between absorbance of the dye in the absence and in the presence of a polymer
(Fig. 12b).
At pH < 2, the spectrum of the IA practically does not differ from the spectrum of
the dye, which indicates the absence of interaction. The greatest deviation from
additivity is observed in the pH range from about 3 to 4, further increase in pH leads
to a decrease in ΔA. At pH> 4.3, the spectra of the dye and associate are identical.
Subsequently, pH 3.6 was chosen as optimal.
Interaction of Bromcresol Purple with polyelectrolyte CPAT. When CPAT is
introduced into the Bromcresol Purpur solution, changes occur similar to those
observed for Bromphenol Blue. (Fig. 13). The intensity of the absorption band of a
doubly ionized form (λ
max
= 590 nm) is increased in the spectrum, and a
hypsochromically shifted band appears at 550 nm. The blue-shifted band of aggregated
dye appears also for the band of a single ionized form at approximately 390 nm. As in
the case of Bromphenol Blue, in the presence of CPAT, an acid-base equilibrium shift
occurs, caused by the formation of IA and amplified by the processes of aggregation
of the dye.
Figure 13. Spectra of Bromcresol Purpur (1), its IA with CPAT (2) and
difference spectrum (3).
C
Dye
= 8 μmol L
-1
, pH 4.65, C
CPAT
= 4.8 mg L
-1
, l = 1 cm
For the Bromcresol Purpur – CPAT system, there is one significant difference.
There is a pH region in which the absorbance of the blank is close to zero (Figure 3.8).
This allows a large excess of dye to be used to completely shift the equilibrium toward
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