Tetrasodium 4-amino-6-[[2,5-dimethoxy-4-[[2-(sulphonatooxy)ethyl]sulphonyl]phenyl]azo]-5-hydroxy-3-[[4-[[2-(sulphonatooxy)ethyl]sulphonyl]phenyl]azo]naphthalene-2,7-disulphonate

Hydrolysis

Description of key information

Substance is not hydrolytically stable at pH 7, 9, and 11 at environmental or use-conditions

Key value for chemical safety assessment

Half-life for hydrolysis:

4 d

at the temperature of:

25 °C

Additional information

The structural analogue, Reactive Black 5, was tested for abiotic degradation according to OECD 111. In addition to pH 4, 7, 9 as required by the guideline, the substance was also tested at pH 11 (dyeing conditions) and pH 1.2.

At pH 1.2 and 37°C, there was no abiotic degradation within the 5 days observation period. At pH 4, 7, 9, and 11, the test substance was not stable at 50°C. The degradation at pH 4, 7, and 9 were hence investigated further.

At pH 4, the half-life at 25°C is ca. 5 years; at pH 7, the half-life at 25°C is ca. 2 days; at pH 9, the half-life at 20°C is ca. 50 minutes.

During degradation of the bis-ester, both isoforms of the mono-vinyl form appear temporarily as degradation product. One main degradation product is Reactive Black 5 bis-vinyl as the active dye-form which further hydrolyses to Reactive Black 5-OH.

For Structural Analogue 01, a study was conducted to determine hydrolysis as a function of pH according to OECD Guideline 111 and EU Method C.7, in compliance with GLP.

Based on the results of a preliminary test which demonstrated >50% hydrolysis within 2.4 h at pH 9.0 and <10% hydrolysis within 5 d at pH 4.0, additional testing was only required at pH 7. Results of the main test at pH 7 and 50°C demonstrated a pseudo first order reaction. Therefore, further testing at pH 7 and 38°C was carried out. From the results, the rate constant (K_obs) and the half-life (t_1/2) of the test substance were derived or calculated for different temperatures and pH conditions.

Under the conditions of the study, the half-life of the test substance at pH 9 was <2.4 h and at pH 4 was >120 h. The half-life of the test substance at pH 7 was found to be 22.6 and 4.6 h at 38 and 50°C, respectively.

The rate constant at 25°C was calculated to be 0.004769 h-1,leading to a half-life of 146 h (6 days).

Due to the fact that Reactive Blue 203 is a combination of both structural analogues, a half-life at pH 7 and at 25°C was estimated to be 4 days.