certain range of pH such a shift leads to an increase in the concentration of the
corresponding intensely colored form of the dye.
2) Aggregation of dye ions. Known situations where such a phenomenon may
occur include: increasing the concentration of the dye, approaching of the dye ions
resulting from the fixation on the certain places of the polymer chain, the concentration
in the micelles of the surfactant, the formation of precipitate or sorption on the sorbent
surface. Such approaching leads to the interaction of π-π electrons, accompanied by the
formation of dimers or more highly aggregated particles having a color that differs
significantly from the color of the dye.
3) Hydrophobic interaction. If cation and anion have groups which are sufficiently
voluminous and hydrophobic (alkyl or aryl radicals), dye ions tend to change aqueous
environment onto organized microenvironment in which interaction with water
molecules is weakened and replaced by the hydrophobic interaction. The resulting
changes in the spectra are often called as the solvatochromic effect.
We believe that it is very useful to clearly distinguish the interaction of dyes and
counterions, accompanied by a change in color, into certain types, in order to describe
what analytical characteristics can be achieved for each analytical effect. It is necessary
to clearly correlate the change in the spectrum and a certain type of interaction, to
develop rules according to which we can check whether the change in the spectrum is
related to a certain type of interaction.
In this article, we intend to give examples of systems in which two of the above-
mentioned types of interaction would be clearly realized, namely, electrostatic and
aggregation.
Aggregation processes of basic dyes initiated by formation of specific ion
association complexes with heteropoly anions.
Malachite Green method for the determination of phosphorus(V). A number
of complex and still unsatisfactorily solved problems arise in analytical chemistry by
using ion-association complexes (IAs) formed between heteropoly anions (HPAs) and
basic organic dyes. Primary and very important problem is the similarity or complete
coincidence of the optical properties of the dye and IA. To solve this problem it is
recommended, as a generally accepted approach, to remove the excess dye that did not
react beforehand. The extraction is preferentially used for this purpose [1]. Other
methods used to separate the excess dye include flotation and centrifugation. However,
separation is generally incomplete, which leads to an excessively high absorbance of
the blank solution and to a deterioration in the reproducibility and sensitivity of the
determination. Organic solvents are highly flammable and dangerous to humans and
the environment. In addition, the determination becomes much more laborious, time-
consuming, and poorly automated. Therefore, the non-extraction methods are highly
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