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EC number: 225-590-9
CAS number: 4948-15-6
At any of the time points mentioned in the TG-318, the influence
of Ca is critical. Regardless of pH, the pigment is categorized at the
24h-sampling time as “instable” in 1 and 10 mM Ca, representing high
At 6h, most media induce “low stability” and only two media (0 mM
Ca, pH 7 and 9) induces a stability around 90%.
At 24h, all media except pH7 and 9 at 0 mM Ca (both intermediate
at 75%) induce a low stability. In the absence of Ca, stability at pH 7
and 9 is equally better than for pH4. In general, two effects can be
observed. A pH-dependent effect with low stability at low pH values and
high stability at higher pH values and a Ca-dependent effects showing
lower stability at higher Ca levels.
Table 1: Full results of the dispersion stability in the presence
Cross-check the apparent stability by a fractionating method
that physically separates particles from dissolved matter, and
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 (SOP
AUC-RI, published by 3). The centrifugation parameters are given in the
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 on the scale of 0.06. This is a fraction of 7% of the
initial absorption but is close to the LOD of the built-in UV/Vis
detector. Considering the LOD, few % of the sample may have been
All evidence combined, the results after centrifugation confirm
that at least 93% of the observed dispersion stability has to be
attributed to the particles, not to dissolution.
apparent dispersion stability may originate from dissolution. This was
excluded by determination of the particle size distribution, which
indicated moderate agglomeration -consistent with the dispersion
stability-, but no dissolution. The remaining absorption after
centrifugal separation of particles was determined to be not more than
7% of the total signal. Thus, dissolution is not the main cause of the
together, the dispersion stability of Pigment Red 149 is low and depends
especially on water hardness and pH. Only in soft water with 0 mM Ca at
pH 7 and 9, the dispersion stability is intermediate. In all other
conditions, the stability is low.
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