[nitrilotris(methylene)]trisphosphonic acid, sodium salt

Hydrolysis

Description of key information

ATMP 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 with ATMP-H from dark and sterile controls from several types of degradation study, in which no degradation (as production of orthophosphate) was observed over time periods of up to 60 days under ambient conditions in water. There is evidence (reliability 4) that partial breakdown (removal of one chain from the N atom, replaced by H) occurs, at a rate of 40% over 4 days (Steber and Wierich, 1987).

 

Schowanek and Verstraete (1991) (reliability 4) report abiotic degradation half-lives of 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 half-life for the reaction of ATMP in the presence of equimolar Mn(II) and in equilibrium with 0.21 atm O2 was 10 min at pH 6.5. The reaction occurs more slowly under more alkaline or acidic conditions.

 

The acid, sodium, potassium and ammonium salts in the ATMP category are freely soluble in water. The ATMP anion can be considered fully dissociated from its sodium, potassium or ammonium cations when in dilute solution. Under any given conditions, the degree of ionisation of the ATMP 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 ATMP-H, ATMP.4Na, ATMP.7K or another salt of ATMP.

 

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

1. ATMP is present as ATMP-H or one of its ionised forms. The degree of ionisation depends upon the pH of the media and not whether ATMP (3-5K) salt, ATMP (3-5Na) salt, ATMP-H (acid form), or another salt was used for dosing.
2. Disassociated potassium, sodium or ammonium cations. The amount of potassium or sodium present depends on which salt was dosed.
3. It should also be noted that divalent and trivalent cations would preferentially replace the sodium or potassium ions. These would include calcium (Ca²⁺), magnesium (Mg²⁺) and iron (Fe³⁺). These cations are more strongly bound by ATMP than potassium, sodium and ammonium. This could result in ATMP-dication (e.g. ATMP-Ca, ATMP-Mg) and ATMP-trication (e.g. ATMP-Fe) complexes being present in solution.

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