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Physical & Chemical properties

Dissociation constant

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Endpoint:
dissociation constant
Data waiving:
study scientifically not necessary / other information available
Justification for data waiving:
the study does not need to be conducted because the substance is insoluble
Justification for type of information:
Adaptation of the standard data requirements is based on REACH Annex IX - section 7.16 column 2: there are no dissociative groups present in the carbon skeleton of Activated Carbon. Therefore a study on the dissociation of the chemical structure of the carbon moiety is waived. N.B.: Information on the dissociation of minor components is given in other study summaries in this IUCLID section. The information does not come from a test report, and the minimum fields required cannot be filled in. Phosphoric acid and its sodium salts are well-studied chemicals with data published in a range of handbooks and although no details are available from these sources, it is scientifically unjustified to repeat these studies.
Endpoint:
dissociation constant
Type of information:
experimental study
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: see 'Remark'
Remarks:
Data were from an internationally recognised Database that is peer reviewed. They were taken from a Handbook that selects suitable sources for each property and is considered as reliable by the scientific community. The reference sources are given but information on the material and method is concise.
Principles of method if other than guideline:
no details presented
Dissociating properties:
yes
No.:
#1
pKa:
2.15
No.:
#2
pKa:
7.09
No.:
#3
pKa:
12.32
Conclusions:
The dissociation constants are:
pKa #1: 2.15
pKa #2: 7.09
pKa #3: 12.32
Executive summary:

The dissociation constants are taken from an internationally recognised Handbook.

The dissociation constants for the phosphoric acid system are: pKa #1: 2.15; pKa #2: 7.09; pKa #3: 12.32

Endpoint:
dissociation constant
Type of information:
experimental study
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: see 'Remark'
Remarks:
Data were taken from an internationally recognised Handbook that selects suitable sources for each property and is considered as reliable by the scientific community. The reference sources are given but information on the material and method is concise. The Handbook contains information obtained from authentic and highly regarded sources and sources are indicated. According to the authors, best efforts have been made to select and verify the data on the basis of sound scientific judgment.
Principles of method if other than guideline:
no details presented
Values refer to dilute aqueous solutions at zero ionic strength
Dissociating properties:
yes
No.:
#1
pKa:
ca. 2.16
Temp.:
25 °C
Remarks on result:
other: no remarks
No.:
#2
pKa:
ca. 7.21
Temp.:
25 °C
Remarks on result:
other: no remarks
No.:
#3
pKa:
ca. 12.32
Temp.:
25 °C
Remarks on result:
other: no remarks
Conclusions:
The dissociation constants at 25 degree C are:pKa
pKa #1: 2.16
pKa #2: 7.21
pKa #3: 12.32
Executive summary:

The dissociation constants are taken from an internationally recognised Standard Handbook.

The dissociation constants for the phosphoric acid system in a dilute aqueous solution at zero ionic strength at 25 degree C are: pKa #1: 2.16; pKa #2: 7.21; pKa #3: 12.32

Endpoint:
dissociation constant
Type of information:
experimental study
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Data were taken from a textbook on Physical Chemistry written by a professor at an internationally respected university. Data were selected from a range of sources that are listed but not connected to the indivudual data.
Principles of method if other than guideline:
no details presented
Dissociating properties:
yes
No.:
#1
pKa:
2.12
Temp.:
25 °C
Remarks on result:
other: Ka1 = 7.5 * 10^-3
No.:
#2
pKa:
7.21
Temp.:
25 °C
Remarks on result:
other: Ka2 = 6.2 * 10^-8
No.:
#3
pKa:
12.67
Temp.:
25 °C
Remarks on result:
other: Ka3 = 2.2 * 10^-13
Conclusions:
The dissociation constants at 25 degree C are:
pKa #1: 2.12
pKa #2: 7.21
pKa #3: 12.67
Executive summary:

The dissociation constants are taken from a textbook on Physical Chemistry.

The dissociation constants for the phosphoric acid system at 25 degree C are:

pKa #1: 2.12;

pKa #2: 7.21;

pKa #3: 12.67.

Description of key information

The carbon skeleton of Activated Carbon does not contain any dissociative groups so the endpoint dissociation is waived.

For the triprotic acid, phosphoric acid, the three dissociation constants in aqueous solution were reported in various sources at 25 degree C:

pKa1 = 2.12 - 2.16

pKa2 = 7.09 - 7.12

pKa3 = 12.32 - 12.67

For the sodium dihydrogen phosphate (Na+ H2PO4-) and disodium hydrogen phosphate (Na2(2+)H2PO4(2-)) the before mentioned pKa2 and pKa3 can be used, respectively.

Key value for chemical safety assessment

Additional information

For the carbon skeleton of Activated Carbon, the adaptation of the standard data requirements is based on REACH Annex IX - section 7.16 column 2: The study is technically not feasible a the carbon skeleton is insoluble and there are no dissociative groups present in the carbon skeleton of Activated Carbon. Therefore a study on the dissociation of the chemical structure of the carbon moiety is waived.

The other constituents are phosphates. These are well-studied chemicals with reliable and verified data published in a range of handbooks and although no details are available from these sources, it is scientifically unjustified to repeat these studies. For hydrogen orthophosphate (phosphoric acid), a triprotic anion, the following dissociation steps are known:

1. H3PO4 (s) + H2O (l) is in equilibrium with H3O+ (aq) + H2PO4(aq)

2. H2PO4(aq) + H2O (l) is in equilibrium with H3O+(aq) + HPO42− (aq)

3. HPO42−(aq) + H2O (l) is in equilibrium with H3O+(aq) + PO43− (aq)

For the triprotic acid phosphoric acid, the three dissociation constants in aqueous solution were reported in various sources at 25 degrees C:

pKa1 = 2.12 - 2.16

pKa2 = 7.09 - 7.12

pKa3 = 12.32 - 12.67

The results taken from the different sources are in line with each other and give confidence in the reliability of the data (Weight of Evidence).

For the sodium dihydrogen phosphate (Na +H2PO4-) and disodium hydrogen phosphate (Na22+H2PO42-),

it is anticipated that the dissociation constants would be those of the parent anions only, so the before mentioned literature values pKa2 and pKa3 can be used, respectively. The concentrations of the sodium salts in the product reflect these acid dissociation constant data.