H
2
C
H
C
*
C
H
2
C
NH
2
O
H
C
C
O
O
H
2
C
H
2
C
+N
CH
3
CH
3
CH
3
n
m
Cl-
We have investigated the interaction of polyelectrolytes CPAT having different
charge density with three representatives of sulfonphtaleine dyes – Phenol Red,
Bromphenol Blue and Bromcresol Purpur.
Interaction of Phenol Red with polyelectrolyte CPAT. Phenol red in solution,
depending on the pH of the medium, exists as a cationic (H
3
R+), neutral (H
2
R), mono-
(HR
-
) and dianionic (R
2-
) forms (Fig. 9).
Figure 9 – Formulas of sulfonphtaleine dyes.
a – Phenol Red; b – Bromphenol Blue; c – Bromcresol purpur
When PE CPAT is added to the solution of Phenol Red in the pH range from 4 to
6, the intensity of the absorption band situated at 560 nm increases proportionally to
the increase in the concentration of positively charged groups of polymer. This band,
in its shape and position of the absorption maximum, coincides with the band of the
double-ionized form of the dye (Fig. 10a).
We believe that the changes in the spectra by the interaction of Phenol Red with
CPAT are due to a single factor – the shift of the acid-base equilibrium toward one of
the protonated forms of the dye caused by the ion-associative interaction of positively
charged groups of polymer and dye anions. The formation of the ion associate occurs
usually in cases when large hydrophobic cations and anions enter the reaction. The
polymer used and the dyes correspond to this rule. Consequently, under certain acidity
a
b
c
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