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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:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to guideline
Guideline:
OECD Guideline 112 (Dissociation Constants in Water)
Deviations:
yes
Remarks:
Only preliminary tests
GLP compliance:
yes (incl. QA statement)
Dissociating properties:
not determined
Remarks on result:
not determinable because of methodological limitations

As Phenothiazine is practically insoluble in water (0.127 mg/l) it was not possible to carry out the titration method.

For the spectrometric method methanol was used as solubiliser. For this, UV/VIS activity of the test item is tested at different pH-values in order to find a wavelength at which the ionised and unionised forms of the compound have appreciably different extinction coefficients.

Despite the high amount of solubilizer (10 % of the solution) it was not possible to dissolve the test item completely. The UV/VIS spectra were measured quickly after preparation of each solution as the test item precipitated slowly. Over time the solution became slightly turbid.

In the UV/VIS spectra of the test item at pH values of 1, 2, 6, 12 and 13 two distinct absorption maxima can be observed at 250 nm and approx. 312 nm. When KOH (c = 0.1 mol/L) is added the solution changes colour from transparent to pink within approx. 10 min. This change is reflected in the UV/VIS spectrum.

The test item appears to decompose at high pH-values (pH approx. 13) as several new bands are detected in the UV/VIS spectrum.

Overall, the absorption spectra show no consistent trend of the absorption of the bands at 312 nm and 250 nm over the tested pH range. While the absorption at both wavelengths is at its maximum at pH 1, the absorption minimum is reached at pH 6 and the absorptions of the solutions with pH 12 and 13 can be found between those of pH 2 and 6. Thus, it is impossible to identify at which pH value the ionised and the unionised form are present.

From these measurements and considering that the test item decomposes at high pH-values neither the dissociation constant nor the start of dissociation can be determined.

The spectrometric method is not applicable for the determination of the dissociation constant.

Conclusions:
According to OECD Test Guideline 112, Dissociation constants in water (1981), no dissociation constant at 20 °C of the test item Phenothiazin D Prills could be determined in the pH range of 1 to 13.

Description of key information

According toOECD Test Guideline 112, Dissociation constants in water (1981), no dissociation constant at 20 °C of the test itemPhenothiazinD Prills could be determined in the pH range of 1 to 13.

Key value for chemical safety assessment

Additional information