Effect of Aging on Aluminum 

Hydroxide Complexes in 

Dilute Aqueous Solutions

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1827-D
Effect of Aging on Aluminum 

Hydroxide Complexes in 

Dilute Aqueous Solutions

By R. W. SMITH and J. D. HEM

CHEMISTRY OF ALUMINUM IN NATURAL WATER 

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1827-D

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1972
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress catalog-card No. 79-189478

For sale by the Superintendent of Documents, U.S. Government Prlntim, Office

Washington, D.C. 20402 - Price 40 cents (paper cover)

Stock Number 2401-2144
CONTENTS

Page

Abstract.__.._._.__..___._._..._....._....._....._..._._._..__..___ Dl

Acknowledgments.________________--___ _______-___-___-_______--_-- 1

Purpose and scope.__________________________________________________ 2

Experimental techniques.____________________________________________ 3

Characteristics of Ala __________________________----_-__--______--___- 5

Characteristics of Alb and Al°_ ________________________________________ 18

Electron microscope observations...___________________________________ 26

Effect of rate of addition of base on Alb ._______________________________ 28

Approach of solutions to equilibrium...._______________________________ 32

Kinetics and structure of nonequilibrium species.___________________ 32

Evaluation of aging process by nonequilibrium thermodynamics. _ _ _ _ _ _ 38

Calculation of the Gibbs free energy of the "face" and "edge" of gibbsite in

equilibrium with its saturated solution.._____________________________ 42

Conclusions relative to aluminum in mildly acid aqueous media.__________ 49

References. ___ ____-_______-______--___-____-__------__--__-------__- 50

ILLUSTRATIONS

Page

FIGURES 1-3. Graphs showing concentration of 

1. Ala as a function of rn value.___________________ D6

2. Ala as a function of pH for solutions B-H at variour

aging times____________________.____________ 8

3. Ala+b as a function of pH at various aging times. __ 9 

4-11. Graphs showing aging time effect on activity of monomeric 

species as a function of pH for 

4. Solution B---_-----___------__---___-_------- 10

5. Solution C-------_---------------_----------- 11

6. Solution D_.___._____________________________ 12

7. Solution E--__--_______-__--___-_--_--------- 13

8. Solution F_____.___________-__--__-----__-_-- 14

9. Solution G_-_-_-_-_.-_-.-_-._-__--__-_-_-__-- 15

10. Solution H_ _________________________________ 16

11. Solution J___________________________________- 17

12-18. Concentration of Alb and Al° as a function of pH and aging 

time for 

12. Solution B___________________________________ 19

13. Solution C-____,___________-_-_-_---_--_---_- 20

14. Solution D______________________________-__-_ 21

15. Solution E_.___._______-_-------_---_-----_-- 22

III
IV CONTENTS

Page

FIGURES 12-18. Concentration of Alb and Al c as a function of pH and aging 

time for Continued

16. Solution F.__________________________________ D23

17. Solution G__-._.___----.--_...__________.____ 24

18. Solution H____.____---.-_._.-__-.___.___.____ 25 

19-22. Electron micrograph of microcrystalline gibbsite f-om 

19. Solution F aged 2 years..___.._._..._.._____.__ 26

20. Solution C aged 2 years..______________________ 27

21. Solution E aged 2 years________________________ 28

22. Solution J2 aged about 2 months._______________ 29

23-25. Graphs showing 

23. Effect of length of time of adding base in p-eparing

solutions on amount of Alb formed.___________ 30

24. First-order rate plot of Alb data from figure 23_._ _ 31

25. Number of aluminum atoms as a functio^ of the

OH:Alb ratio of the Alb particle. ______________ 34

26. Schematic representation of the ring structure formed by six

aluminum hydroxide octahedra.______________________ 35

27. Sketch showing growth of polynuclear aluminum hydroxide 36 

28-31. Graphs showing 

28. Concentration of Alb residual as a function of aging

time for solutions D, E, F, G, and H_ __________ 37

29. Advancement of the reaction (^Aib ) as a function of

aging time for solutions D, E, F, G, and H_____ 40

30. Reaction affinity as a function of reaction velocity

(d£A ib /<&)__ ________________________________ 43

31. Relationship between the D dimension of the Alb

particles and the OH:Alb ratio of these particles.. 46

TABLES

TABLE 1. Concentrations of various types of aluminum in relation to age___ D4

2. Concentrations of monomeric aluminum (Ala )__________________ 5

3. Gibbs free energies of formation of monomeric aluminum snecies__ 5

4. Activity coefficients of monomeric species for ~10~2 ionic strength

solutions._______________________________________________ 7

5. Reaction affinities of solutions in relation to age________________ 39

6. Advancement of the reaction (^Aib) values and Alb residual values of

solutions in relation to age._______________________________ 39

7. Estimated order of aging reaction relative to Alb _ ______________ 42

8. OH: Alb ratios for Alb particles and corresponding reaction affinities 47

9. Calculated yE and yF values________._______-_______----_____ 48
CHEMISTRY OF ALUMINUM IN NATURAL WATER

EFFECT OF AGING ON ALUMINUM HYDROXIDE 

COMPLEXES IN DILUTE AQUEOUS SOLUTIONS

By R. W. SMITH and J. D. HEM

ABSTRACT

Aqueous aluminum solutions containing 4X10~5 mole/liter aluminum and a 

constant total ionic strength of 10~2 , but with varying ratios of hydroxide to aluminum (OH:A1), were prepared. Progress of these solutions toward equilibrium conditions over aging periods of as much as 2 years was studied by determining the 

composition and pH of the solutions at various time intervals. The solution-?, after 

mixing, were supersaturated with respect to both crystalline and amorphous forms of 

aluminum oxides and aluminum hydroxides. The compositions of the solutions 

were determined by use of a timed colorimetric analytical procedure which allowed 

the estimation of three separate forms of aluminum that have been designated Ala , 

Alb, and Al°. Form Ala appeared to be composed of monomeric species such as 

A1(H2 0) 6+3, A1(OH)(H2 0) 6 +2, A1(OH)2 (H2 0) 4 +1 and A1(OH)«-. Form Alb was polynuclear material containing perhaps 20-400 aluminum atoms per structure. It 

appeared to be a metastable material. Form Al° was composed of relatively large, 

microcrystalline, clearly solid A1(OH) 3 particles.

For each OH:A1 ratio, the concentration of Ala remained constant with agir<* time, 

Alb decreased, and Alc increased. It appeared that Alb particles were increasing in 

size and ultimately were converted to Alc particles. After a few weeks' ag'ng, Alc 

particles had the structure of gibbsite.

In all solutions, equilibrium was only very slowly achieved, and the time required 

depended on the OH:A1 ratio and how rapidly the solution was initially prepared 

(mixing time). Lower ratios caused a slower approach to equilibrium; sometimes 

equilibrium was not achieved even after several years' aging. The more slowly base 

was initially added (to obtain the proper OH:A1 ratio), the more slowly was equilibrium approached. Ultimate equilibrium values of dissolved aluminum concentration 

and pH were consistent with known thermodynamic data on monomeric aluminum 

species.

From data determined during the aging study and by considering Alb material to 

consist of extremely small solid gibbsite particles, it was possible to estimate the 

Gibbs free energy of the (001) crystal face (yp, the gibbsite "face") and th^> Gibbs 

free energy of the (110) and (100) crystal faces (yE, the gibbsite "edge") of gibbsite 

in equilibrium with its saturated solution. These values were: 

7P = 140±24 ergs/cm2 , and 

75 = 483±84 ergs/cm2 .

ACKNOWLEDGMENTS

Much of the material used in this paper was taken from a drsertation submitted by R. W. Smith in partial fulfillment of the requirements for the Ph. D. degree, Stanford University.

Dl
D2 CHEMISTRY OF ALUMINUM IN NATURAL WATER

The help of and many discussions with Prof. G. A. Parks of Stanford 

University are gratefully acknowledged.

PURPOSE AND SCOPE

The experiments described in chapter A of this water-supply paper 

(Hem and Roberson, 1967) were concerned with dilute aqueous solutions of aluminum in the presence of perchlorate, hydroxide, hydrogen, and sodium ions. When these solutions were partly neutralized 

by addition of sodium hydroxide so that the molar ratio (r) of combined hydroxide to total aluminum (OH:A1) was between 1.0 and 

3.0, the solutions contained both monomeric and polymeric forms of 

aluminum hydroxide complexes. When the ratio exceeded 2.0, suspended gibbsite crystals about O.lOju (microns) in diameter could be 

detected in the solutions after 10 days of aging. Smaller aggregates 

and monomeric ions were also present.

The properties of the solutions and the suspended or dissolved 

material that had formed in them after 10 days continued to change 

slowly with further aging. The pH generally tended to decrease, with 

little or no change in the total concentration of the monomeric dissolved aluminum. This effect was attributed to an increasing orderliness of the polymerized material which resulted as the hydroxide in 

the polymer was incorporated into bridging positions between the 

aluminum ions. Bridging hydroxide presumably would not be involved in H+-OH~ equilibria in the solution. Somewhat similar effects 

can be expected as the crystalline particles grow in size. The longest 

aging time in the solution studies by Hem and Roberson was between 

4 and 5 months.

A more extensive and detailed study of the aging process, with 

longer aging times, was undertaken by the senior author in order to 

evaluate more closely the chemistry of the polymerization process 

and explore more fully the nature of unstable aluminum species. The 

results of the work are summarized in this paper and are given in 

greater detail in a larger paper by Smith (1969). The results of these 

experiments, which involved some solutions aged as long as 3 years, 

show that the solid formed in solutions whose pH is below neutrality 

eventually attains a solubility similar to that of bayer'te which is 

formed relatively rapidly in alkaline solutions and which characteristically consists of crystals that are l.Oju or more in diameter (Hem 

and Roberson, 1967; Schoen and Roberson, 1970). The greater solubility of microcrystalline gibbsite that forms earlier in the aging process 

is essentially a particle-size effect.

Although microcrystalline gibbsite is metastable, it is altered very 

slowly in solutions similar to natural water and therefore has considerable significance in the chemistry of aluminum in natural systems.
AGING OF ALUMINUM HYDROXIDE COMPLEXES D3

The approach used in this study was to identify as closely as" possible the form of all dissolved aluminum species and to follow the 

changes in concentration of each with time. The pH of all sections 

also was measured when aluminum determinations were made.

Recently the form of aluminum in acid aqueous media ha.s been 

studied by several investigators using timed spectrophotometric 

procedures (Smith, 1969; Smith, 1970; Turner, 1969). Using similar 

analytical techniques both Smith and Turner conclude that in such 

systems aluminum can be present (often simultaneously) in three 

distinct forms. Smith designated these forms Ala, Alb, and AK Form 

Ala consists of simple monomeric dissolved species such as A1(F2 0) 6+3, 

A1(OH)(H20) 5+2, A1(OH) 2 (H2 0) 4+, and A1(OH) 4-; form Alb consists 

of polynuclear solute species with OH: Al > 2.0; and form Alc consists of submicroscopic crystalline particles of A1(OH) 3 displaying the 

characteristic behavior of a solid.

EXPERIMENTAL TECHNIQUES

The work was initiated by preparing a series of solutions containing the same total concentration of aluminum but differing amounts 

of added base and by determining the composition and pH of the 

solutions after various periods of aging. For convenience, the?e solutions will be designated "aging-study solutions." Electron microscopy 

was used to help determine the nature of colloidal material that 

formed in some of the solutions.

The solutions studied all contained 4.54 X 10~4 moles/liter aluminum and total ionic strength was 10~2, the remainder of the total 

ionic strength being sodium and perchlorate ions. The OH:A1 ratio 

in the solutions as made up (nominal r value or rn) varied from 0.55 

to 3.01. In preparing these aging-study solutions, three standard 

solutions were prepared initially and mixed together in corr3ct proportion to achieve the desired rn value. These solutions were designated solutions 1, 2, and 3. The procedure for doing this has been 

described by Hem and Roberson (1967). The solution containing base, 

but no aluminum (solution 2), was always added last in solution preparation. Reagent grade chemicals were used throughout. Solutions 

were prepared having rn values of 0.55, 0.94, 1.36, 1.84, 2.13, 2.47, 

2.76, and 3.01 and were designated, respectively, solutions B, C, D, 

E, F, G, H, and J. At various aging times aliquots of the solutions 

were analyzed for amounts of Ala, Alb and AK The procedure used 

for determination of the three types of aluminum was that described 

by Smith (1969, 1970). The procedure depends on the variation of 

rate and type of reaction of the three forms of aluminum with ferron 

(7-iodo-8-hydroxyquinoline-5-sulfonic acid). At the same time pH of 

the solutions was measured.
D4 CHEMISTRY OF ALUMINUM IN NATURAL WAT^R

TABLE 1. Concentrations of various types of aluminum in relation to age

[Concentration X 10"1 moles per liter. Asterisk indicates old solution of similar makeup]

Age

Hours 

1.1- ..

23..... ..

48.......

168-__--_.

505 .- -

41 .-.

77-.....-

116-......

188.......

254- .-

1001* .-

1.5.---.

23-----.-

96-- . 

288- 

625_-- _-

Days 

46- 

82.......

121..-..-.

193 

259- .-

1038* - -

Hours 

1 Q

23 . .-

96

168.......

288...--..

Days 

46---....

82 . 

121-..-.-.

193-.-.-..

259.......

1038*--..-.

Hours 

1.0-.__.

23. ------ 48_ _ -

168.-...-.

Days

41.......

77.--.-.-

116,-..--_

1 OO

254. ------

1038* 

Al» Alb

Solution B

[rn =0.55]

. 3.90 0.31

. 3.64 .57

. 3 . 69 .40

. 3.57 .54

. 3.72 .64

. 3.84 .48

. 3.80 .50

. 3.76 .35

. 3.77 .32

. 3.75 .11

[rn =0.94]

. 3.40 0.65

- 3.24 .94

- 3.20 1.06

. 3.31 1.10

. 3.12 1 . 00

_ 2.94 1.19

- 3.35 1.14

. 3.20 .97

. 3.23 .79

- 3.07 .65

- 3 . 10 .76

. 3.23 .43

Solution D

[rn =1.36]

_ 2.84 1.06

. 2.42 1.54

. 2.35 1.66

- 2.30 1.63

2 Q1 111

- 2.34 .34

. 2.39 .06

. 2.43 .09

. 2.30 .02

. 2.46

Solution E

[rn =1.84]

. 2.24 1.50

- 1.56 2,10

1

OO O AA

_ 1.46 1.86

- 1.44 2.14

- 1 . 50 1 . 81

- 1.53 1.45

1.51 .49

- 1.50 .12

- 1.79

Al°

0.34

.34

.46

.44

.19

.23

.25

.44

.46

.40

69

0.49

.36

.28

.13

.42

.42

.05

.37

.52

.82

.68

.89

0.64

.46

.58

.60

.53

.61

1 19

2.09

2.02

2.22

2.08

0.50

.88

.72

1 99

96

1.23

1.56

1. 74 

2.54

2 Q9

2.77

PH

4.46

4.44

4.49

4.45

4.46

4.43

4.43

4.43

4.42

4.43

4.43

4.52

4.46

4 43

4.43

4.46

4.48

4.46

4.48

4.47

4.47

4.47

4.36

4.66

4.56

4.52

4 49

4.49

4.40

4

qo

4.26

4.23

4.14

4.12

4.09

4.79

4.68

4.61

4.56

4.54

4.54

4.52 

4.44

4.32

4.13

Age

Hours 

1.2 ...

23... _.

96 .

168-......

288-....,.

625.......

46 ..-..

82.......

121 .

193- --

259_- _- 967*

Hours

1.2____.

23 _ _-

48 . 

168. _--._-

505- 

Days 

41 -

77----

116-.--.

188. 

254 . .-

389*-- 

Hours 

1.1 

23-..---.

96 - 

168_--_--.

288. 

625- 

Days 

46__-_-.

82_-_.-_.

121-.-.---

1 QQ

OCQ

Hours 

1.0-_-_-

23.--.---

48-- - ---

168_.-_---

505-------

41.--.-..

77 - 

116. -__---

188_------

Al» Alb

Solution F

[r. =2.13]

. 1.46 1.78

. 1.11 2.05

. 1.12 1.86

. 1.12 1.86

. 1.05 1.82

. 1.05 1.35

_ 1.02 .78

. 1.20 .26

- 1.21 .10

. 1.07 .04

. 1.02 .02

Solution G

[f» -2.47]

- 1.17 1.97

.60 1.20

.60 1.15

.55 1.05

.64 .81

.68 .40

.68 .13

. 66 . 043

.61 .019

.62 .007

.46

Solution H

[rB =2.76]

. 0.78 1.36

.20 .50

.20 .40

.23 .39

.24 .36

.24 .30

.28 .17

.29 .05

.284 .011

.232 .008

OQ r

Solution J

[rn =3.01]

.08 .22

.07 .16

.05 .07

.05 .03

.065 .01

.08

.057

.019

Al°

1.30

1.38

1.56

1.56

1.67

2.04

2.74

3.08

3.23

3.43

3.50

3 qq

1.40

2.74

2.79

2.94

3.09

3.46

3.63

3.84

3.91

3.91

4.68

3.40

3.84

3.94

3.92

3.94

4.00

4.09

4.20

4.24

4.30

2

AO

4.24

4.31

4.42

4.46

4.47

4.46

4.48

4.52

PH

4.88

4.75

4.63

4.60

4.52

4.42

4.37

4.30

4.28

4.22

4.20

4 19

5.02

4.82

4.77

4.75

4.49

4.42

4.40

4.35

4.31

4.28

4.29

5.23

5.16

5.00

4.76

4.72

4.62

4.53

4.51

4.49

4.45

4.45

7 1 1

6.64

6.59

6.52

6.54

6.43

6.28

6.40

6.21

Table 1 lists the values of Al a, Alb, Al c, and pH obtained for the 

solutions at various aging times. The "old" solutions were solutions 

of somewhat similar composition that had been prepared 1 year 

previously. It should be noted that for most solutions Al a remains 

virtually constant after 23 hours' aging and that Alb decreases and
AGING OF ALUMINUM HYDROXIDE COMPLEXES D5

Al c increases as functions of aging time. Values for Ala from the various solutions are listed in table 2, and the relationship between 

initial r value (rn ) and Ala value is shown graphically in figure 1.

TABLE 2. Concentrations of monomeric aluminum (Ala )

[Numbers in parentheses beneath solutions indicate rn values. Asterisk indicates old solutions of

similar makeup]

Age

Hours 

23..... _ .............

48-..---.---...--..-...

96. ....................

168. ...................

288....................

505.. ..................

625..------.-----.---..

Days 

41-46-.-.----..---..-.

77-82..................

116-121-.--.. ..........

188-193. ---.----_---.-.

254-259.. .- -.-.... .

Average. ---._---.

Solution

B

(0.55)

.---. 3

._-.. 3

.-- 3

.---. 3

..... 3

.---. 3

.---. 3

.---. 3

..... 3

..... 3,

...-- 3

.64 

.69

,57

.72

.84 

.80 

.76 

.77 

.56

.71

,75

C 

(0.94)

3.24

3.20 

3.31 

3.12

2.94

3.35 

3.20 

3.23 

3.07 

3.10

3.18

3.23

D

(1.36)

2.

2 

2, 

2,

2.

2. 

2, 

2,

2. 

2

2,

2.

.50

.42 

,32 

.35

.30

.31 

.34 

.31 

.43 

.30

.36

,46

E

(1.84)

1 

1

1 

1

1 

1 

1 

1. 

1

1

1,

.56

.82

.46 

.44

.50 

.53 

.60 

.51 

.50

.55

,77

F

(2.13)

1.11

1.12 

1.12 

1.05

1.05

1.02 

1.20 

1.21 

1.07 

1.02

1.10

1.16

G

(2.47)

0.60 

.60

.55 

.64

.68 

.68 

.66 

.61 

.62

.63

.46

H

(2.76)

0.20

.20 

.23 

.24

.24

.28 

.29 

.28 

.23 

.23

.24

--

J

(3.01)

0.08 

.07

.05 

.05

.07 

.08 

.06 

.02

.06

--

CHARACTERISTICS OF Ala

The form of aluminum, Al a, that reacts almost instantly with ferron 

can be reasonably assumed to consist of only simple morjomeric 

species; that is, A1+3 , A1(OH)+2, A1(OH) 2+, and A1(OH) 4- with appropriate coordinated water molecules. These forms do not include 

any bridging hydroxide, which was shown by Hem and Roberson 

(1967) to stabilize the structure of aluminum hydroxide pdymers 

and make them slow to react to changes in the solution. Standard 

Gibbs free energies of formation (A(7°) for the monomeric species are 

available in the chemical literature. Table 3 lists some of these values, 

in kilocalories, plus a Gibbs free energy value for gibbsite, oAl(OH) 3 , 

taken from the literature. This value corresponds to the most negative values for AG° for aluminum hydroxide solids reported by Hem 

and Roberson (1967).

Concentrations of Al a measured in the aging solutions can be assigned to the four monomeric solute species by chemical equilibrium

TABLE 3. Gibbs free energies of formation of monomeric aluminum species

Species

Al+3

A1(OH) +2

Al(OH),+...__. __________

Al(OH) 4-_. ..________.___

aAl(OH)s-_--_ _

Standard Gibbs 

free energies 

of formation 

(AG° kcal)

____ -115.0

____ -164.9

____ -215.1

.__- -311.7

____ -273.9

Source

Latimer (1952).

Raupach (1963a, b)

Do.

Latimer (1952).

(1967).

5-125 O - 72 - 2
D6 CHEMISTRY OF ALUMINUM IN NATURAL WATER

4.0

2

X

UJ

_J 

O

y 2.0

ce

UJ

o

o 

o

1.0

Total Al=4.54 X 10-4 mole/liter

0123

rn value

FIGURE 1. Concentration of Ala as a function of rn value.

calculations, based on earlier, more detailed investigations of these 

forms.

The Gibbs free energy values for A1+3 , A1(OH)+2 and A1(OH) 4- 

were selected to be consistent with the work of Hem and Roberson 

(1967). Few values for A1(OH) 2+ were available, and Raupach's was 

selected for consistency because of the use of his value for A1(OH)+2 .

Hem and Roberson (1967) determined a value of 10~4 - 75 for *Ki 

(the first hydrolysis constant of A1+3 ) and reported that this gave a 

satisfactory fit of their data without considering the species A1(OH) 2+. 

The calculations here were refined by including both A1(OH)+2 and
AGING OF ALUMINUM HYDROXIDE COMPLEXES D7

A1(OH)2+ species, but the total amount of hydroxide bound in Ala 

species comes out about the same by either procedure. 

From data of table 3 the following can be written:

aAl(OH) 3 +3H+^>Al+3 +3H20 (1)

aAl(OH) 3 +2H+^A10H+2 +2H2 0 (2)

*Ksi = 10+3 - 22

aAl(OH) 3 +H+^Al(OH)2++H20 (3)

*KS2 = 10-1 - 54

aAl(OH) 3 +H20<=±Al(OH) 4-+H+ (4)

*KS 4 = 10-13 - 9

Al+3 +H2CMAl(OH)+2 +H+ (5)

*#i = 10-5 - 00

+ H20<=>Al(OH)2++H+ (6)

Using the above equations plus estimated activity coefficients 

(table 4), the theoretical concentrations of the monomeric species 

can be calculated and plotted as a function of pH. Further, actual 

concentrations of Ala for the various solutions and aging times can 

be plotted on the same chart. This combined plot is shown in figure 

2. Increasing aging time is indicated by the arrows.

Figure 2 shows that Ala concentration remains nearly constant 

while pH drops, at least for solutions D-H, as a function of aging 

time until the theoretical equilibrium line for Al+3 +Al(OH)+2+ 

A1(OH) 2++A1(OH) 4~ is reached. The sum of the Al a specie? thus 

conforms to the thermodynamic solubility of gibbsite.

Figure 3 shows curves similar to those of figure 2 except that 

Ala+b concentrations are plotted rather than Ala concentrations alone. 

The trend of the total aluminum concentrations in figure 3 is downward during aging, owing to decreases in Alb.

From equations 1-6 and the several activity coefficients it is also

TABLE 4. Activity coefficients of monomeric species for ~10~z ionic strength solutions

Species Coefficient

Al+3 _ ___--_-____..___________ ____________________________ » 0.427

Al(OH) +2__ .__.._-_.___.__._.-......__..__.._ .____________ 2 .678

Al(OH),+_-__-___-_____ ______-___-___-. ____________________ 2 .907 Ai(OH) 4- ----------------------------------------------- 3 .sro

1 From Debye-Hiickel limiting law using parameter a = 9.

2 From Debye-Huckel limiting law using parameter a =6.

3 From Debye-Hiickel limiting law using parameter a =3.

NOTE. The use of the above values for parameter a is in accord with Butler (1964). The activity 

coefficients used for A1+3 and A1(OH)+2 were identical to the values used by Hem and Robersin (1967) 

for similar solutions.
D8 CHEMISTRY OF ALUMINUM IN NATURAL WATER

10

3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

FIGURE 2. Concentration of Ala as a function of pH for solutions B-H at various

aging times.

possible to calculate activities of the various possible monomeric 

species at all aging times using formulas of the type:

[Al+3 ]=- Al*

1.47xlO- 5 + 1.91XlO-10 7.6X10-23 (7)

[H- [H+12 [H+14

where [H+] is hydrogen ion activity. Such formulas are derived from 

complexing equilibria and stoichiometric or ion-balance equations 

like those described by Hem (1968) for aluminum fluorde systems. 

The calculated activity values can then be plotted on a graph of log 

activity as a function of aging time. These plots are shovn in figures 

4-11. Equilibrium lines shown are based on equations 1-4.

Note for solutions D, E, F, G, and H that all species nove toward 

the equilibrium lines with increasing aging time. The data, from other, 

older solutions similar to B and C (darkened points) indicate that 

these solutions, too, are very slowly moving toward equilibrium.
AGING OF ALUMINUM HYDROXIDE COMPLEXES D9

10

4.5 5.0 5.25

pH

FIGURE 3. Concentration of Ala+b as a function of pH at various aging times.

Solution J with its high r n value is acting in a somewhat different 

manner, although there is evidence that it also is moving toward 

proper equilibrium values.

The experimental results consistently show that for each solution 

the total concentration of monomeric aluminum remains constant
DIG CHEMISTRY OF ALUMINUM IN NATURAL WATER

Al a = 3.7 X 10~ 4 mole/liter

O AI +

O Al (OH)

A Al

O Al (OH) 4

PH

FIGURE 4. Aging time effect on activity of monomeric species as a function of pH

for solution B.
AGING OF ALUMINUM HYDROXIDE COMPLEXES Dll

Al a ~ 3.2 X 10- 4 mole/liter

O AI+3 

O Al (OH)+2 

A Al (OH)2 

O Al (OH)4

PH

FIGURE 5. Aging time effect on activity of monomeric species as a function of pH

for solution C.
D12 CHEMISTRY OF ALUMINUM IN NATURAL WATER

T

I I 1 , , ,

a = 2.4 X 10^ 4 mole/liter

O AI+3 

O Al (OH)+ 2 

A Al (OH) 2 

O Al (OH) 4

PH

FIGURE 6. Aging time effect on activity of monomeric species as a furction of pH

for solution D.
AGING OF ALUMINUM HYDROXIDE COMPLEXES D13

a:

UJ i 

cr

UJ 

Q_

O

<

Al a = 1.6 X 10~4 mole/liter

O AI+3 

O Al (OH)+2 

A Al (OH) + 

O Al (OH)4

10-9 _

10-io _

pH

FIGURE 7. Aging time effect on activity of monomeric species as a function of pH

for solution E.

466-125 O - 72 - 3
D14 CHEMISTRY OF ALUMINUM IN NATURAL WATFR

ce

LJIce

LJ 

Q_

1C- 3

10-4

1C- 5

10-6

10- 7

o

<

10-

I 'I '

1.1 X 10~4 mole/liter

O AI + 3

O

A AI(OH)

O AI(OH)Z

PH

FIGURE 8. Aging time effect on activity of monomeric species as a function of pH

for solution F.
AGING OF ALUMINUM HYDROXIDE COMPLEXES D15

10-3

10- 4 -

10~ 5 -

10- 6 -

UJ 

D_

O 

<

io- 8 -

10~ 9 -

Al a = 0.06 X 10 4 mole/liter

O AI+3

O Al (OH)+2

A Al (OH)£

O Al (OH)!

PH

FIGURE 9. Aging time effect on activity of monomeric species as a function of pH

for solution G.
D16

10

10-5

o:

LU 

Q.

io- 7

o 

< 10-

lO-io

CHEMISTRY OF ALUMINUM IN NATURAL WATER

1 \ I ' I ' I ' I 

Al a = 0.2 X 10^4 mole/liter

O AI+3 

O Al (OH)+2 

A Al (OH)I 

O Al (OH)4

PH

FIGURE 10. Aging time effect on activity of monomeric species as a function of pH

for solution H.
AGING OF ALUMINUM HYDROXIDE COMPLEXES D17

10-

io-4 -

10-5 _

cr

UJ

o;

UJ

o_

UJ

O

10~6 -

10-7 -

o

<

10~ 9 -

lO-io

Al a =0.06 X 10- 4 mole/liter

O AI+3

O Al (OH)+2

A Al (OH)£

O Al (OH)?

PH

FIGURE 11. Aging time effect on activity of monomeric species as a function of pH

for solution J.
D18 CHEMISTRY OF ALUMINUM IN NATURAL WATEP

with age. The pH of the solution, however, changed during aging, 

finally attaining a value that represents equilibrium with gibbsite 

having the free energy shown in table 3. At intermediate aging times 

Al a concentration is greater than it should be if solid, Al a, and the 

observed pH were in equilibrium. Further, all major adjustments of 

the system have to do with pH, Al b, and Al c. Changes in monomeric 

species' activities merely reflect an essentially instantaneous adjustment of these activities to changing pH, with an increase in proportion of A1+3 to hydroxylated forms as pH decreases.

CHARACTERISTICS OF Alb AND Alc

Figures 12 through 18 have been constructed to show clearly 

progress of the amounts of Al b, Al c, and Al b+c with time and pH. 

Darkened points are for old solutions aged 1-3 years. Since Al c appears to be a solid material it is actually incorrect to think of Al c 

in terms of concentration "in solution." However, such a designation 

is convenient and helpful in showing the amount of solid species 

produced and in following the system as a function of time. These 

curves of minus log concentration of Al b and Al c versus pH with appropriate times indicated again show that Alb decreases in concentration with time and Al c increases. As a corollary to corstant Al a 

it should be noted that Alb+c concentration stays constant with aging 

time. Equilibrium is reached in the systems when the Al c and the 

Al b+c lines reach the dotted line which indicates the amount of crystalline gibbsite that should be present at equilibrium as a function of 

pH. It is noted that, after a few days' aging, the equilibrium pH 

for these systems can be accurately predicted by extrapolating the 

Al b+c curve, which generally appears to be a straight line parallel 

to the pH axis, to its intersection with the dotted gibbsite concentration curve.

Form Al a appears to be composed of monomeric species, and its total 

concentration is approximately constant after about 23 hours' aging. 

Form Al c, from both analysis and aging data and from electron microscope observation, consists of crystalline solid (colloidal) particles, 

and its concentration increases as a function of aging time until equilibrium is achieved. Form Al b is present in all the solutions at 23 

hours' aging arid often in considerable concentrations. Its concentration then drops as a function of aging time until it finally disappears 

from solution when equilibrium is reached. It is obviously not permanently stable material and is ultimately converted to Al c. It reacts 

with ferron in the analytical procedure according to a first-crder rate 

law which suggests that all aluminum atoms in the material are individually reacting with ferron in a way that does not change as the 

Al b structures react and dimmish in size in the procedure. Tl is would
S f\j cn rv) 

_ X X XX 

to i i i > i >i 

 i O OOO 

1 1 III 

^^Ol O1O1O1 

§ b

o

to

3

h-J

&3 .^

f ^

a

"o- £»

&3

a ^

S- jp.

§ J '<* 

td »

P T3

S* 

3 -^

§-. °° o'

Oi-t.

>rt U1 W b

3Pw ^

3

TO

£?. 

S cn

i 111 

__ 

~ r~ 

q era" 

5 » ^ _ 

 Q) n± ^ iv i>- 

 ^ ^ .^. OO ^AP

era ^ *" 

 

- 

 M

1 III 

S4 ^ >-t
o5H

i i

co

i *

§ b

0§

<rh 

>-»

tt ^

o ro 3O

£ *

* ^ » £

las'- 1 

*

13 p CT>

O » 

' P TD

3- ^

S *"

a bo

o 

3 O1-H

 O Ol

W b

P3

&.

w P1 

5' N>

oq

rtCD CJl

 --------,.-- ...- 

K> (n X V ^ 

£ £ x 

o ^t '-' '-' 

° ? ? ° 

 f ;* ^ - ___

- nr 

1 - ° 2 ~ ^

5 CD 

0) rt- . ^

_ i/i 5'

3 3

OQ »

-

^ ' " 1 ______
AGING OF ALUMINUM HYDROXIDE COMPLEXES

5X10- 4

D21

10-4

cr

UJ

cr

°- 5X10-5

2xlO- 5

o

8

5X10-6

2X10- 6

I I_ _L -L- \---\-~-.

xi__ Theoretical solid Gibbsite 

f concentration at equilibrium

A

Age time

D Al b + Al c

A Al b

O Al c

259 days 23 hours

I I I

4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4

PH

FIGURE 14. Concentration of Alb and Alc as a function of pH and aging time for

solution D.
D22 CHEMISTRY OF ALUMINUM IN NATURAL WATER

5X10-4

I__J_ _J .1 I ..

~~ ~" ^\_Theoretical solid Gibbsite 

concentration at equilibrium

2X10- 5

10-5

254 days 23 hours

I I I I

4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4

2X10- 6

FIGURE 15. Concentration of Alb and Alc as a function of pH and aping time for

solution E.
CONCENTRATION, IN M

O

§

» S 

f-S.

§-.{> 

o "r1

i-t> HI

c
CONCENTRATION, IN M

O

o

3

O 3

c « 

f fc

o

I

01

00

CJI

b

tn

fo

en 

X
AGING OF ALUMINUM HYDROXIDE COMPLEXES 

5X10~4

D25

2X10- 4

5X10-5

en

LU

en

LU

O

p 2X10-5

enhLU

O

I io- 5

5X10- 6

/^Theoretical solid Gibbsite 

/concentration at equilibrium D A|b + Al l

A Al b

O Al c

259

I I

Age time

days 23 hours

1 1 1 1

4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4

pH

FIGURE 18. Concentration of Alb and Alc as a function of pH and aging time for

solution H.
D26 CHEMISTRY OF ALUMINUM IN NATURAL WATER

indicate that the structures comprising Alb are not very large. One 

would expect large units to react at a rate proportional to the surface 

area exposed and to follow a rate law of order less than one. However, 

since it takes ferron a finite length of time to break down the Alb structures as compared with the time required to break down the Al a 

structures, it would appear that the structures are rather strongly 

bound together. A structure of limited size containing 100-400 aluminum atoms in coalesced six-membered aluminum hydroxide rings, as 

suggested by either Hem and Roberson (1967) or Hsu and Bates 

(1964a, 1964b), would appear reasonable.

ELECTRON MICROSCOPE OBSERVATIONS

Particles of Alc are colloidal and can be observed under an electron 

microscope. Figures 19 through 21 show electron micrographs of Alc 

from solutions F, G, and E. X-ray diffraction work has indicated that 

the hexagonal platelets are gibbsite (Schoen and Roberson, 1970). 

The solutions were aged about 2 years. Also shown on the micrographs are gold sol particles (small, dark spheres and tetrahedrons).

FIGURE 19. Electron micrograph of microcrystalline gibbsite from solution F aged 

2 years with negative gold particles adsorbed.
AGING OF ALUMINUM HYDROXIDE COMPLEXES D27

FIGURE 20. Electron micrograph of microcrystalline gibbsite from solution C aged 

2 years with negative gold particles adsorbed.

Figure 22 shows an electron micrograph of particles from solution 

J2 which was 4.55 X 10~3 molar in aluminum, had an ionic strength of 

about 0.1, and had an r value of 2.98. The solution was aged for about 

2 months. The figure shows much of the detail of the particles, such 

as their typical crystalline shape and imperfections. The gold sol 

particles as prepared in this work are negatively charged (Thiessen, 

1942, 1947). It is interesting to note that these particles tend to adsorb on edges, corners, and imperfection points of the gibbsite crystals, 

particularly at the last two types of sites. The suggestion is, then, that 

corners and imperfection points are the most highly positive sites 

on the gibbsite surface.

The electron micrograph of the gibbsite of solution F is of particular 

interest since it apparently shows edges (rectangular shapes) as well 

as faces and indicates the relative thickness of the particles. Also, 

it can be seen that the particles appeared layered much in the manner 

of microscopic mica particles. The electron micrograph of material 

from solution C indicates that microcrystalline gibbsite is ultimately 

formed even if rn value is as low as 0.94 and pH is near 4.
D28 CHEMISTRY OF ALUMINUM IN NATURAL WATER

FIGURE 21. Electron micrograph of microcrystalline gibbsite particles from solution 

E aged 2 years with negative gold particles adsorbed.

Thus, the electron micrographs show conclusively that colloidal 

material is formed in the aging study solutions. Further, this material 

appears to be microcrystalline gibbsite.

EFFECT OF RATE OF ADDITION OF BASE ON Alb

In preparing the experimental aging solutions the two stock aluminum solutions, 1 and 3, which were both distinctly acid, were mixed 

together first, and the basic solution 2 was then added in the amount 

desired. An experiment was performed to determine whether the rate 

at which the base was added would influence the composition of the 

aluminum species in the final mixture. Four replicates of solution H 

were prepared, which differed only in the rate at which solution 2 was 

added to the mixture. One solution was prepared by very rapid addition requiring only 10 seconds. The other three were made by slower 

addition of the basic solution. The longest period of addition with 

mixing was 91 minutes.
AGING OF ALUMINUM HYDROXIDE COMPLEXES D29

FIGURE 22. Electron micrograph of microcrystalline gibbsite from solution J2 aged 

about 2 months with negative gold particles adsorbed.

As figures 23 and 24 show, the four solutions did display significant 

differences. Figure 23 represents the results of the aluminum determinations. The common intercept of all the curves on the y axis 

indicates the concentration of Ala was the same in all solutions and 

was evidently independent of the manner of preparation. However, 

the amount of Al b, the fraction reacting during the first 100 minutes 

or so, is obviously greater in the solutions prepared by slower addition 

of base. A more easily interpreted representation of Al b concentrations is given in figure 24, where the Al b concentrations at time zero 

can be seen to range from about 10~4 - 25 to 10~3 - 75. The higher Al b 

concentrations were accompanied by higher pH values.

All these solutions were aged for 7 days before the aluminum determinations were made.
D30 CHEMISTRY OF ALUMINUM IN NATURAL WATER 

3.0

oi (

X 

cr

0=2.0

UJ 

Q_

C/)

GJ 1.0 

cr

LJ

O

o

UJ

cr

I \ I I I 

SOLUTION H :

Total Al = 4.54X10-4 mole/liter - 

91 minutes . j_

I I I I

20 40 80 120 160 

ANALYSIS TIME, IN MINUTES

FIGURE 23. Effect of length of time of adding base in preparing solutions on amount 

of Alb formed aluminum recovered as a function of analysis time.

In general the results of this experiment show that the more slowly 

the base is added the farther away from equilibrium the mixture is 

when the aging period begins. The solutions where addition of base 

was made very slowly have the highest concentration of Al b, produced 

at the expense of AK For a particular rn value Al a is constant and 

independent of rate of base addition. Also, pH at the end of 1 week's 

aging is higher.

Rapid addition promotes formation of solid A1(OH) 3 particles, 

possibly originally of a partly amorphous character. These particles 

appear to soon start organizing themselves into crystalline form 

(gibbsite under our experimental conditions). Slow addition gives 

less opportunity for local excess of base and formation of amorphous 

solid A1(OH) 3. With slower addition of base, more monomeric and 

polynuclear species are initially formed. The monomeric species very 

rapidly convert to polynuclear microparticles or macroions in roughly 

the manner outlined by Hem and Roberson (1967).
AGING OF ALUMINUM HYDROXIDE COMPLEXES D31

0 20 40 60 80 

ANALYSIS TIME, IN MINUTES

FIGURE 24. First-order rate plot of Alb data from figure 23.

100
D32 CHEMISTRY OF ALUMINUM IN NATURAL WATER 

APPROACH OF SOLUTIONS TO EQUILIBRIUM

The aging studies show that the amount of Ala stay^ nearly constant after 1 day even though pH drops and that concentration of 

Al b decreases as Al c increases. Meanwhile, activities of monomeric 

species, as A1+3 and A1(OH)+2, adjust themselves according to whatever pH change occurs in the system. It appears that individual 

A\zb (QH) n:c+v particles must grow by combining with each other 

accompanied with the "splitting out" of protons from combined 

water molecules. The relation between the monomeric species and 

the pH of the solution can be summed up in the equation (obtained 

by combining eq. 1-4)

Al+3 +Al(OH)+2 +Al(OH) 2++Al(OH) 4-+5H2 0 <± (8)

4A1(OH) 3 +5H+.

The A.lz(OB.) nz+y intermediates are, of course, left out of the above 

equation because they are not of thermodynamic significance, disappearing from solution before equilibrium is attained.

The existence and distributions of activities of monomeric species 

of aluminum such as those on the left side of equation 8 seem to fit 

reasonably well at the equilibrium conditions attained after long 

aging. The possibility of dimeric or other small forms of aluminum 

hydroxide complexes is not ruled out by this equilibrium. Hem and 

Roberson (1967, p. 33) studied the kinetics of dissolution in acid of 

aluminum hydroxide species prepared in the same way as the ones 

described here. The kinetic behavior of hydroxide present in bridging 

positions was shown to differ sharply from that assignable to monomeric or unbridged positions. The studies of Hem and Roberson indicated that essentially all structural (bridging) hydroxide is associated 

with polymerized forms, or the equivalent of what is termed Al b and 

Al° in this report. Essentially no bridging hydroxide was left over to 

be assigned to AK

It seems logical to conclude that Al a consists of monomeric aluminum hydroxide complexes with negligible amounts of dimers, such as 

A12 (OH) 2+4, trimers, tetramers, or higher products of polymerization. 

Formation of polymers requires the initial step of forming a double 

hydroxide bridge between two monomers. This step appears to require 

an energy input because it does not occur in these solutions at a 

significant rate after the very early stages of the aging process (Ala 

remains nearly constant).

KINETICS AND STRUCTURE OP NONEQUILIBRIUM SPECIES

The procedure used here for determining the amount of Al b that 

is present during aging is based on the distinctive kinetics of this 

fraction of the aluminum when participating in complexing reactions
AGING OF ALUMINUM HYDROXIDE COMPLEXES D33

with ferron. In earlier studies by Hem and Roberson (1967) the 

kinetics of the reaction of aluminum hydroxide species with H+ were 

used to identify the equivalents of Alb and Al°. The structural or 

bridging hydroxide was the substance whose quantity was directly 

determined; the molar proportion of bridging hydroxide to aluminum 

in this kind of material is directly related to the size of polymeric 

units if the units are considered to be evolving toward crystalline 

gibbsite. In this process there are strict geometric constraints that 

such structures impose on the way individual aluminum ions can be 

bound into the unit.

The work of Hem and Roberson further demonstrated that after 

the first few days the hydroxide contained in polymerized forms of 

aluminum hydroxide was nearly all in bridging positions. One could 

possibly explain the behavior of the Al b toward ferron by postulating 

that the material is a poorly organized polymer of indefinite str.icture 

and size which contains many imperfections, perhaps missirg hydroxide bridges or nonbridging hydroxide present in randorr positions. A more logical explanation, however, would seem to b^ that 

Al b represents reasonably well organized units having gibbsite structures, and containing only minor amounts of unbridged hydroxide, 

which differ from Al c units only in being smaller. This explanation 

would appear to be the better one in view of the experimental observations of Hem and Roberson, which appear to rule out any large 

proportion of nonbridging hydroxide in polymeric material.

The number of aluminum ions per Al b unit can be calculated if the 

gibbsite structural pattern is imposed and if the mole ratio of hydroxide to aluminum in the units is known. The relationship is expressed graphically in figure 25. The simplest single ring structure 

Al6 (OH)i2+6 (fig. 26) has six aluminum ions and a ratio of OH:A1 of 

2.0 (fig. 25). By the time the ratio reaches 2.95 the structure contains 

about 600 aluminum ions bound in a system of coalesced rings. The 

process of increasing size of structure is shown in figure 27.

The coalescing of structures as shown in figure 27 represents deprotonation of water molecules along edges of the units as the hydroxide double bridges are formed. It is to be expected that this 

process will affect the pH of the system, and therefore the amount of 

change in H+ activity during aging should be a rather direct indication of the amount of polymerization that is taking place.

From data in table 1 showing the amount of Al b present at various 

times during aging and the pH measurements made at the same 

times, an insight into the limiting composition of Al b can be obtained. 

Data from the table are shown graphically in figure 28, where the 

negative log of residual concentration of Al b is the ordinate and aging
D34 CHEMISTRY OF ALUMINUM IN NATURAL WATER 

1000

100

10

I ' I ^ T T I

I . 1 1

1.8 2.0 2.2 2.4 2.6 2.8 3.0

OH:Al b

FIGURE 25. Number of aluminum atoms as a function of the OH:Alb ratio of the

Alb particle.
AGING OF ALUMINUM HYDROXIDE COMPLEXES D35

FIGURE 26. Schematic representation of the ring structure formed by six al"minum

hydroxide octahedra.

time in days is the abcissa. This is the form of a first-order kinetic 

plot, and it is of interest to note that four of the five solutions give 

approximate straight lines of equal slope for a long period of time 

(40-120 days). The data for shorter aging (up to 40 days) evidently 

represents the period in which small Al b units are coalescing but few 

particles are reaching a sufficient size to become AK During the 

period when the disappearance of Al b is more rapid, the reaction seems 

to obey a psuedo first-order mechanism. The straight-line parts of 

the rate curves in figure 28 represent the stage of polymerization 

where the polymeric units are reaching the upper limit of size for 

behavior as Al b. Although the polymerization process would seem 

to be basically an encounter of two aluminum hydroxide units, and 

therefore possibly a second-order mechanism should govern, it is
D36 CHEMISTRY OF ALUMINUM IN NATURAL WATER

AI 12 (OH)22 (H 2 0){68 

Q QO

A|6(OH) 12 (H2 0);2b

AI 24 (OH)60 (H 2 0) +2412

FIGURE 27. Growth of polynuclear aluminum hydroxide.

evident that the measurements of Al b give no indication of the number 

of units involved. A psuedo first-order behavior may therefore be 

within reason.

The decline in rate after 120 days probably is caused by lack of 

precision in measurements, as well as by the probability that the 

polymerization mechanism itself can operate at a relatively rapid 

rate only when Al b units are plentiful enough so that random encounters between units occur very frequently. As concentrations decrease the probability of encounters between units drops off rapidly.

As a further observation, it might be noted that the upper size 

limit of the Al b particles seems to be fairly definite and specific. At 

least the solutions acted very much alike after the point in the aging 

process where significant losses of Al b began to take place.

The aging data provide a means for calculating the average size of 

Al b units during the time these were being most rapidly transformed 

into AK The concentration of Al b remaining in solution after 120 days' 

aging, subtracted from the amount present after about 40 days' 

aging, gives the quantity of aluminum converted from Al b to Al c 

during this period. The Alc is assumed to contain three hydroxide 

ions per aluminum ion. The change in pH of the solution during this 

aging period can be equated to the hydroxide demand of the Al b 

that was converted to Al c. When this is done the amcunt of hydroxide originally present in the converted Al b can readihr be calculated, and the value of the mole ratio OH:Al b determined for the 

material just before it became AK The size of the units can then be 

estimated from figure 25.

For example, in solution F the decrease in concentration of Al b 

between the 46-day and 121-day observations was 0.68 XlO~4 moles/ 

liter, and the pH change indicated a gain of O.llXlO"4 moles/liter 

in concentration of H+ during this period. The amount of hydroxide 

required for 0.68X1Q- 4 moles of A1(OH) 3 is 2.04 X10~4 moles. The
AGING OF ALUMINUM HYDROXIDE COMPLEXES D37

o

W

H

'I 

as

q (6

10

uS

(N

06

ivnaiS3dgiv 001 SHNIIAI
D38 CHEMISTRY OF ALUMINUM IN NATURAL WA^ER

amount of hydroxide already present in the Al b lost was (2.04 0.11) X 

10~4 or 1.93XlO~4 moles. The value of the molar ratio OH:Al b is 

1.93/0.68=2.84. In this particular solution the total hydroxide tied 

up in monomeric Al a species was the same at 46 and at 121 days. This 

factor does need to be considered in some solutions, however, as the 

loss of hydroxide from monomers as the pH declines can be a significant source of hydroxide for incorporation into Al b structures.

From similar calculations the values of the ratio OH:Al b at the 

upper size limit for this species was computed for solutions D-H. 

Results were as follows:

Solution Molar ratio OH:Alb Approximate formula A1P

C___.......................... 2.81 Al2 oo(OH) 662 +38

D.......__.........._......_.- 2.87 Al 4 oo(OH) 116 o+50

E......._.._...._...._........ 2.83 AWOH) 708+42

2.84 Al 30 c(OH)-+47

G....._......_...........-_... +40 2.80 Al20 o(OH) 560

H..._...._._...._-...-_....--. 2.73 Al10 o(OH) 273 +27

J...._._...__.........___...... 2.86 Al35o(OH) 10 on +50

In calculating the formula of Al b it is necessary to assume all the 

hydroxide that is found in the final structures is present in bridging 

positions. Prior experiments with forms of aluminum hydroxide 

equivalent to Al b and Al° described by Hem and Roberson (1967, 

p. 40) showed that after a few days' aging this assumptior is in accord 

with observed behavior.

This approach to calculating the formula of Al b suggests that the 

largest units have from 100 to 400 aluminum ions. A sirgle sheet of 

octahedra would thus be about 50 to 100 A (angstrom) in diameter 

(predicted from fig. 29). These data also tend to confirm the idea that 

the microcrystalline particles observed in the electron micrographs 

are gibbsite platelets.

EVALUATION OF AGING PROCESS BY NONEQUILIBRIUM THERMODYNAMICS

Chemical reactions that are slowly approaching a state of equilibrium can be studied by techniques of nonequilibriuir thermodynamics. In this section the types of information that can be obtained 

in this way will be shown.

The relationship governing the final equilibrium in the aged solutions (eq. 8) can be used for calculation of reaction affinity. Results 

of this calculation are given in table 5. By utilizing nor equilibrium 

thermodynamic concepts (van Rysselberghe, 1963; Morgan, 1967), it 

is possible to calculate advancement of the reaction, £, where £ is the 

ratio of the amount of some species that has reacted at a particular 

aging time to the total amount that will have reacted at equilibrium. 

The two species that react and are apparently "used up" as the aging
AGING OF ALUMINUM HYDROXIDE COMPLEXES D39

TABLE 5. Reaction affinities in kilocalories per mole, of solutions B through H in

relation to age

Hours

1__ ____ 7

QO C 

LI& O

48_______ 7

505 6

625.......

Days

41__.__._ 6

77. _. 6

254 6

B

0

5

3

6

~9

5

5

5

3

3

7

6

6

6

6

6

6

6

6

6

6

c

6

6

1

?,

.5

.6

6

9

6

6

6

9

7

6

6

6

4

3

?,

?,

D

?.

4

8

1

.2

.8

6

7

3

9

5

E

10.4

8.0

8.1

6.2

6.0

6.0

5.8

4.5

2.6

10

8

6

6

4

3

?,

1

1

F

5

?,

5

1

.8

.3

5

8

5

3

0

G

11.6

7.6

7.0

5.8

3~2

2.3

1.9

1.1

.3

.0

12

8

7

4

4

2

1

1

1

H

,8

9

?,

7

.1

.7

8

6

?,

.2

?,

process proceeds are Alb and OH~. Table 6 shows calculated £A i b values 

for solutions D, E, F, G and H.

It should be noted that in all cases the amount of Al b did not change 

until a certain aging time had elapsed. For calculating £A i b which is 

the ratio of the Al b reacted at a particular aging time to total amount 

originally present, an average value for total Alb noted was taken. 

From data of table 6, the curves of figure 29 were constructed. This 

figure shows £A i b as a function of time. A tangent to a curve at any 

aging time is a measure of d^/dt at that particular aging time. From

TABLE 6. Advancement of the reaction (fAib) values and Alb residual values of solutions

in relation to age

[Alb, residual, concentrations X1(H]

Hours 

23... __

48____.-.

96-...--.

168. __.--.

288.......

505-.....-

625-....--

Days 

41.......

46.......

77-.--.-.

82.......

116.......

121.......

188.......

193.__._._

254.......

259--.....

]

Alb

. 1.58

_ 1.54

. 1.62

1.66

. 1.63

. 1.11

.34

.06

.09

.02

D

{Alb

0.000

.000

.000

.000

.000

.323

.789

.963

.938

.987

Alb

2.10

2.00

1.86

2.14

1.82

1.45

1.20

.49

.12

E

lAlb

0.000

.000

.000

.000

.104

.286

.408

.758

QAO

Alb

2

nc

1.86

1.86

1.82

1 Qfi

.78

.26

.10

.02

F

lAlb

0.000

.000

.000

OQQ

cqn

.863

947

.990

i

Alb

1.20

1.15

1.04

.81

.40

1 9

019

.007

G

|Alb

.000

.000

.289

.649

.887

.962

QQO

All

0.50

.40

.39

.36

on

.17

.0)

.111

.00")

H

lAlb

0.000

.000

.000

.000

.268

.585

.878

1.000
D40 CHEMISTRY OF ALUMINUM IN NATURAL WATER

these d^/dt values plus the Al b concentrations at the same aging time, 

it is possible to estimate the order of the reactions over various discrete time intervals. 

In general,

dx 7

 = k xn , 

dt

(9)

where x is the concentration of some species (in our case Al b), t is time, 

A; is a constant, and n is the order of the reaction relative to species x.

_d\,x (total)/

dt dt

Thus, for a time interval such as between 12 and 26 days' aging, values 

of d^/dt and x can be substituted in equation 9, k can be eliminated, 

and the value of n can be determined. By this general procedure the 

data of table 7 were determined. Solution E was not included because 

of anomalies in the data for this solution. From the table it can be 

seen that the reaction involving the loss of Al b from solution is very 

close to the first order with respect to Al b over much of the aging time.

1.0 

0.8 

0.6 

0.4 

0.2 

0

SOLUTION D

i i i i i i i i

"0 20 40 60 80 100 120 140 160 180 200 220 240 260

AGE TIME, IN DAYS 

FIGURE 29. Advancement of the reaction (£AI»>) as a
AGING OF ALUMINUM HYDROXIDE COMPLEXES D41

Only at the start, when there is little loss of Alb from the solution and 

later as equilibrium is closely approached, does the reaction deviate 

significantly from first order. At short aging times there is a pH drop 

indicating some considerable combination of Al b particles. However, 

these particles do not become sufficiently large and do not change 

character sufficiently at this stage to be considered Al c particles. Near 

the finish of the approach to equilibrium, perhaps the mass-action 

law influences the rate of reaction because of the very few Alb particles left and the problems related to bringing these few particles

I_____I ! I I I I 1 I

0 20 40 60 80 100 120 140 160 180 200 220 240 260

AGE TIME, IN DAYS 

function of aging time for solutions D, E, F, G and H.
D42 CHEMISTRY OF ALUMINUM IN NATURAL WATER 

TABLE 7. Estimated order of aging reaction relative to AIb

a , .. Age time interval Order of reaction 

Solutlon (days) (n)

D.. _._.._._.......____.

F

G. ___.....__.._.____. _.

H. .__.._.-___.___.____

..-.___...__..__ 46- 82

82-121 

121-193 

........._...... 26- 46

46- 82 

82-121 

121-193 

._..--._....__._ 21- 41

41- 77 

77-121 

121-188 

...-._...--.-..- 26- 46

46- 82 

82-121 

121-193

0.8

1.0

0)

.9

.9 

1.5 

3.2 

1.0

1.2 

1.5 

1.4 

.9

.8 

1.2 

4.4

1 Negative value.

together. Figure 30, which shows affinity for the overall thermodynamic reaction versus d^/dt, tends to confirm this idea. As equilibrium 

is approached closely, affinity appears roughly proportional to d^/dt 

in accord with the form of the Marcelin-de Bonder equation suitable 

for use near equilibrium; that is,

(10)

where V is the reaction velocity or d^/dt, Ve is the reaction velocity 

in the forward direction at equilibrium, A is affinity, R is the gas 

constant, and T is absolute temperature.

The general first-order rate of disappearance of Al b is confirmed 

by figure 28, where there is a rough straight-line relationship for several 

solutions over long aging periods.

CALCULATION OF THE GIBBS FREE ENERGY OF THE "FACE" AND

"EDGE" OF GIBBSITE IN EQUILIBRIUM WITH ITS

SATURATED SOLUTION

In the work of Hem and Roberson (1967) a solubility fcr gibbsite 

material was determined from the results of Al a and pH measurements on samples aged from 1 to about 5 months. The value obtained, 

a solubility product for A1(OH) 3 of 10~32 65, was denoted by them as 

applicable to microcrystalline gibbsite a material having particles 

from a few hundredths to a few tenths of a micron in diameter. Probably this solubility determination was influenced by Alb, as this material is not fully precipitated until after longer aging. The difference 

between AG° for the microcrystalline gibbsite of Hem and Roberson 

(-272.3 kcal/mole) and the one given here (-273.9 kcal/mole) 

represents, in one way of looking at the data, a particle-size effect.
AGING OF ALUMINUM HYDROXIDE COMPLEXES D43

8 12 16 2 

d(Mh /dt (1/hr X103 )

EXPLANATION

O SOLUTION D V SOLUTION G 

O SOLUTION F D SOLUTION H

FIGURE 30. Reaction affinity as a function of reaction velocity (d%A.v>/dt).

From thermodynamic reasoning (Adamson, 1967) it can be rhown, 

in principle at least, that small crystals should exhibit a greate" solubility than large ones. The age study solutions, over much of the aging 

time, appeared supersaturated with respect to monomeric aluminum 

species. During the same period, there appeared to be polynuclear 

aluminum present. If the polynuclear particles are considered to be 

extremely small crystals, it should be possible from the data ot tained 

in the course of experimentation to calculate the Gibbs free energy 

of the (001) crystal face (which will be designated the gibbsite "face") 

and the (110) and (100) crystal faces (which together will be designated the gibbsite "edge"). These calculations will be made with the 

gibbsite in equilibrium with its saturated solution. It will also be 

necessary to estimate the dimensions of the polynuclear particles.

If the Al b and Al c particles are considered to have the shape of 

hexagonal platelets of roughly the following form
D44 CHEMISTRY OF ALUMINUM IN NATURAL WA^ER

and if surface area = QtD+kD2, (11) 

volume = V = kDH, (12) 

k =2.598,

p= particle density, and 

M= molecular weight of solid,

it should be possible to write a modified expression of the Kelvin 

equation for solids (Adamson, 1967), relating reaction affinity to interfacial energies. Thus,

A =^(QtDyE +2kD2 7F)M/VP, (13) 

o

where A =affinity for the overall reaction 

Al+3 +Al(OH)+2 +Al(OH) 2++Al(OH) 4-+5H20^±4Al(OH) 3 +5H+ (8)

and where 7#=Gibbs energy of the "edge" of a gibbsite particleperchlorate solution interface, and

7f=Gibbs energy of the "face" of a gibbsite particleperchlorate solution interface.

Eliminating t from equation (13),

A =¥±+ZkD»>YM/Vp. (14)

Holding V and p constant and taking the derivative of A with 

respect to D,

<M = 2/-6y7g+4H) \M/V (15)

dD 3\ kD* ) '

To test whether this is a minimum or maximum, take the second 

derivative

TO A n /1 r>T7 \

(16)

and thus the extremum is a minimum. Setting the first derivative 

equal to zero and putting t back in the equation,

(17) kD* 

and thus,
AGING OF ALUMINUM HYDROXIDE COMPLEXES D45

Substituting back into equation 13 and also eliminating V,

_tAp. (19) 7F 4M

Similarly,

(20)

6M 6M 

Equation 18 can also be written as

s 2fcD (21)

Close observation of the electron micrograph in figure 19 indicates 

that the ratio D/t has a numerical value of about 2. If this value is 

assumed, equation 21 reduces to

"y f

 =3.46 (22)

7F

Equations 19-22 can now be used to estimate JE and yF . Affinity 

for a particular solution at a particular aging time can be readily 

obtained from the aging study. The density of gibbsite can be used 

for p. The problem, then, in solving these equations revolves about 

estimating D.

According to Hem and Roberson (1967), the probable way in which 

six or more aluminum hydroxide-water octahedra are bound together is through sharing of two adjacent hydroxide ions. Six-membered rings of the form shown in figure 26 can be produced owing to 

the angles at which the octahedra are joined. The maximum dimension of such a ring is about 9 A, and a single ring probably has the 

formula Al 6 (OH) 12 (H2 0)i2+6 (Hem and Roberson, 1967; Hsu and 

Bates, 1964b) if all hydroxide is assumed to be present in br'dging, 

or shared positions. This type of structure should grow by coalescence 

of such rings which would accompany deprotonation of water in the 

shared positions. Hsu and Bates (1964b) proposed a coalesced-ring 

structural pattern of the types shown in figure 27 for such enlarged 

structures. The OH :A1 ratio of the single-ring structure is 2.0, and as 

the rings coalesce into larger structures, the OH:A1 ratio becomes 

large, approaching a maximum of 3.0 for very large structures. Figure 

25 shows the relationship between the number of A1+ 3 ions per structural unit and OH:A1 ratio. Considering that the maximum dimen-
D46 CHEMISTRY OF ALUMINUM IN NATURAL WATER

sion across a ring is 9 A, the following simplified figure can be constructed :

9A

|*-4.5A-*|

7.8A

_L

Using Hsu and Bates' coalescence structure, it can be reen that, if 

there are six aluminum atoms per structure, the D dimension is about 

4.5 A; if there are 24, the dimension D is about 15.6 A; if there are 54, 

the dimension D is about 23.4 A; and so forth. From this information, 

figure 31 was constructed. If, then, the OH:A1 ratio of the small

2.8

2.4

< 

o

2.0

1.6

1.2

I I I

0 20 40 60 80

DIMENSION Z>,IN ANGSTROMS

FIGURE 31. Relationship between the D dimension of the Alb particles and the 

OH:Alb ratio of these particles.
AGING OF ALUMINUM HYDROXIDE COMPLEXES D47 

TABLE 8. OH:Alb ratios (r) for Alb particles and corresponding reaction affinities (A)

[For each solution, A values, upper row of figures; r values, lower row. Italic figures indicate values used

for calculation of yE and yp]

Solution 23 

hr

B--____

C------

D--_---_

£-._--_

P.......

G ....

EL......

----- 6.

1. 

----- 6.

2,

----- 7.

2.

----- 8,

2.

----- 8,

2.

----- 7.

2.

.-- - 8.

2.

R

.8 

.6 

.9 

.4 

.7 

0

.5 

.2 

,5 

6

.3 

.9 

,1

48-96 

hr

7.3

1.2 

6.1 

2.9

6.8 

2.6 

8.1

2.7 

6.5 

2.6 

7.0

2.3 

7.2 

1.6

168 

hr

6.6

1.6 

6.2 

3.2 

6.1 

2.5 

6.2

2.4 

6.1 

2.6 

5.8

2.2 

4.7 

2.2

288 

hr

6~5 

2.6 

6.2 

2.6

U.8 

2.6

U-1 

2.3

505-625 

hr

6.9 

2.2 

6.6 

2.2 

U.8 

2.6 

6.1 

2.5 

3.3 

2.8 

3.2 

2.6 

2.7 

2.4

41-46 

days

6.5 

2.7 

6.6 

3.2 

3.6 

2.6 

6.0 

2.5 

2.5 

2.2 

2.3 

2.8 

1.8 

2.9

77-82 

days

6.5 

2.6 

6.9 

2.7 

2.7 

2.3 

6.0 

2.4 

1.8 

3.1 

1.9 

4.8 

1.6 

3.7

116-121 

days

6.5 

2.1 

6.6

2

0 

. O

2.3 

2.7 

5.8 

2.5 

1.5 

3.8 

1.1

1~2

188-193 

days

6.3 

2.2 

6.6 

2.2 

0.9 

6.4 

4.5 

1.4 

0.3

0~3 

0~2

254-259 

days

6.3 

2.2 

6.6 

2.3 

0.5 

2.0 

2.6

0~00

o~6

0~2

particles present that give rise to supersaturation can be determined, 

it should be possible to estimate the average size of these particles. 

Assuming that the material that causes supersaturation is the Alb 

material determined in the analyses, it should be possible to estimate the OH :A1 ratios in the various solutions at all aging times by 

making hydroxide balances. These balances involved assigning the 

proper amounts of hydroxide to monomeric species and to the Al c 

present, assuming the Al c to be A1(OH) 3. The OH that could not be 

accounted for otherwise was then assigned to Alb. The OH :A1 values 

obtained are obviously different from the ones calculated based on 

pH drop. They are for a particular solution at a particular agirg time, 

and the size of the Al b units calculated from these ratios should be 

average size Al b structures for these particular solutions and aging 

times. The Al b structures calculated by the pH drop method should 

be for the maximum size Alb units formed. For use with the modified 

particle-size equation, it is obvious that the Alb size as calculated for 

a particular aging time and solution must be used with the affinity 

value for the same solution and aging time. Table 8 lists the OH :Al b 

values obtained. Also shown in the table for convenience are appropriate affinity values. For calculation of yE and yF only part of the 

data was used. Aging times of 96 hours or less (168 hr for solutions 

B and C) were not considered because it was felt that for such short 

aging time the solutions possessed greater instability than fo^ longer 

aging times and could well give poorer values of yF and yE. Data at 

the other extreme (large initial r value and very long aging times) 

were rejected since so little Al b was present that relatively small 

errors in the hydroxide balance would give rise to large errors in the 

value of the OH:Alb ratio. Solution C, aged 46 days, was rot used 

because using it would have resulted in an impossible answer.

From equations 19-22 and from data of table 8, the values of yE 

and jF listed in table 9 were obtained. The 90-percent confidence
D48 CHEMISTRY OF ALUMINUM IN NATURAL WATER

TABLE 9. Calculated J-E and JF values, in ergs per square centimeter

Age re VF

Solution B

505 hr.._._._

41 days

77 days. . . _

116 days____

188 days. . . .

254 days. . . .

280

940

__.._ 740

_____ 210

_____ 270

970

82 

273 

213 

60

78 

78

Solution C

288hr....._.

625 hr_.__._

82 days....

121 days

193 days. . . .

259 days_...

._... 740

---_ 280

_.___ 1010

1470

_-.._ 280

_____ 360

213 

82 

291 

425 

82 

104

Solution D

168hr_-__.__

288 hr_ _ . . _ _

625hr...._._

82 days_...

121 days....

540

_..__ 700

__.__ 540

_____ 400

..... 150

.._._ 330

156 

204 

156 

117 

44 

96

Age re 7F

Solution E

168hr_.._.._

505hr_..._._

77 days...,

116 days____

..... 420

_____ 540

..... 520

...._ 400

-..._ 510

121 

156 

151 

117 

147

Solution F

168 hr.......

288 hr. .....

625 hr._...._

46 days _

_____ 690

-_.-. 540

790

_____ 100

200 

156 

208 

30

Solution G

168 hr.......

CAC >.,.

41 days

_..__ 240

o/?A

_____ 510

69 

104 

147

Solution H

168hr..__ .

288hr._..__.

625 hr__.__._

46 days

_____ 200

-_... 180

..... 180

__.._ 800

57 

52 

52 

230

interval of the mean of yE, using the t distribution, is 483 ±?4 ergs/cm2 

(ergs per square centimeter). The similar confidence interval of the 

mean of JF is 140±24 ergs/cm2 . It is of interest to compare these 

values with other Gibbs energy values of crystals in equilibrium with 

their saturated solutions as measured by solubility versus particlesize studies. Eniistun and Tiirkevich (1960) reported the value of 85 

ergs/cm2 for SrS04-H20 system. Schindler (1967) reports 770±330 

ergs/cm2 for ZnO-0.2M NaC10 4 solution, 690 ±150 for CuO-0.2M 

NaC10 4 solution, and 410±130 for Cu(OH) 2-0.2 NaC10 4 solution. 

The last value is of particular interest considering a hydroxide is 

being measured and the closeness to our value for the gibHite edgeperchlorate solution system.

Several comments should be made regarding 7^ and JE- Probably in 

addition to edge and face energies a "corner" energy should be taken 

into account. Considering figures 26 and 27 it will be noticed that 

all "edge" sites cannot be the same. One could, perhaps, think of the 

single six-membered ring as consisting only of "faces" and "corners." 

Such "corners," of course, must have dimensions of area. When two 

rings become bound together, an "edge" also is introduced. As the 

structure becomes larger, this "edge" area becomes relatively greater 

compared to "corner" area. Many of the electron micrographs showing negative gold particles adsorbed on gibbsite edges, imperfections, 

and, in particular, corners tend to confirm the importance of corner 

adsorption sites and the greater energies associated with these locations.
AGING OF ALUMINUM HYDROXIDE COMPLEXES D49

CONCLUSIONS RELATIVE TO ALUMINUM IN MILDLY 

ACID AQUEOUS MEDIA

From the aluminum hydrolysis work and electron microscopy, 

several conclusions can be reached relative to the behavior of aluminum in acid aqueous systems. These conclusions are:

1. If aluminum perchlorate solutions are made up to contain 

4.54xlO~4 moles/liter aluminum (total ionic strength about 

10~2 molar), provided the nominal ratio of bound hydroxide to 

aluminum (rn value) is 3.00 or less, there will initially be present 

in the system three different forms of aluminum which can be 

designated Al a, Al b, and AK Form Al a appears to be composed 

entirely of the monomeric species A1+3, A10H+2, A1(OH) 2+, and 

A1(OH) 4~. Form Al b appears to consist of polynuclear aluminum 

hydroxide species probably of a general six-membered ring structure in which each aluminum is bonded to its neighbor through 

shared pairs of OH ions. The individual rings tend to coalesce 

into larger structures until they ultimately become large enough to 

be filtered out and identified by electron microscopy as gibbsite 

crystals. The manner in which the rings coalesce appears to be governed by a first-order rate law relative to the Alb material. The 

Alb particles appear to range in size up to Alioo(OH) 27 3+27 to 

Al 4 oo(OH)ii50+50. Form Alc is solid material which may be initially all or partly amorphous, but which rapidly becomes 

crystalline and takes on the structure of gibbsite.

2. For a particular rn value the amount of Ala is nearly constant, at 

least after 23 hours' aging, and is independent of how long the 

solution has aged and the rate at which base was added to the 

solution in the initial solution preparation. A corollary to this 

statement is that the amount of Alb+c also is constant. The 

significance of these statements is great for they indicate that 

the Gibbs free energy of formation of Al b is not much less than 

solid AH In fact, from the affinity calculations the difference 

appears to be only a few kilocalories. Although the concentration 

of Al a remains constant with aging time the activities of the 

individual monomeric species adjust themselves almost instantly 

to change in pH of the solution.

3. The pH of the solutions decrease with aging time until equilibrium 

pH values are ultimately achieved. The equilibrium pH values 

are consistent with known thermodynamic values for monomeric species and solubility products. The equilibrium pH 

achieved in a particular solution depends on its initial rn value. 

The lower the rn value the lower is the equilibrium pH. The 

rate at which equilibrium is approached depends on rn value 

and also on the rate at which base is added during solution make
D50 CHEMISTRY OF ALUMINUM IN NATURAL WATER

up. The lower the rn value the slower is the progress toward 

equilibrium. For example, if rn is around 2.8 equilibrium may 

be achieved in 6 or 8 months (depending on how much Al b was 

initially formed during base addition). If r» value is 1.0 or less, 

equilibrium will probably not be achieved after several years' 

aging.

4. The amount of Al b present in solution after 23 hour*'' aging depends on the rate of base addition during solution preparation. 

The slower the base is added, the greater the quant: ty of Ala+b 

formed and the less Al c initially formed. Some of the Al a formed 

is rapidly converted into Al b, but this conversion is limited by 

the particle-size solubility effect. Thus, as soon as enough Al b is 

formed to bring this effect into operation, the concentration of 

Al a remains constant. The amount of Al b present then decreases 

as a function of aging time. Over considerable time intervals 

during aging, the disappearance of Al b is approximately first 

order with respect to concentration of Al b . As Al b disappears 

from the system, an equivalent amount of Al c is formed. It 

seems that Al b particles are coalescing into larger particles as a 

function of time. At some critical size the character of the Al b 

particles appears to change, and they behave as the less reactive 

Al° particles. After equilibrium is achieved, the quartity of Al b 

present is below the detection limit.

5. If Al b is considered to be small solid particles holding the solution 

at apparent supersaturation relative to activities of monomeric 

species, it is possible to estimate the Gibbs free energy of the 

"face" (TF) and "edge" (7^) of a gibbsite particle-perchlorate 

interface. From the present work, assuming the overall reactions taking place to be

Al+3 +Al(OH)+2 +Al(OH)2++Al(OH) 4-+5H20<=±4Al(OH) 3 +5H+,

the values (90-percent-confidence-level intervals for the mean 

of values obtained) for these excess energies are

7£=483±84 ergs/cm2, and 

7F = 140 ±24 ergs/cm2 .

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Publishers, 747 p.

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nitrate: Am. Chem. Soc. Jour., v. 82, p. 4502-4509.
AGING OF ALUMINUM HYDROXIDE COMPLEXES D51

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 1963b, Solubility of simple aluminum compounds expected in scils, IV. 

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 1971, Relations among equilibrium and non-equilibrium aqueous species of 

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U.S. GOVERNMENT PRINTING OFFICE: 1972 O 466-125