occurring in the spectrum of the dye. One or two new bands appear in the spectrum
together with disappearance of the initial dye band (Fig. 1). The color of the primary
solution is appreciably changed and can be easily fixed with the naked eye.
A lot of useful properties make IAs formed between polymethine dyes and HPAs
one of the most valuable analytical form for the determination of P(V), As(V), Si(IV)
and Ge(IV). Method has a good reproducibility that together with high molar
absorptivity up to 2 10
5
mol
-1
L cm
-1
results in exclusively high sensitivity. The
rapidity and low concentration of the dye favorably distinguish the developed simple
and sensitive sequential injection method for the determination of orthophosphate
among other flow methods [9, 10].
1
2
3
4
480
500
520
540
560
580
600
0
0,4
0,8
1,2
1,6
2,0
λ, нм
A
Figure 1. Influence of orthophosphate concentration on the absorption
spectrum of the dye by interaction of 12-molybdophosphate with Astra Phloxine.
С
P(V)
, mol L
-1
: 0 (1), 3.0×10
-7
(2), 4.5×10
-7
(3), 6×10
-7
(4). С
AP
= 2.8×10
-6
mol L
-1
; С
Mo(VI)
=
2.8×10
-3
mol L
-1
; С(H
+
) = 0.16 mol L
-1
; l = 5 cm
An additional complex separation stages are absent in this approach. The reaction
is fast, contrast change of the IA color occurs when IA is formed, and the solution of
IA remains stable for a long time, which is completely sufficient to obtain highly
reproducible measurements of the absorbance. The sensitivity achieved with this
approach is highest from the known methods developed for the spectrophotometric
determination of phosphorus(V), silicon (IV), and arsenic(V). Using this effect,
extremely sensitive methods for the determination of phosphorus(V), silicon (IV), and
arsenic(V) were developed with the detection limit at approximately 10
-8
mol L
-1
level
[6, 7]. Very high selectivity inherent to this approach allows, for example, to determine
phosphorus and silicon in steels and alloys just after dissolution. Even extremely high
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