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Dissociation constant

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Reference
Endpoint:
dissociation constant
Type of information:
experimental study
Adequacy of study:
key study
Study period:
september 2012 - november 2012
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: performance of the study was not inspected
Qualifier:
according to
Guideline:
OECD Guideline 112 (Dissociation Constants in Water)
GLP compliance:
yes
Specific details on test material used for the study:
Name: BLANCOLEN HP Disodiumhydroxy(sulfonato)acetate
Batch no.: 09102303
Designation in Test Facility: 12092101G
Dissociating properties:
yes
No.:
#1
Remarks on result:
other: determination not possible

Main Test 1

The intercept of the linear fit of the Kohlrausch correlation represents the limit conductivity

of the test item (conductivity in unlimited dilution): 234.71 S*cm2*Mol-1.

Concentration.

Molar Conductivity.

Limit Conductivity

Degree of Dissociation

Mol/L

S*cm2*Mol-1

S*cm2*Mol-1

α

6.249E-04

225.62255

234.71

0.9612900

1.250E-03

221.62215

234.71

0.9442458

2.500E-03

215.62155

234.71

0.9186796

5.000E-03

207.82077

234.71

0.8854435

9.999E-03

197.61975

234.71

0.8419809

With the calculatedα,KDwas determined.

Degree of Dissociation

Dissociation Constant

- log (Dissociation Constant)

α

KD

pKD

0.9612900

0.014918

1.8262782

0.9442458

0.019988

1.6992407

0.9186796

0.025943

1.5859748

0.8854435

0.034216

1.4657714

0.8419809

0.044859

1.3481484

 

The mean of the measured values would be 1.59, standard deviation is 0.19 (RSD=11.9%), but the values showed a decreasing tendency with increasing concentration.

Main Test 2

The intercept of the linear fit of the Kohlrausch correlation represents the limit conductivity of the test item (conductivity in unlimited dilution): 233.92 S*cm2*Mol-1.

Concentration.

Molar Conductivity.

Limit Conductivity

Degree of Dissociation

Mol/L

S*cm2*Mol-1

S*cm2*Mol-1

α

6.249E-04

224.34243

233.92

0.9590727

1.250E-03

221.62215

233.92

0.9474434

2.500E-03

215.62155

233.92

0.9217906

5.000E-03

208.02079

233.92

0.8892971

9.999E-03

197.41973

233.92

0.8439771

With the calculatedα,KDwas determined

Degree of Dissociation

Dissociation Constant

- log (Dissociation Constant)

α

KD

pKD

0.9590727

0.014045

1.8524738

0.9474434

0.021347

1.6706541

0.9217906

0.027158

1.5660978

0.8892971

0.035716

1.4471389

0.8439771

0.045649

1.3405704

The mean of the measured values would be 1.58, standard deviation is 0.20 (RSD=12.6%), but the values showed a decreasing tendency with increasing concentration.

Conclusions:
A dissociation constant could not be determined according to the OECD 112 guideline.
Executive summary:

The conductometric approach for the determination of the dissociation constant was chosen

on behalf of the properties of the test item. A titrimetric determination was not possible

as no point of equivalence could be observed neither by titration with acid nor with base. A

photometric determination was not possible as the UV/Vis spectrum did not change with

the pH.

The evaluation of the conductometric method showed a linear Kohlrausch correlation, indicating

that the test item is a strong electrolyte, but the calculated values, derived with

equation 2, showed a downward tendency. For a valid evaluation the calculated values

would show an asymptotic approximation with decreasing test item concentration (as it is

the case for e.g. acetic acid). The limit value determined in such a manner is then called

dissociation constant KD, its negative common logarithm is called pKD. For the test item,

such a limit value could not be determined. Therefore, the conductometric method yields

no reliable result, but confirms that the test item BLANCOLEN HP

Disodiumhydroxy(sulfonato)acetate is a strong electrolyte. With the conductometric

method it is not possible to distinguish if the sodium ions are dissociated either from the

carbonic acid or the sulfonic acid group. In conclusion it is not possible to determine a

dissociation constant following the OECD 112 guideline, neither using the titrimetric nor

the photometric nor the conductometric method.

As the test item is a sodium salt of a strong acid containing vincinal chemical groups with

high electronegativities it can be concluded that the test item would be virtually completely

ionised at environmentally relevant pH’s.

From the result of the titrimetric tests - no point of equivalence could be determined - it can

be further concluded that the anionic derivatives of the substance do not exhibit

noteworthy basic properties.

Description of key information

The test item behaves like a strong electrolyte (like a salt), which means that it dissociates quite completely.
It is not suspected to exhibit noteworthy basic properties.
Determination of a dissociation constant according to the OECD guideline 112 was not possible.

Key value for chemical safety assessment

Additional information

In conclusion it is not possible to determine a dissociation constant following the OECD 112 guideline, neither using the titrimetric nor the photometric nor the conductometric method.