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The dispersion stability of Pigment Red 53:1 was of intermediate stability and strongly depended on water hardness. We found that the organic pigment is rather insensitive to pH changes, with little differences in dispersion stability between pH 4 and pH 9. This was a significant difference against the metal oxide (TiO2) that was proposed as benchmark material of intermediate stability by the OECD TG 318.

Dissolution was excluded as the main cause of the apparent stability. Only in very hard water with 10 mM Ca(NO3)2, the dispersion stability of the tested substance was low. In all other conditions, the dispersion stability was at least intermediate.

Additional information

The determination was conducted by the OECD Guideline 318.

The TG 318 on Dispersion Stability of Nanomaterials in Simulated Enviromental Media is the first nano-specific test guideline, published 2017. Here we applied it to an organic pigment in order to map the ranges of stability in different media of surface water.