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EC number: 221-264-5
CAS number: 3049-71-6
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 10 mM Ca, representing high water
At 6h, most media induce “high stability”, and only few media (1
mM Ca at pH 4 and all in 10 mM Ca) induces low stability.
At 24h, stability map is similar. pH 7 is generally induces a
slightly higher stability than pH9 and much higher than pH 4. Stability
at pH 4 is the most sensitive to the water hardness. 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 low agglomeration at 1
mM Ca, pH7, with). 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 c.a. 0.06. This is a fraction of 5% of the initial
absorption, but actually is close to the LOD of the built-in UV/Vis
detector. Considering the LOD, between 0% and 5% of the sample may have
All evidence combined, the results after centrifugation confirm
that at least 95% of the observed dispersion stability has to be
attributed to the particles, not to dissolution.
Elevated apparent dispersion stability may originate from
dissolution. This was excluded by determination of the particle size
distribution, which indicated low agglomeration -consistent with the
dispersion stability- , but no dissolution. The remaining absorption
after centrifugal separation of particles was determined to be not more
than 5% of the total signal. Thus, dissolution is not the main cause of
the observed stability. Taken together, the dispersion stability of
Pigment Red 178 depends especially on water hardness and pH. Only in
very hard water with 10 mM Ca and under acidic conditions with 10 mM Ca,
the dispersion stability is low. At all other conditions stability was
intermediate or high.
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