3-amino-2,4,6-triiodobenzoic acid

Dissociation constant

Administrative data

Endpoint:

dissociation constant

Type of information:

experimental study

Adequacy of study:

key study

Study period:

09 March 2001

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Data source

Reference

Reference Type:

study report

Title:

Unnamed

Year:

2001

Report date:

2001

Materials and methods

Test guideline

Qualifier:

according to guideline

Guideline:

OECD Guideline 112 (Dissociation Constants in Water)

Deviations:

no

GLP compliance:

yes

Test material

Test material information

Constituent 1

Reference substance name:

3-amino-2,4,6-triiodobenzoic acid

EC Number:

221-493-0

EC Name:

3-amino-2,4,6-triiodobenzoic acid

Cas Number:

3119-15-1

Molecular formula:

C7H4I3NO2

IUPAC Name:

3-amino-2,4,6-triiodobenzoic acid

Test material form:

solid: bulk

Specific details on test material used for the study:

SOURCE OF TEST MATERIAL
- Source and lot/batch No.of test material: Wirkstoffbetrieb F Bergkamen, Schering AG; 60708021

Results and discussion

Dissociating properties:

yes

Dissociation constant

No.:

#1

pKa:

> 2.7 - < 2.9

Remarks on result:

other: 20 % by Vol. CH3CN

Any other information on results incl. tables

Carboxylic group

The results of the experiment are provided in Table 1.

 

A superposition of the spectra measured at different pH-values is shown in Figure 1 attached.

pH value	A230
1.94	420.2
1.98	425.2
2.23	437.4
2.49	461.4
2.7	476.8
2.85	489.1
3.05	501.8
3.29	514.6
3.67	527
4.16	536.9
4.64	541.9
5.16	552.2
5.66	550.5
6.11	561.4
6.49	564
6.8	560.5
Ka	1.7 · 10-3
Aun,λ	404.6
Ai,λ	552.7

A₂₃₀= absorbance at 230 nm (mAU)

Using a nonlinear regression based on Equ.1 the following pKa value was obtained (with 95%

confidence interval)

 pKa (20% CH3CN) = 2.8 ± 0.1

Estimation of the pKa-value of the anilinium group

For anilinium ions the following Hammett equation was published (Ref.2)

1.) pKa = 4.58 - 2.88⋅Σσ

Σσ = sum of substituent constants for the Hammett equation.

Table 2: Substituent constants for the Hammett equation of the testing substance

group	σ
2 Iortho	2.68
Ipara	0.21
COOHmeta	0.35
Σσ	3.24
pKa (Equ.1)	-4.8

 

Inserting the sum of the individual substituent constants into Equ.1 the pKa value of the protonated anilinium group was calculated to be: pKa = . 4.8

 

The result was also supported by a calculation with the ACD program giving a pKa-value of the protonated anilinium group pKa = . 1.4.

 

These results give evidence, that the amino group of the test item cannot be protonated in a pH range 2 .10, which is of relevance for biological systems.

Applicant's summary and conclusion

Conclusions:

The acid dissociation constants of the test item was determined with a modified photometric method according to OECD 112 in aqueous solution containing 20 % by Vol. CH3CN. The following pKa-value was obtained (with 95 % confidence interval): pKa (20% CH3CN) = 2.8 ± 0.1.

Executive summary:

The acid dissociation constants of the test item was determined with a modified photometric method according to OECD 112 in aqueous solution containing 20 % by Vol. CH3CN.

The following pKa-value was obtained (with 95 % confidence interval): pKa (20% CH3CN) = 2.8 ± 0.1

A calculation with the Hammett equation for substituted anilinium ions gives evidence, that the amino group of the test item cannot be protonated in a pH range 2 - 10, which is of relevance for biological systems.