Calcium 3-hydroxy-4-[(4-methyl-2-sulphonatophenyl)azo]-2-naphthoate

Additional information on environmental fate and behaviour

Administrative data

Endpoint:

additional information on environmental fate and behaviour

Remarks:

Dispersion stability in simulated environmental media

Type of information:

experimental study

Adequacy of study:

key study

Study period:

2020

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Data source

Reference

Reference Type:

study report

Title:

Unnamed

Year:

2020

Report date:

2020

Materials and methods

Test guideline

Qualifier:

according to guideline

Guideline:

other: OECD 318

GLP compliance:

yes (incl. QA statement)

Test material

Test material information

Constituent 1

Reference substance name:

Calcium 3-hydroxy-4-[(4-methyl-2-sulphonatophenyl)azo]-2-naphthoate

EC Number:

226-109-5

EC Name:

Calcium 3-hydroxy-4-[(4-methyl-2-sulphonatophenyl)azo]-2-naphthoate

Cas Number:

5281-04-9

Molecular formula:

C18H14N2O6S.Ca

IUPAC Name:

calcium 3-hydroxy-4-[(1E)-2-(4-methyl-2-sulfonatophenyl)diazen-1-yl]naphthalene-2-carboxylate

Results and discussion

Any other information on results incl. tables

At any of the time points mentioned in the TG-318, the influence of Ca is critical. Regardless of pH, the pigment is least stable in 10 mM Ca, representing high water hardness.

After 6h, the samples showed high dispersion stability in 0 mM Ca and 1 mM Ca at pH 4. The stability at all other conditions was intermediate.

After 24 hours the stability in 0 mM Ca and 1 mM Ca at pH 4 remained high. For the samples at oder conditions the stability decreased to the lower end of intermadiate level with exception of the sample in 10 mM Ca at pH 7 which became instable.

Table 1: Full results of the dispersion stability in the presence of NOM

	Ca(NO3)2	Stability after 6h	Standard deviation	Stability after 15h	Standard deviation	Stability after 24h	Standard deviation
	[mM]	[%]	[%]	[%]	[%]	[%]	[%]
pH 4	0	100	0.4	100	0.5	100	0.5
pH 4	1	100	0.1	100	0.1	100	0.1
pH 4	10	24.3	1.7	15.6	1.6	13.1	1.8
.
pH 7	0	100	0.2	100	0.2	100	0.2
pH 7	1	31.6	6.9	19.6	6.1	15.8	5.7
pH 7	10	19.0	2.2	10.2	0.9	8.2	0.6
.
pH 9	0	100	0.5	100	0.6	100	0.5
pH 9	1	35.0	2.4	22.3	1.1	19.4	0.3
pH 9	10	23.9	3.4	15.8	0.7	13.6	0.9

 

 

To rationalize the observed dispersion stability, we finally checked the particle size distribution directly in the environmental medium (exact same sample preparation as for the UV/VIS measurements). We applied the NanoDefine method of Analytical Ultracentrifugation.

 

The observed size distributions confirm the moderate agglomeration at 1 mM Ca, pH7, with NOM. If the particles would have been significantly dissolved, no size distribution would be observable at all by this method, which relies on the detection of the movement of particles during centrifugal separation.

 

Additionally, the centrifugation methods include a determination of the remaining absorption after centrifugation, fully consistent with the conventional determination of the dissolved fraction after centrifugation as recommended by the TG-318. The remaining absorption was measured at ca. 0.06. This is a fraction of 4.6% of the initial absorption, but actually is close to the LOD of the built-in UV/Vis detector. Considering the LOD, between 0% and 4.6% of the sample may have been dissolved.

 

All evidence combined, the results after centrifugation confirm that at least 95.4% of the observed dispersion stability has to be attributed to the particles, not to dissolution.

Applicant's summary and conclusion

Executive summary:

Elevated apparent dispersion stability may originate from dissolution. The dissolution in environmental media has not yet been assessed, but the particle size distribution was measured in all media. The remaining absorption after centrifugal separation of particles was determined to be not more than 4.6% of the total signal. Thus, dissolution is not the main cause of the observed stability.

 

Taken together, the dispersion stability of Pigment Red 57:1 depends on pH and water hardness. The stability in 0 mM Ca and 1 mM Ca at pH 4 is high. The stability is at pH 7 and 9 in 1 mM Ca and at pH 4 and 9 in 10 mM Ca is intermediate and at pH 7 in 10 mM Ca is low