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The new ECHA CHEM database has been released by ECHA, and it now contains all REACH registration data. There are more details on the transition of ECHA's published data to ECHA CHEM here.

Diss Factsheets

Physical & Chemical properties

Dissociation constant

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Endpoint:
dissociation constant
Type of information:
experimental study
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
data from handbook or collection of data
Principles of method if other than guideline:
No details are available on the guideline used to determine this result.
GLP compliance:
no
Dissociating properties:
yes
No.:
#1
pKa:
1.03
Remarks on result:
other: Temperature not stated
No.:
#2
pKa:
2.08
Remarks on result:
other: Temperature not stated
No.:
#3
pKa:
3.11
Remarks on result:
other: Temperature not stated
No.:
#4
pKa:
4.15
Remarks on result:
other: Temperature not stated
No.:
#5
pKa:
5.19
Remarks on result:
other: Temperature not stated
No.:
#6
pKa:
6.23
Remarks on result:
other: Temperature not stated
No.:
#7
pKa:
7.23
Remarks on result:
other: Temperature not stated
No.:
#8
pKa:
8.3
Remarks on result:
other: Temperature not stated
No.:
#9
pKa:
11.8
Remarks on result:
other: Temperature not stated
No.:
#10
pKa:
12.58
Remarks on result:
other: Temperature not stated

In a source giving no experimental details, DTPMP is described as having ten pKa values, of: 

pKa1 = 1.03 

pKa2 = 2.08 

pKa3 = 3.11

pKa4 = 4.15

pKa5 = 5.19

pKa6 = 6.23

pKa7 = 7.23

pKa8 = 8.30

pKa9 = 11.8

pKa10 = 12.58

(Ref: Tomson et.al. 1994). These values are a more complete set of data, are consistent with the values from the standard source, and are the best available set.

Endpoint:
dissociation constant
Type of information:
experimental study
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
data from handbook or collection of data
Principles of method if other than guideline:
No information on method stated
GLP compliance:
no
Dissociating properties:
yes
No.:
#3
pKa:
2.8
Remarks on result:
other: Temperature not stated
No.:
#4
pKa:
4.45
Remarks on result:
other: Temperature not stated
No.:
#5
pKa:
5.5
Remarks on result:
other: Temperature not stated
No.:
#6
pKa:
6.38
Remarks on result:
other: Temperature not stated
No.:
#7
pKa:
7.17
Remarks on result:
other: Temperature not stated
No.:
#8
pKa:
8.15
Remarks on result:
other: Temperature not stated
No.:
#9
pKa:
10.1
Remarks on result:
other: Temperature not stated
No.:
#10
pKa:
12.04
Remarks on result:
other: Temperature not stated

DTMTP has ten possible ionisations. In a recognised data source for information on pKa values and stability constants (Martell and Sillen), eight pKa values of DTPMP are reported: 

pKa3 = 2.8
pKa4 = 4.45
pKa5 = 5.5
pKa6 = 6.38
pKa7 = 7.17
pKa8 = 8.15
pKa9 = 10.1
pKa10 = 12.04

The ionisation number has been assigned by the reviewer to align with those from a more complete source (below). These were measured in 0.1 M potassium chloride. The original source (a 1967 paper in Russian) is cited in the data book.

Description of key information

Dissociation constant: DTPMP acid has ten ionisable phosphonate groups.

Key value for chemical safety assessment

Additional information

In aqueous solution, DTPMP-xNH4 will ionise to form DTPMP and ammonium ions. Dissociation constants are obtained for the salt by valid read-across from the parent acid. Ten dissociation constants as listed below were obtained for the parent acid using an appropriate method. The result is considered reliable and selected as key study.

pKa1 = 1.03 
pKa2 = 2.08 
pKa3 = 3.11
pKa4 = 4.15 
pKa5 = 5.19 
pKa6 = 6.23 
pKa7 = 7.23 
pKa8 = 8.30 
pKa9 = 11.8 
pKa10 = 12.58

Eight of the ten pKa values were determined in a second reliable study (Martell 1968) and are listed below.

pKa3 = 2.8
pKa4 = 4.45
pKa5 = 5.5
pKa6 = 6.38
pKa7 = 7.17
pKa8 = 8.15
pKa9 = 10.1
pKa10 = 12.04

The two sources are considered to be reliable and are used as weight of evidence.

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 between NH3 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/(10pKa-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 pKa 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).