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According to the column II of Annex VIII, an experimental study on hydrolysis as a function of pH is not required for aluminium dihydrogen triphosphate (CAS 13939-25-8) as the substance is poorly soluble.

Basically aluminium dihydrogen triphosphate is subject to hydrolysis when the substance is released to water. During hydrolysis, triphosphate decomposes into orthophosphate ion and pyrophosphate ion in a parallel step. The pyrophosphate ion undergoes further hydrolysis and is converted into orthophosphate ions. Thus the hydrolysis reaction leads directly to the formation of orthophosphate.

The hydrolysis of well soluble sodium tripolyphosphate was investigated in sterile aqueous buffers at pH 3, 4, 5 and 7, and at temperatures between 40 and 70 °C, by Zidner, Hertz and Oswald (1984). The experimental data were in good agreement with the following mechanism and gave a pseudo first-order reaction law.

P3O105- + H2O → PO43- + P2O74- + 2 H

P2O74- + H2O →2 O43- + 2H+

In sterile water the predicted half-life of triphosphate at pH 7-8 and at 20 °C is in the order of years.

The kinetics of hydrolysis of triphosphate and pyrophosphate were also studied in sterile lake water and sterile algal culture media and in non-sterile media at 25 °C by Clesceri and Lee (1965a, 1965b),and compared to published results obtained in distilled water. The results showed that triphosphate and pyrophosphate were hydrolysed in orthophosphate in a period of several days. Addition of glucose increased the rate of hydrolysis, indicating that microbial activity was one of the primary mechanisms of hydrolysis.

The hydrolytic half-life anticipated for the anions of specified phosphates, pyrophosphates and triphosphates has been addressed by read across from a single example for each chemical class, where applicable. The ionic substances were assumed to undergo dissociation in aqueous solution and the resulting cation to have negligible influence on the hydrolysis rate of the anion within the buffer solutions used as instructed by Method 111 of the OECD Guidelines (Harlan, 2011).

The available data demonstrate that triphosphates and triphosphates are hydrolytically stable under environmental conditions with a half-life > 1 year.

Chemical Class

Substance Used for Experimental Determination

Anticipated Half-Life at 25°C

 

Phosphates Orthophosphates

Not applicable, no possible mechanism for hydrolysis

N/A

Pyrophosphates Diphosphates

Tetrapotassium Pyrophosphate

CAS 7320-34-5

pH4 > 1 year pH 7 > 1 year

Triphosphates Tripolyphosphates

Pentapotassium Triphosphate

CAS 13845-36-8

pH 9

> 1 year

pH4

pH 7

14.5 days > 1 year

pH 9

> 1 year

 

The presence of multivalent metal ions, such as aluminum ion, catalyzes hydrolysis reaction and increase hydrolysis rate, as hydrolysis of aluminium itself forms insoluble hydrolytic precipitantes, such as aluminium hydroxide or oxide. Orthophosphate can be incorporated into either biological solids (e.g. microorganisms) or chemical precipitates and removed from water,

The hydrolysis of aluminium strongly depends on pH and on the aluminium concentration. 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 to 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. Szabo et al. (1978) determined the change of water activity due to the variation of the aluminium content of alkaline aluminate solutions. In systems of the same total alkali concentration an increase of the aluminium content leads to a general increase in the water activity of the solution. The water activity at low aluminium concentrations showed a smaller, almost linear, increase. In case smaller mole ratios (greater aluminium concentrations) a greater increase of water activity was determined leading to a steeper straight line.

References:

Clesceri N.L. and Lee G.F. (1965a) Hydrolysis of Condensed Phosphates – II : Sterile Environment, Int. J. Air Wat. Poll. 9, 743-751.

Clesceri N.L. and Lee G.F. (1965b) Hydrolysis of Condensed Phosphates – I : Non-Sterile

Zinder B., Hertz J. and H. R. Oswald (1984) Kinetic Studies on the Hydrolysis of Sodium Tripolyphosphate in Sterile Solution, Water Res. 18 (5), 509-512.