Aluminium tris(dihydrogen phosphate)

Hydrolysis

Link to relevant study record(s)

Reference

Endpoint:

hydrolysis

Type of information:

migrated information: read-across based on grouping of substances (category approach)

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Well documented non.guideline study

Principles of method if other than guideline:

The effect of pH on aluminium chloride hydrolysis was determined.

GLP compliance:

no

Details on sampling:

A test substance concentration of 1.5E-4 mol/L was prepared. The pH of the solution was adjusted to 4.0, 4.8, 5.0, 5.2, 5.8, and 6.4 respectively

Buffers:

- pH: 4.01 and 7.00
- Type of buffer: Tetramethylammmonium hydroxide pentahydrate and HCL solution

Transformation products:

not specified

Remarks:

see "any other information on results"

Remarks on result:

not measured/tested

Remarks on result:

not measured/tested

When AlCl3 was diluted to a concentration of 1.5*E-4 mol/L hydrolysis occurred immediately At pH 4 mono- and dimeric aluminium species were detected as main products. With increasing pH the hydrolysis and polymerisation increased. Monomeric and dimeric aluminium species hydrolysed and polymerised into small polymeric aluminium species at pH 4.8. At pH 5 the small polymeric aluminium species polymerised into median polymeric species. At pH 5.8 metastable median and large polymers decomposed into small aluminium species and disaggregated into dimeric species. At pH 6.4 the majority of aluminium species formed Al(OH)3 amorphous flocks.

Additional information

An experimental study on hydrolysis as a function of pH is not available for aluminium tris(dihydrogen phosphate) (CAS 13530-50-2) as the substance dissociates in aqueous solution. Aluminium undergoes hydrolysis forming various species strongly as a function of pH. The pH dependency of aluminium hydrolysis was investigated by Zhao et al. (2009). The hydrolysis of aluminium chloride was tested at pH values ranging from 4 - 6.4. When AlCl3 was diluted to a concentration of 1.5*E-4 mol/L hydrolysis occurred immediately. At pH 4 mono- and dimeric aluminium species were detected as main products. With increasing pH the hydrolysis and polymerisation increased. Monomeric and dimeric aluminium species hydrolysed and polymerised into small polymeric aluminium species at pH 4.8. At pH 5 the small polymeric aluminium species polymerised into median polymeric species. At pH 5.8 metastable median and large polymers decomposed into small aluminium species and disaggregated into dimeric species. At pH 6.4 the majority of aluminium species formed Al(OH)3 amorphous flocks.

Phosphoric acid is a weak acid that does not fully dissociate in water. Salts containing the anion H2PO4¯ are weakly acidic. When dihydrogen phosphate salt is dissolved in solution, equilibria are established among the four species H3PO4 (phosphoric acid itself), H2PO4-1 (dihydrogen phosphate anion), HPO4-2 (hydrogen phosphate anion), and PO4-3 (phosphate anion). The tendency of the H2PO4¯ ion to dissociate is greater than its tendency to hydrolyse to HPO4-2. Whereas the salts of HPO4-2 are weakly basic, and the tendency of this ion to hydrolyse is greater than its tendency to dissociate. Various phosphate ions maintain a dissociation equilibrium state and are present as dominant phosphorus species in water under the normal environmental conditions. 

H3PO4 + H2O < --- > H2PO4¯ + H3O+                   pK1 = 2.12

H2PO4¯ + H2O < --- > HPO4-2 + H3O+                  pK2 = 7.21

HPO4-2 + H2O < --- > PO4-3 + H3O+                    pK3 = 12.44

Further dissociation forming tertiary phosphates, such as orthophosphate, give a strongly alkaline solution with pH > 12. However, this is unlikely for aluminium tris(dihydrogen phosphate) under environment conditions. Monohydrogen and dihydrogen phosphates act as a buffer over a pH range of 6 – 8 and have an important biological function in the aquatic environment.

As ions monohydrogen and dihydrogen phosphates are well soluble in water. However, they can be incorporated into either biological solids (e.g. microorganisms) or chemical precipitates and thus are removed from water e.g. by the formation of insoluble aluminum hydroxide at a pH value between 6 and 8.