[nitrilotris(methylene)]trisphosphonic acid, ammonium salt

Dissociation constant

Link to relevant study record(s)

Reference

Endpoint:

dissociation constant

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

data from handbook or collection of data

Principles of method if other than guideline:

not stated

GLP compliance:

no

Dissociating properties:

yes

No.:

#1

pKa:

< 2

Temp.:

25 °C

No.:

#2

pKa:

< 2

Temp.:

25 °C

No.:

#3

pKa:

4.3

Temp.:

25 °C

Remarks on result:

other: ± 0.05

No.:

#4

pKa:

5.46

Temp.:

25 °C

Remarks on result:

other: ±0.05

No.:

#5

pKa:

6.66

Temp.:

25 °C

Remarks on result:

other: ±0.04

No.:

#6

pKa:

12.3

Temp.:

25 °C

Remarks on result:

other: ±0.1

ATMP has six possible ionisations.

pK1 <2
pK2 <2
pK3 4.3
pK4 5.46
pK5 6.66
pK6 12.3

These were measured in 1 M potassium nitrate.
These data are from a recognised data source for information on pKa  values and stability constants (Martell and Sillen).

Conclusions:

Dissociation constant values of <2, <2, 4.3, 5.46, 6.66, 12.3 are reported at 25 °C in a handbook which has been subject to peer-review and in which the original data sources are traceable. The result is considered to be reliable.

Description of key information

Dissociation constant [ATMP-xNH4]: ATMP acid has six ionisable phosphonate groups.

Additional information

In aqueous solution, ATMP-xNH4 will dissociation to form ATMP and potassium ions. Dissociation constants are obtained for the salt by valid read-across from the parent acid. Six dissociation constants as listed below were reported for ATMP-H from a handbook or collection of reliable data. The result (Martell 1968) is considered reliable and selected as key study.

pKa1 = <2
pKa 2 = <2
pKa 3 = 4.3
pKa 4 = 5.5
pKa 5 = 6.7
pKa 6 = 12.3

In Lacour 1999, dissociation constant valuesat 25°C were reported for ATMP-H, using a potentiometric method.

pKa1 = 0.5

pKa2 = 1.6

pKa3 = 4.6

pKa4 = 5.9

pKa5 = 7.2

pKa6 = 12.5

Similarly, in a secondary source to which reliability could not be assigned, six dissociation constant values as listed below are reported for ATMP-H.

pKa1 = 2.2
pKa2 = 2.2 
pKa3 = 4.5
pKa4 = 5.9 
pKa5 = 7.3 
pKa6 = 11.4

 

An approximate experimental determination, using stoichiometric titration and pH determination of aqueous solutions, has been conducted. This is considered to provide suitable and practical information, which is fit for purpose in the context of REACH chemical safety assessment. This data is reported in section 4.20.

Ammonia has a pKa of 9.25 at 25°C. Under ambient environmental conditions, ammonia is a stable substance that shows normal acid/base chemical activity with the following equilibria:

NH₄⁺ + H₂O ↔ NH₃ + H₃O⁺

NH₃ + H₂O ↔ NH₄⁺ + OH^-

The ammonia/ammonium ion in aqueous solution exists in equilibrium betweenNH₃ and NH₄⁺, depending on the pH. Where the term ammonia is used throughout this dossier, it refers to the equilibrium mixture of ammonia/ammonium unless otherwise stated. In general, as pH increases, the fraction of the total ammonia which is unionised increases. The fraction of unionised ammonia can be calculated using the following equation:

fractionunionised = 1/(10^pKa-pH +1)

At pH 8.5, the proportion of unionised ammonia is approximately 10 times that at pH 7.5. The concentration of unionised ammonia will be lower at higher ionic strengths of very hard freshwater or saltwater environments. This effect can be significant in estuarine and marine waters. Moreover, the pK_a is reciprocally related to temperature. For every 9°C increase in temperature, the proportion of unionised ammonia approximately doubles (EA 2007).

Reference:

Environment Agency (2007). Proposed EQS for Water Framework Directive Annex VIII substances: ammonia (un-ionised). Science Report: SC040038/SR2; SNIFFER Report: WFD52(ii).