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Physical & Chemical properties

Dissociation constant

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Reference
Endpoint:
dissociation constant
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Justification for type of information:
REPORTING FORMAT FOR THE ANALOGUE APPROACH
See read-across justification report under Section 13 ‘Assessment Reports’.

1. HYPOTHESIS FOR THE ANALOGUE APPROACH
In accordance with REACH Annex XI, Section 1.5, of Regulation (EC) No. 1907/2006 (REACH) the standard testing regime may be adapted in cases where a grouping or read-across approach has been applied.

The similarities may be based on:
(1) a common functional group
(2) the common precursors and/or the likelihood of common breakdown products via physical or biological processes, which result in structurally similar chemicals; or
(3) a constant pattern in the changing of the potency of the properties across the category


Read-across is justified on the basis that pyrophosphoric acid is the parent acid for all inorganic pyrophosphates.

2. SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES)
See read-across justification report under Section 13 ‘Assessment Reports’.

3. ANALOGUE APPROACH JUSTIFICATION
See read-across justification report under Section 13 ‘Assessment Reports’.

4. DATA MATRIX
See read-across justification report under Section 13 ‘Assessment Reports’.
Reason / purpose:
read-across: supporting information
Qualifier:
no guideline followed
Principles of method if other than guideline:
No experimental determination of the dissociation constants in water was performed for the test materials as it was anticipated that on performance of the test procedures, as detailed in Method 112 of the OECD Guidelines for Testing of Chemicals, 12 May 1981, that the resulting dissociation constants determined would be that of the parent anions only, for which literature values are available. For example, it is anticipated that different types of orthophosphate will demonstrate significantly different pH values in water due to increasing numbers of protons being substituted with the particular cation on titration with acid or base as appropriate, the actual dissociation constants determined for each compound would be common, i.e. that of the triprotic acid anion. Titrations would also be expected to be similar irrespective of the counter ion.
GLP compliance:
no
Dissociating properties:
yes
No.:
#1
pKa:
0.9
Temp.:
25 °C
No.:
#2
pKa:
2.31
Temp.:
25 °C
No.:
#3
pKa:
6.69
Temp.:
25 °C
No.:
#4
pKa:
9.42
Temp.:
25 °C
Conclusions:
This study is considered to be scientifically justified for use as a key study. Read-across is justified on the basis that pyrophosphoric acid is the parent acid for all inorganic pyrophosphates (diphosphates).

Description of key information

No experimental determination of the dissociation constants in water was performed for the test materials as it was anticipated that on performance of the test procedures, as detailed in Method 112 of the OECD Guidelines for Testing of Chemicals, 12 May 1981, that the resulting dissociation constants determined would be that of the parent anions only, for which literature values are available. For example, it is anticipated that different types of orthophosphate will demonstrate significantly different pH values in water due to increasing numbers of protons being substituted with the particular cation on titration with acid or base as appropriate, the actual dissociation constants determined for each compound would be common, i.e. that of the triprotic acid anion. Titrations would also be expected to be similar irrespective of the counter ion. Read-across is justified on the basis that pyrophosphoric acid is the parent acid for all inorganic phosphates. 

Key value for chemical safety assessment

Additional information

Results:

 No. pKa  Temperature (°C)
 #1  0.9  25
 #2  2.31  25
 #3 6.69  25
 #4  9.42  25