desirable capable to determine phosphate, silicate, and arsenate directly in aqueous
solution without preliminary stage of dye separation.
A wide application when using IAs of HPAs in the analysis showed a method
based on the formation of ion association complex between triphenylmethane dye and
HPA in conditions, in which the dye is converted to a colorless protonated form [2, 3].
In this approach, the dye solution is first acidified to completely form a colorless
protonated form. Subsequently, the addition of HPA leads to the displacement of the
proton, because a single charged dye cation forms a more stable IA with HPA. Since
monocationic form of the dye is intensely colored, this transformation from a slightly
colored, doubly positively charged form to a singly charged is accompanied by a sharp
color change.
PMo
12
O
40
3-
+ 3HMG
2+
= (MG
+
)
3
(PMo
12
O
40
3-
) + 3H
+
(yellow) (yellow, λ
max
= 446 nm) (green, λ
max
= 650 nm)
where MG – Malachite Green.
This method was first reported by Itaya and Ui [4]. They developed a simple and
sensitive spectrophotometric method for the determination of phosphate in serum in
acidic medium with malachite green. The method was subsequently many times
modified and applied to the determination of phosphate in serum, plasma and urine [5].
Low soluble IAs are usually formed in the reaction between HPAs and basic
organic dyes. Various surface-active substances (SAS) were used to solubilize the
precipitate of IA [6]. Molecules or more large particles of IAs Dye-HPA are stabilized
by the hydrophobic interaction with SAS micelles. Thus, a stable transparent solution
of IA is formed, for which the absorbance or intensity of luminescence can be
measured.
Very seldom mentioned feature of this reaction is the fact that the spectrum of IA
is markedly distinguished from that for singly charged form of the dye. We correlate
this fact with another factor determining the changes in the spectra by formation of IA
Dye-HPA. It was found by us and others [5] that solutions of IA Dye-HPA can be
stabilized without addition of surfactants or other solubilizers. Moreover, surfactants
did not always prevent some agglutination from occurring. In the presence of many
solubilizers, a decrease in absorbance is observed with time.
Approaching of the dye ions naturally occurs when the precipitate of IA Dye-HPA
is formed. This creates the conditions for the π-π interaction of aromatic systems of the
dyes interacted. In this way, formation of dimers of more highly ordered aggregates of
dye is promoted. Such aggregate can be destructed by solubilization with surfactants.
Due to hydrophobic interaction, a dye molecules are efficiently solubilized in the
microenvironment created by SAS micelles in contrast to the situation in aqueous
solution where there is a clear tendency to aggregation of dye molecules.
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