Sodium salts of [[(phosphonomethyl)imino]bis[ethane-2,1-diylnitrilobis(methylene)]]tetrakisphosphonic acid (1-3 Na:1)

Hydrolysis

Link to relevant study record(s)

Reference

Endpoint:

hydrolysis

Data waiving:

study scientifically not necessary / other information available

Justification for data waiving:

other:

Description of key information

DTPMP-H and its salts are not hydrolytically unstable under ambient conditions.

Additional information

The requirement to test the substance for hydrolysis was waived as the substance is not susceptible to hydrolytic degradation.

This is supported by results from dark and sterile controls, in which no degradation (as production of orthophosphate) was observed over time periods of up to 50 days under ambient conditions in water (Schowanek and Verstraete (1991)).

Schowanek and Verstraete (1991) report abiotic degradation half-lives in the order of 30 days in the presence of calcium, magnesium and iron divalent ions. Nowack (2003) remarks that degradation of amine containing phosphonates is negligible in metal ion-free oxygenated solutions; the presence of metal ions increases the rate of degradation to approximately 1% per day. The degradation is also negligible in the absence of dissolved oxygen indicating that redox reactions play a role.

The acid and salts in the DTPMP category are freely soluble in water and, therefore, the DTPMP anion is fully dissociated from its cations when in solution. Under any given conditions, the degree of ionisation of the DTPMP species is determined by the pH of the solution. At a specific pH, the degree of ionisation is the same regardless of whether the starting material was DTPMP-H, DTPMP (1-3Na), DTPMP (5-7Na), DTPMP-xK, DTPMP (xNH4) or another salt of DTPMP.

 

Therefore, when a salt of DTPMP is introduced into test media or the environment, the following is present (separately):

1. DTPMP is present as DTPMP-H or one of its ionised forms. The degree of ionisation depends upon the pH of the media and not whether DTPMP-H, DTPMP (1-3Na), DTPMP (5-7Na), DTPMP-xK, DTPMP (xNH4), or another salt was used for testing.

2. Disassociated ammonium, potassium or sodium cations. The amount of ammonium, potassium or sodium present depends on which salt was added.

3. Divalent and trivalent cations have much higher stability constants for binding with DTPMP than the sodium, potassium or ammonium ions so would preferentially replace them. These ions include calcium (Ca²⁺), magnesium (Mg²⁺) and iron (Fe³⁺). Therefore, the presence of these in the environment or in biological fluids or from dietary sources would result in the formation of DTPMP-dication (e.g. DTPMP-Ca, DTPMP-Mg) and DTPMP-trication (e.g. DTPMP-Fe) complexes in solution, irrespective of the starting substance/test material.

Nowack, B. (2003). Review: Environmental chemistry of phosphonates. Water research (37), pp 2533-2546.