Reaction products of disodium 3,5-diamino-2-[(2-sulfonatophenyl)diazenyl]benzoate and  diazotised sodium 2-[(4-aminophenyl)sulfonyl]ethyl sulfate

Dissociation constant

Link to relevant study record(s)

Reference

Endpoint:

dissociation constant

Type of information:

calculation (if not (Q)SAR)

Adequacy of study:

supporting study

Study period:

Study completion date - 15 July 2003

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

accepted calculation method

Qualifier:

no guideline required

Principles of method if other than guideline:

Considering that the substance is a sulfonic acid sodium salt and thus hydrolytically unstable in water as supported by the water solubility test, experimental determination of the dissociation constant is not required and instead the ionic nature at environmental relevant pH is supported by an expert statement based on assessment of the substance's functional groups.

GLP compliance:

no

Specific details on test material used for the study:

Identity: FAT 40810/A
Batch: WP 6/02
Purity: approx. 75 %
Appearance: Solid, dark brownish powder
Expiration date: 12 December 2010
Storage: At room temperature at about 20 °C

No.:

#1

pKa:

-3.84

Remarks on result:

other: ± 0.23

Remarks:

Primary amine #1

No.:

#2

pKa:

-6

Remarks on result:

other: ± 0.29

Remarks:

Primary amine #2

No.:

#3

pKa:

-1.25

Remarks on result:

other: ± 0.30

Remarks:

Sulfonic acid group

No.:

#4

pKa:

-3.91

Remarks on result:

other: ± 0.18

Remarks:

Sulfuric acid ester (2x)

No.:

#5

pKa:

2.42

Remarks on result:

other: ± 0.50

Remarks:

Carboxylic acid group

The behaviour of the test item in aqueous solution is dominated by the strong acidic sulfonic and carboxylic acid and sulfate ester groups. In the whole environmentally relevant pH range (pH 5 to pH 9) all acidic groups will be more than 99 % dissociated and the primary amines will not be protonated.

Conclusions:

FAT 40810/A is ionic under environmental conditions of pH 5 to 9 due to its strongly negative pKa.

Executive summary:

The purpose of this study was to estimate the dissociation constants (pKa-values) of FAT 40810/A. The molecular structure of FAT 40810/A was used for the estimation of the dissociation behaviour. The compound has six sites, which can be protonated or dissociated. The pKa-values of FAT 40810/A were computed using the pKa prediction module of ACD Inc. LogD Solubility Suite v.7.0 [8]. The behaviour of FAT 40810/A in aqueous solutions is dominated by the strong acidic sulfonic and carboxylic acid and sulfate ester groups. In the whole environmental relevant pH range (pH 5 to pH 9) all acidic groups will be more than 99 % dissociated and the primary amines will not be protonated.

 

Acid/Base	pKa
Primary amines (2 x)	-3.84 ± 0.23 and -6.00 ±0.29 respectively
Sulfonic acid group	-1.25±0.30
Sulfuric acid ester (2 x)	-3.91±0.18
Carboxylic acid group	2.42±0.50

Description of key information

The purpose of this study was to estimate the dissociation constants (pKa-values) of FAT 40810/A. The molecular structure of FAT 40810/A was used for the estimation of the dissociation behaviour. The compound has six sites, which can be protonated or dissociated. The pKa-values of FAT 40810/A were computed using the pKa prediction module of ACD Inc. LogD Solubility Suite v.7.0 [8]. The behaviour of FAT 40810/A in aqueous solutions is dominated by the strong acidic sulfonic and carboxylic acid and sulfate ester groups. In the whole environmental relevant pH range (pH 5 to pH 9) all acidic groups will be more than 99 % dissociated and the primary amines will not be protonated.

 

Acid/Base	pKa
Primary amines (2 x)	-3.84 ± 0.23 and -6.00 ±0.29 respectively
Sulfonic acid group	-1.25±0.30
Sulfuric acid ester (2 x)	-3.91±0.18
Carboxylic acid group	2.42±0.50

Key value for chemical safety assessment

pKa at 20°C:

-6