analytical effects. It was confirmed that in very many cases no change in the color of
the dye occurred by mixing organic dyes and large anions originating from metalic and
non-metalic elements. At present, we do not understand many of the reasons that lead
to the appearance of color. Only some factors were found, which affect the aggregation
of dye ions. One of the prerequisites is the need to create conditions in which the dye
ions would be more or less close contacted. The energy of π-π dispersion interaction is
not enough to make dye aggregates stable in aqueous solutions without additional
factors. In our case, the determining cause is the formation of sparingly soluble
substances. In crystals of the precipitate, contact between the ions of the dye is naturally
intensified and conditions arise for the formation of aggregates. Indeed, in all cases
found by us, as the concentration of the ion associate increases, the moment comes
when the aggregative stability is violated and the solutions become cloudy or a
precipitate is formed.
Analysis of literature has shown that typical changes in the spectra inherent to the
phenomenon of dye aggregation were observed in a few systems. It is worth to say that
in none of these works authors do not explain nature of the spectral changes. In the
determination of inorganic species, two systems are clearly distinguished among
others. Reaction of halide and thiocyanate complexes of some metal ions with
rhodamine or polymethine dyes is accompanied with significant changes in spectra,
which are similar to those observed in the spectra of IA dye-HPA.
Many of elements for which formation of stable rhodamine and halide is typical
form specific IAs with basic organic dyes. Such examples include iodide complexes of
Cd(II) with Quinaldine Red [21], Pyronine G [22] or Rhodamine B [23], Pt(IV) with
Pyronine G [24], Hg(II) with Rhodamine 6G [25] and Rhodamine B [26], Pb(II) with
Rhodamine B [23], thiocyanate complexes of Hg(II) with Rhodamine B [27], Ag(I)
with Rhodamine B [28], Mo(V) with Rhodamine 6G [29] and 2-[2-(4-dipropylamino-
phenyl)-vinyl]-1,3,3-trimethyl-3H-indolium [30].
Indirect spectrophotometric determination of Se(IV) is based on the reaction with
iodide with formation of triiodide ion, which is capable to form specific IAs with
Rhodamine B [31]. Determination of W(VI) is based on the direct reaction with
polymethine dye 2-[4-(dimethylamino) styryl]-1-ethylquinolinium iodide [32].
However, we do not agree with conclusion of authors that hydrotungstate ion HWO
4
-
is responsible for the color reaction with dye. Tungstate ions have small dimensions
and strongly hydrophilic. Isopoly tungstate anions can be formed even in diluted
solutions with C(W(VI) < 10
-4
mol L
-1
. Structure of many isopoly tungstate anions is
similar with that for Keggin heteropoly anions, for example, metatungstate H
2
W
12
O
40
6
.
This or related compounds are more probable candidates for interaction.
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