Form and stability of aluminium hydroxide complexes in dilute solution
Laboratory studies of solutions 4.53 x 10/sup -4/ to 4.5 x 10/sup -5/ molal (12.2 -1.2 pp.) in aluminum, in 0.01 molal sodium perhlorate, were conducted to obtain information as to the probable behavior of aluminum in natural water. When the solutions were brought to pH 7.5 -9.5 and allowed to stand for 24 hours, a precipitate was obtained which was virtually amorphous as shown by x-rays, and which had a solubility equivalent to that of boehmite. This precipitate had a hydrolysis constant (ks4) of 1.93 x 10/sup -13/. When solutions were allowed to stand at this pH range for 10 days, their precipitates gave the x-ray pattern of bayerite (ks4=1.11 x 10/sup -4/). These hydrolysis constants were obtained at 25 C. and corrected to zero ionic strength and are in close agreement with other published values. The predominant dissolved form in this pH range is Al(OH)/sup 4 -/. Below neutral pH (7.0) the dissolved aluminum species consist of octahedral units in which each aluminum ion is surrounded by six water molecules or hydroxide ions. Single units such as Al(OH/sub 2/)/sup 6 +3/ and AlOH*OH/sub 2/)/sup 5 +2/ are most abundant below pH 5.0, and where the molar ratio (r) is greater than 1.0, polymerization of the octahedral units occurs. Particles whose diameters were greater than 0.10 micrometers were identified by x-ray diffraction as gibbsite. Particles smaller than 0.10 micrometers were also present and were shown by means of the electron microscope to have a hexagonal crystal pattern. Structured material consisting of sheets of coalesced six-membered rings of aluminum ions held together by double OH bridges has a distinctive kinetic behavior. The kinetic experiments and stoichiometric data for solutions aged for long periods provided a means of determinng activities of unpolymerized aluminum. From these values the solubility product for microcrystalline gibbsite was determined to be 2.24 x 10/sup -33/, and its free energy of formation, -272.3+/sup =/0.4 kcal per mole.
- OSTI ID:
- 5607253
- Journal Information:
- Geol. Surv. Water-Supply Pap. (U.S.); (United States), Vol. 1827-A
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ALUMINIUM COMPLEXES
STABILITY
STRUCTURAL CHEMICAL ANALYSIS
ALUMINIUM
ALUMINIUM COMPOUNDS
FREE ENERGY
GIBBSITE
HYDROLYSIS
PH VALUE
SODIUM PERCHLORATES
SOLUBILITY
WATER
WATER POLLUTION
X-RAY DIFFRACTION
ALKALI METAL COMPOUNDS
ALUMINIUM HYDROXIDES
CHEMICAL REACTIONS
CHLORINE COMPOUNDS
COHERENT SCATTERING
COMPLEXES
DECOMPOSITION
DIFFRACTION
ELEMENTS
ENERGY
HALOGEN COMPOUNDS
HYDROGEN COMPOUNDS
HYDROXIDES
LYSIS
METALS
MINERALS
OXIDE MINERALS
OXYGEN COMPOUNDS
PERCHLORATES
PHYSICAL PROPERTIES
POLLUTION
SCATTERING
SODIUM COMPOUNDS
SOLVOLYSIS
THERMODYNAMIC PROPERTIES
520200* - Environment
Aquatic- Chemicals Monitoring & Transport- (-1989)