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Physical & Chemical properties

Dissociation constant

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Reference
Endpoint:
dissociation constant
Type of information:
other: Peer reviewed official report.
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Peer reviewed official report.
Qualifier:
no guideline followed
Principles of method if other than guideline:
official risk assessment, peer reviewed
Dissociating properties:
yes
Remarks on result:
other: See the field "Any other information on results incl.table" below

At low pH values, dissolved aluminium is present mainly in the aquo form (Al3+). Hydrolysis occurs as pH rises, resulting in a series of less soluble hydroxide complexes (e.g. Al(OH)2+, Al(OH)2+). Aluminium solubility is at a minimum near pH 6.5 at 20 °C and then increases as the anion, Al(OH)4-, begins to form at higher pH (Driscoll and Schecher, 1990; Witters et al., 1996). Thus, at 20 °C and pH <5.7, aluminium is present primarily in the form Al3+ and Al(OH)2+. In the pH range 5.7 to 6.7, aluminium hydroxide species dominate, including Al(OH)2+ and Al(OH)2+, and then Al(OH)3. Typically, at a pH of approximately 6.5, Al(OH)3 predominates over all the other species. In this range, aluminium solubility is low, and availability to aquatic biota should also be low. At pH >6.7, Al(OH)4becomes the dominant species. It is important to note that the various aluminium species described above are always present simultaneously at any pH value. The influence of pH in aquatic systems is mainly to change the proportion of all the species as the pH changes (2008 email from Canadian Wastewater Association to J. Pasternak, Environment Canada; unreferenced).

Mononuclear aluminium hydrolytic products combine to form polynuclear species in solution (Bertsch and Parker 1996). Aluminium begins to polymerize when the pH of an acidic solution increases notably over 4.5:

 

     2 Al(OH)(H2O)52+                      Al2(OH)2(H2O)84+   +   2 H2O

 

Polymerization gradually proceeds to larger structures, eventually leading to the formation of the Al13 polycation (Parker and Bertsch 1992a, 1992b).

 

Driscoll CT, Schecher WD. 1990. The chemistry of aluminum in the environment. J. Environ Perspect Health 12: 28–49.

Witters HE, Van Puymbroeck S, Stouthart AJHX, Bonga SEW. 1996. Physicochemical changes of aluminium in mixing zones: mortality and physiological disturbances in brown trout (Salmo truttaL.). Environ Toxicol Chem 15: 986–996.

Bertsch PM, Parker DR. 1996. Aqueous polynuclear aluminum species. In: Sposito G, editor, The environmental chemistry of aluminum. 2nd edition. Boca Raton, Florida: CRC Press p. 117–168.

Parker DR, Bertsch PM. 1992a. Identification and quantification of the “Al13” tridecameric polycation using ferron. Environ Sci Technol 26: 908–914.

Parker DR, Bertsch PM. 1992b. Formation of the “Al13” tridecameric polycation under diverse synthesis conditions. Environ Sci Technol 26: 914–921.

 

Description of key information

Behaviour of aluminium in water is very complex. Aluminium speciation depends strongly on pH:

pH < 5.7:        Al3+ and [Al(OH)2]+

pH 5.7 to 6.7:       [Al(OH)]2+, [Al(OH)2]+ and Al(OH)3

pH > 6.7:       [Al(OH)4]-, polymerization

Key value for chemical safety assessment

Additional information

The behavior and dissociation of aluminium is common knowledge, as described in the assessment report for the three aluminium salts (chloride, nitrate and sulphate) under the Canadian Environmental Protection Act:

At low pH values, dissolved aluminium is present mainly in the aqua form (Al3+). Hydrolysis occurs as pH rises, resulting in a series of less soluble hydroxide complexes (e.g. Al(OH)2+, Al(OH)2+). Aluminium solubility is at a minimum near pH 6.5 at 20 °C (Al(OH)3) and then increases as the anion, Al(OH)4-, begins to form at higher pH. It is important to note that the various aluminium species described above are always present simultaneously at any pH value. The influence of pH in aquatic systems is mainly to change the proportion of all the species as the pH changes.

Mononuclear aluminium hydrolytic products combine to form polynuclear species in solution. Aluminium begins to polymerize when the pH of an acidic solution increases notably over 4.5.