Nevertheless, the described approach has its own limitations. The reactions of the
protonation or deprotonation of the dye are slow, strongly acid medium is the reason
why the hydrolysis of the organic and inorganic forms of phosphorus(V) is strongly
intensified, competition between the dye cations and pseudo cations of the non-ionic
SAS causes the considerable decreasing of the molar absorptivity of IA.
Use of IAs of polymethine dyes with molybdenum heteropoly anions for the
determination of phosphorus(V), silicon and arsenic(V). Lately, we have drawn
attention to other relatively seldom used possibility solving the basic problem
appearing by use of IA in spectrophotometric analysis i.e. interference from the dye
excess [6, 7]. Under certain conditions, dye aggregation processes are favored by a
number of factors including increase of the dye concentration, decrease of the medium
polarity, decrease of the IA solubility because of salting-out effect, localization of IA
on the surface of the crystals or inside of organized medium created by micelles of
monomeric or polymeric SAS. The absorption or luminescent spectra as well as other
properties of the dyes are significantly changed due to the formation of the dimers or
more highly aggregated particles. Such phenomenons can become a basis for a
development of simple, fast and effective analytical methods.
Capability of the dyes to aggregate is determined by various factors, role of which
is not sufficiently clarified at the present time. In most of cases, even the big
concentrations of counterions don`t change the spectral properties of dyes. Therefore,
it is interesting to find the systems for which such phenomenon can occur.
Heteropoly anions occupy the special place among other anions. They have
several properties essentially facilitating the aggregation of the cationic dyes when IA
are formed. Heteropoly anions having Keggin structure are large symmetric particles
with very low surface density of negative charge. For 12-molybdophosphate HPA
(12-MPC) PMo
12
O
40
3-
, the three negative charges are quite uniformly distributed
among 40 oxygen atoms. Hence, partial charge on the oxygen atoms is relatively small
3/40 = 0.075. The Keggin anions have large dimensions and their diameter is equal
to approximately 10 Å. These factors determine capability of HPAs to form stable ion
association bond in aqueous solutions and explain the very low solubility of the IAs
formed with basic dyes. Low solubility of the IAs of HPAs with nitrogen containing
organic species is widely used by the development of the ion selective electrodes [8].
In addition, specific IAs formed in the reaction between pseudocations of non-ionic
surfactants and HPAs are used for their gravimetric, photometric and direct
potentiometric determination.
Ion associates formed between HPAs and dyes are stable during at least several
hours at concentrations of HPA lower than 10
-6
mol L
-1
. Reason for this is not fully
understood at the moment. Formation of stable phase of IA in the solution is
accompanied by several effects. The most valuable consequence is drastic change
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