m l metal-free w a t e r b l a n k a n d necessary s t a n d ards. 3 ) A d d 2.0 m l N H O H H C 1 r e agent to b l a n k standards a n d s a m p l e a n d let s t a n d 30 m i n u t e s . 2

2)

253

Complexes

S a m e as s t a n d a r d p r o c e d u r e .

3 ) A d d 2 m l N a C H 0 to 5 m l ferron-orthophenanthroline re agent ( r e a g e n t is t w i c e as s t r o n g i n f e r r o n as s t a n d a r d p r o c e d u r e ) . 2

3

2

4 ) A d d 5.0 m l f e r r o n - o r t h o p h e n a n t h r o l i n e reagent a n d stir.

4 ) A d d ( 3 ) a b o v e to b o t h b l a n k a n d s a m p l e a n d stir.

5 ) A d d 2.0 m l N a C H 0 . Stir a n d let s t a n d for at least 10 m i n utes b u t not m o r e t h a n 30 m i n u t e s before t a k i n g a r e a d i n g of color.

5 ) A s q u i c k l y as possible ( a t least w i t h i n 4 m i n u t e s ) , a d d 2.0 m l N H o O H H C 1 reagent, stir q u i c k l y , a n d at the same t i m e start t i m i n g .

6 ) D e t e r m i n e the a b s o r b a n c y of the test s a m p l e a n d standards against the b l a n k at 370 χημ.

6 ) A s q u i c k l y as possible, r e a d a b s o r b e n c y against the b l a n k at 370 m/i. T i m e the r e a d i n g a n d c o n t i n u e to take t i m e d r e a d i n g s (at 3 - 4 m i n i n t e r v a l s for the first h a l f h o u r , t h e n at w i d e r i n t e r v a l s for as l o n g as n e c e s s a r y ) .

2

Downloaded by UNIV OF MISSOURI COLUMBIA on March 15, 2013 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0106.ch010

Hydroxy

3

2

It s h o u l d b e n o t e d i n the m o d i f i e d p r o c e d u r e that the last reagent a d d e d is the h y d r o x y l a m i n e h y d r o c h l o r i d e , w h i c h b r i n g s the p H to a b o u t 5. T h u s , a p H l o w e r t h a n this v a l u e is not o b t a i n e d at a n y stage of a d d i n g a n a l y t i c a l reagents, u n l i k e i n the s t a n d a r d p r o c e d u r e w h e r e the h y d r o x y l a m i n e is a d d e d at the start of the p r o c e d u r e a n d u s u a l l y l o w e r s p H of the s a m p l e to a b o u t 1.5.

H e m and Roberson (8)

discuss at some l e n g t h

the possible c o m p l e x f o r m e d b e t w e e n f e r r o n a n d a l u m i n u m . W h e n o p t i c a l d e n s i t y of a l u m i n u m standards p r e p a r e d b y d i s s o l v i n g e i t h e r a l u m i n u m w i r e i n H C 1 s o l u t i o n or a l u m i n u m sulfate i n w a t e r is m e a s u r e d as a f u n c t i o n of a l u m i n u m c o n c e n t r a t i o n u s i n g the m o d i f i e d p r o c e d u r e , curves of the t y p e s h o w n i n F i g u r e 1 are o b t a i n e d .

Optical

d e n s i t y values p l o t t e d o n this figure, w h i c h are for d u p l i c a t e sets of ex p e r i m e n t s , w e r e r e a d a b o u t 30 m i n u t e s after h a v i n g a d d e d the 2 m l of hydroxylamine hydrochloride.

T h e straight l i n e c u r v e appears to o b e y

Beer's l a w at least u p to 0.05-0.06 m g of a l u m i n u m . T h e p H of

the

s t a n d a r d solutions b e f o r e analysis w a s i n a l l cases b e l o w 3. T h e m o d i f i e d p r o c e d u r e a l l o w e d for the e s t i m a t i o n of three different types of a l u m i n u m present i n a p a r t i c u l a r s a m p l e at a n y p a r t i c u l a r a g i n g t i m e , b a s e d o n the m a n n e r i n w h i c h the v a r i o u s types of a l u m i n u m r e a c t e d w i t h f e r r o n . T h e w a y i n w h i c h the e s t i m a t i o n is m a d e is i l l u s t r a t e d b y F i g u r e s 2 a n d 3, w h i c h h a v e b e e n c a l c u l a t e d f r o m r a w a n a l y t i c a l d a t a . S h o w n o n F i g u r e 2 is a l u m i n u m r e c o v e r e d as a f u n c t i o n of analysis t i m e after h a v i n g a d d e d the h y d r o x y l a m i n e reagent. cal absorbance

I n this figure, o p t i

r e a d i n g s h a v e b e e n c o n v e r t e d to m o l a r i t i e s a n d

u s i n g p r o p e r factors to a c c o u n t for a l i q u o t s t a k e n , etc.

ppm's

C u r v e s are for



254

NONEQUILIBRIUM SYSTEMS IN N A T U R A L

.01

.02 .03 ALUMINUM

.04 (mg.)

.05

WATERS

.06·

Figure 1. Optical density as a function of aluminum concentration for aluminum standards using the mod ified ferron procedure solutions w i t h r

n

values r a n g i n g f r o m 0.94 to 2.76 a g e d 625 h o u r s , a n d

i n a d d i t i o n curves are s h o w n for a s o l u t i o n w i t h a n r v a l u e of 2.13 a g e d n

f r o m 23 h o u r s to 961 days. c o n c e n t r a t i o n of 4.54 χ

A l l these solutions h a d a t o t a l a l u m i n u m

10" m o l e / l i t e r . A l s o s h o w n are s i m i l a r t i m e d 4

d a t a for 2 5 - m l s t a n d a r d solutions c o n t a i n i n g 2, 6, a n d 10 p p m a l u m i n u m . T h e fact that the c o l o r i m e t r i c r e a d i n g s for the s t a n d a r d solutions c h a n g e d l i t t l e d u r i n g the three h o u r s of analysis t i m e is g o o d e v i d e n c e t h a t a m e r e c o l o r c h a n g e of f e r r o n w i t h t i m e is n o t w h a t is b e i n g o b s e r v e d w h e n the a m o u n t of a l u m i n u m r e c o v e r e d increases w i t h analysis t i m e . A d d i t i o n a l e v i d e n c e is to b e f o u n d i n the w a y i n w h i c h the curves for the s o l u t i o n with r = n

2.13 c o n v e r g e w h e n e x t r a p o l a t e d to z e r o analysis t i m e .


10.

SMITH

Equilibrium

in Aluminum

Hydroxy

255

Complexes

F i g u r e 2 shows that the solutions c o n t a i n e d three different types o f a l u m i n u m species, one o f w h i c h d i s a p p e a r e d s l o w l y d u r i n g l o n g a g i n g . F o r c o n v e n i e n c e , the three species w i l l b e r e f e r r e d t o as A P , A P , a n d A l , c

a n d f r o m the s t o i c h i o m e t r y o f p r e p a r a t i o n o f t h e solutions, i t is k n o w n that i n e a c h case A l = A l + A P + A P = 4.54 X IO" m o l e / l i t e r . I t i s a

4

i n t e r e s t i n g t o note t h a t T u r n e r ( 2 2 ) ,

using his similar analytical tech-

n i q u e , also c o n c l u d e d t h a t three different types o f a l u m i n u m c a n exist i n aqueous media. Downloaded by UNIV OF MISSOURI COLUMBIA on March 15, 2013 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0106.ch010

T h e fastest r e a c t i n g f o r m , A P , i s c o n v e r t e d t o the f e r r o n

complex

almost i m m e d i a t e l y , a n d for a p a r t i c u l a r r v a l u e is present i n n e a r l y the n

same a m o u n t regardless o f a g i n g t i m e , as s h o w n b y t h e c o n v e r g e n c y o f a l l t h e d e t e r m i n a t i o n s a t z e r o t i m e for t h e s o l u t i o n w i t h a n r v a l u e o f n

2.13. T h e slowest r e a c t i n g m a t e r i a l , A P , i s represented b y the n e a r l y flat

80 ANALYSIS

TIME

Figure 2. Aluminum recovered as a function of analysis time for selected aging study solutions plus several standards


256


slope s h o w n b y the s a m p l e r e a d i n g s after a b o u t 120 m i n u t e s . M e t a s t a b l e m a t e r i a l , A P , is t h e m a t e r i a l w h i c h reacts w i t h a n i n t e r m e d i a t e r a t e , g i v i n g the c u r v e d p o r t i o n s o f the lines o f F i g u r e 2. F r o m these curves, i t is a s i m p l e m a t t e r t o measure t h e a m o u n t s present o f the t h r e e types o f a l u m i n u m . B y e x t r a p o l a t i n g t h e essentially s t r a i g h t p o r t i o n s o f the c u r v e s ( A l sections) b a c k to z e r o t i m e , one c a n c

estimate the t o t a l q u a n t i t y o f A l

a

+

A P present a t a p a r t i c u l a r a g i n g

t i m e . A l c a n b e o b t a i n e d d i r e c t l y f r o m the zero t i m e v a l u e o f a l u m i n u m a

r e c o v e r e d a n d A P o b t a i n e d b y s u b t r a c t i n g the v a l u e o f A l f r o m the v a l u e Downloaded by UNIV OF MISSOURI COLUMBIA on March 15, 2013 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0106.ch010

a

of A l

a

+

A P . H o w e v e r , i t is s o m e w h a t

difficult t o extrapolate

these

curves a c c u r a t e l y , a n d thus the curves o f the t y p e o f F i g u r e 3 w e r e c o n s t r u c t e d t o d e t e r m i n e m o r e a c c u r a t e l y A l a n d A P concentrations a n d to a

obtain

additional information

o n the ferron

reaction

with

type

b

aluminum.

625 HRS.

505 HRSAGE

I I I I I I I

AGE

ι ι ι ι ι ι I I

Δ SOLUTION Β Ο

«

Ε

•

Il

6

1

J6

1

ia

1

J6

1

jo jo ANALYSIS

Figure

3.

TIME

1

46

A)

1

8*6

(min.)

Minus log At residual as a function of analysis time for age study solutions aged 505-625 hours 3

F i g u r e 3 shows first o r d e r rate plots o f t h e n e g a t i v e l o g o f A P r e s i d u a l vs. t i m e f o r solutions a g e d 505 to 625 h o u r s . T h e i n d i v i d u a l A P r e s i d u a l points w e r e d e t e r m i n e d b y s u b t r a c t i n g d a t a p o i n t s o f F i g u r e 2 f r o m the v a l u e s o f the e x t r a p o l a t e d ( d o t t e d ) fines a t the same analysis r e a c t i o n times. B e c a u s e the curves o b t a i n e d w e r e consistently g o o d , s t r a i g h t l i n e s , e v e n w h e n 9 0 % o f the r e a c t i n g A P h a d b e e n c o n s u m e d , i t w a s a s i m p l e m a t t e r t o extrapolate t h e m t o zero t i m e t o o b t a i n m o r e exact values o f A P a n d therefore also A P . T h u s , the k i n e t i c b e h a v i o r o f the a l u m i n u m


10.

Equilibrium

SMITH

in Aluminum

Hydroxy

257

Complexes

species p r o v i d e d a c o n v e n i e n t means of i d e n t i f i c a t i o n . A l s o of interest is the f a c t t h a t A P reacts w i t h f e r r o n a c c o r d i n g to a first o r d e r rate l a w .

General Equation The

for

Curves

of Figure

2

f o l l o w i n g e q u a t i o n s h o u l d b e g e n e r a l for the curves of F i g u r e 2,


a s s u m i n g a l l of t y p e a a l u m i n u m to react w i t h f e r r o n i n s t a n t a n e o u s l y η >

0

( a n d η is p r o b a b l y close to z e r o )

— H Al - ~ -

c

= W

(2b)

w h e r e k is a rate constant of some u n k n o w n o r d e r for t y p e c a l u m i n u m . c


258


Table I.

First O r d e r Rate Constants for Α Γ Aging


Solution Β C D Ε F G H J a

23 Hours

48 Hours

9.9 7.8 5.5 5.8 5.1 5.1 4.4 3.2

8.5

-

-

4.6

-

4.4 -

4.8 5.1

-

7.4 5.8 5.8 5.3 5.3 5.1 4.4

—

5.5 4.8

288 Hours

168 Hours

96 Hours

5.5 5.5

-

5.1

-

3.9

-

-

Average (48 hours-259 days) : Β 7.0; C 5.3; D 5.2; Ε 5.3; F 4.2; G 4.2; H 3.8.

Integrating between the limits 0 a n d t a n d A 1 Al^"

= Alo "

n

c l

n

0

C

and Al*

c

- T ^ — 1 — η

and A V

= (AV

1 - 7 1

(4)

γ ^ ) ^

-

Substituting Equations 3 and 4 into E q u a t i o n 1 *A1

T

= A P + [AU 6

Al e-H

Al°

pH

1.0 23 48 168 505 41 77 116 188 254

hrs hrs hrs hrs hrs days days days days days

2.24 1.56 1.82 1.46 1.44 1.50 1.53 1.60 1.51 1.50

1.50 2.10 2.00 1.86 2.14 1.81 1.45 1.20 0.49 0.12

0.80 0.88 0.72 1.22 0.96 1.23 1.56 1.74 2.54 2.92

4.79 4.68 4.66 4.61 4.56 4.54 4.54 4.52 4.44 4.32

O l d solution : r„ v a l u e n e a r Ε 1038 d a y s

1.79

-

2.77

4.13


262


T a b l e II.

Continued

Solution (r» =

F 8.18)

Concentration


Age

Time

1.2 h r s

X 10*

Al*

Al

Al'

pH

1.46 1.11 1.12

1.78 2.05 1.86

1.30

4.88

1.38 1.56

4.75

1.12

1.86 1.82

b

23

hrs

96 168

hrs hrs

288

hrs

625

hrs

1.05 1.05

1.35

1.56 1.67 2.04

46

days

1.02

0.78

2.74

82 121

days days

1.20 1.21

3.08

4.37 4.30

193

days

3.23 3.43

4.28 4.22

259

days

1.07 1.02

0.26 0.10 0.04 0.02

3.50

4.20

-

3.33

4.19

4.63 4.60 4.52 4.42

Old solution: r v a l u e near F n

967

days

1.16

Solution (r

n

G

= 247) Concentration

Age

Time

1.2 h r s

X 10*

Al*

AI»

AI"

1.17 0.60 0.60

1.97 1.20 1.15

1.40 2.74

1.05

2.79 2.94

0.81

3.09

4.49

0.40 0.13 0.043 0.019

3.46 3.63 3.84

4.42 4.40

0.007

3.91

23 48

hrs hrs

168

hrs

505

hrs

0.55 0.64

41 77 116 188 254

days days days days days

0.68 0.68 0.66 0.61 0.62

3.91

pH 5.02 4.82 4.77 4.75

4.35 4.31 4.28


10.

Equilibrium

SMITH

in Aluminum

T a b l e II.

Hydroxy

263

Complexes

(Continued)

Solution H (r« = 2.76)


Concentration Age

Time

1.1 23 96 168 288 625 46 82 121 193 259

hrs hrs hrs hrs hrs hrs days days days days days

X 10*

Al*

Al

Al

pH

0.78 0.20 0.20 0.23 0.24 0.24 0.28 0.29 0.284 0.232 0.235

1.36 0.50 0.40 0.39 0.36 0.30 0.17 0.05 0.011 0.008

3.40 3.84 3.94 3.92 3.94 4.00 4.09 4.20 4.24 4.30 4.30

5.23 5.16 5.00 4.76 4.72 4.62 4.53 4.51 4.49 4.45 4.45

e

b

Solution J ( r . = 3.01) Concentration

Age

Time

1.0 23 48 168 505 41 77 116 188

hrs hrs hrs hrs hrs days days days days

Of

X 10*

Al*

Al>

Al"

pH

0.56 0.08 0.07 0.05 0.05 0.065 0.08 0.057 0.019

1.90 0.22 0.16 0.07 0.03 0.01

2.08 4.24 4.31 4.42 4.46 4.47 4.46 4.48 4.52

7.15 6.64 6.59 6.52 6.54 6.43 6.28 6.40 6.21

--

interest f r o m this t a b l e is the f a c t t h a t for most solutions A l

a

c o n c e n t r a t i o n r e m a i n s constant after 23 hours of a g i n g , A P decreases as a f u n c t i o n of a g i n g t i m e , a n d A P increases. T h e r e l a t i o n s h i p b e t w e e n

r

n

v a l u e a n d A P v a l u e is s h o w n g r a p h i c a l l y i n F i g u r e 4. Experimental

Character

of

Al

&

S i n c e A P reacts almost i n s t a n t l y w i t h f e r r o n , i t w o u l d seem r e a s o n able

that

Al(OH)

2 +

,

i t consists Al(OH)

water molecules). (AG°)

2

+

of , and

only

simple

Al(OH) ~ 4

S t a n d a r d G i b b s free

monomeric (with

species—i.e.,

appropriate

energies

of

Al , 3 +

coordinated

formation

values

for the species are a v a i l a b l e i n the c h e m i c a l l i t e r a t u r e . T a b l e I I I



264


r

value

Figure 4. Concentration of Al* as a func tion of r value for solutions containing 4.54 X 10~ mole/liter total aluminum n

4

lists some of these values p l u s a G i b b s free e n e r g y v a l u e f o r g i b b s i t e [«Al(OH) ]. 3

T h e values for A l , A l ( O H ) 3 +

2 +

, A l ( O H ) " , and « A l ( O H ) 4

3

w e r e se

l e c t e d to b e consistent w i t h the w o r k of H e m a n d R o b e r s o n . F e w v a l u e s for A l ( O H )

2

+

w e r e a v a i l a b l e , a n d R a u p a c h s w a s selected for consistency

because of the use of his v a l u e for A l ( O H ) . 2+

F r o m d a t a of this t a b l e , the f o l l o w i n g equations c a n b e w r i t t e n . A 1 ( 0 H ) . (gibbsite) + 3 H + *± A l + + 3 H 0 3

*K

B0

=

10+ · 8

2

22

Table III. Gibbs Free Energies of Formation of Monomeric A l u m i n u m Species

Species Al + Al(OH) + Al(OH) + Al(OH) α Α 1 ( Ο Η ) (gibbsite) 3

2

2

4

3

Standard Gibbs Free Energies of Formation (AG°kcal)

Reference

-115.0 -164.9 -215.1 -311.7 -273.9

(29) (18,19) (18,19) (8) (29)


(7)

10.

Equilibrium

SMITH

Al(OH)

8

in Aluminum

Hydroxy

265

Complexes

(gibbsite) + 2 H +

Relations Among Equilibrium and Nonequilibrium Aqueous Species of

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