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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 (5-7Na) will ionise to form DTPMP and sodium 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.