salinity of Dead Sea water does not influence on the determination of orthophosphate
with IA Astra Phloxine – 12-MPC [7]. Rapidity of the reaction and broadening of the
formation range from strong to weakly acid medium explain why this reaction is best
suited for the determination of phosphate in the presence of labile phosphorus esters,
including e.g. creatine phosphate. The isolation and change in chemical properties of
the dye being the constituent of IA allow using several new analytical effects described
later in this article.
Formation of extremely low-soluble particles of basic dyes with HPAs is of high
importance to promote the dye aggregation. In the presence of HPAs, even those dyes
interact with each other that have a feebly marked capability of forming aggregates.
For instance, voluminous isopropylidene groups present in the molecule of Astra
Phloxine (AP) strongly complicate the processes of aggregation. Aggregation of this
dye occurs only in very low polar medium containing 95% of hexane in dichlorethane
[11]. Addition of 2∙10
-4
mol L
-1
of Fe(CN)
6
4-
or other bulky inorganic anions has no
influence on the spectrum of AP [12]. At the same time, extremely low concentrations
of PMo
12
O
40
3-
or other HPAs induce the formation of stable aggregates of dye cations.
Isobestic point is present in the spectra that is evidence of equilibrium between free
and aggregated dye ions (Fig. 1).
Aggregates of IA dye-HPA are destructed by addition of critical concentration of
polar solvents such as ethanol, acetone, acetonitrile (>10-15%). The spectrum of IA
coincides then with the dye spectrum, which proves that only solvated individual dye
ions are present in such solutions.
Dissociation of the aggregates proceeds also by interaction with micellar phase of
non-ionic surfactants. Not only dye but also the HPAs can be effectively solvated in
micellar phase. Stability constant of 12-MPA determined for the micellar solutions of
non-ionic surfactants is several orders of magnitude greater than that calculated for
aqueous solution. Difference in stability constants was explained by binding of
12-MPA with micellar phase. Constant of the binding of HPA with micellar phase was
evaluated by magnitude of 10
4
.
Application of exciton theory for the explanation of aggregation processes of
dyes by the formation of specific IAs with heteropoly anions. B
esides electrostatic
forces, hydrophobic interaction may also be involved in the binding process and assist
in the formation of stable dispersed phase in the solution. However, these interactions
cannot be reason for the big changes in the spectrum. Close contact between IA
molecules in low-soluble aggregates favours the non-covalent dipole-dipole bonding
interaction between π-electronic systems of the dye molecules [13]. Formation of
charge transfer complex was proposed as an explanation of changes in the spectrum of
the IA formed between Rhodamine 6G and tetraphenylborate anion. However, the
same changes in the spectrum appeared when HPAs are used instead of
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