Chemistry Reference
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reagents, giving rise to a binary or ternary fluorescent complex. Borate is the most
common anion determined by this methodology. Neither of these methods is based on the
phenomenon of quenching.
Although many anthraquinones have been described as photometric reagents of borate,
only a few have been applied to the fluorometric analysis of this anion. Some of these
anthraquinones are shown below.
Name
Substituent
Carminic acid
Quinizarin
Alilzarin Red S
R
1
OH
OH
H
R
2
CO(CHOH)
4
CH
3
SO
3
Na
H
R
3
OH
H
OH
R
4
OH
OH
OH
R
5
COOH
H
H
R
6
OH
H
H
R
7
H
H
H
R
8
CH
3
H
H
1-Amino-4-hydroxyanthraquinone was the first anthraquinone tested for the qualitative
fluorometric determination of boron. The complex shows up an intense orange-brown
fluorescence in daylight in concentrated sulphuric acid [61,62].
A method for the determination of boron with carminic acid in buffered neutral
solution has been described [63], with a standard deviation of 0.03µg mL
−1
. Initially the
method involves the use of cochineal, a preparation obtained from the dried female insect
Coccus cacti L.,
and of its chief active constituent, carminic acid [64].
Holme [65 carried out a systematic investigation of complex formation in concentrated
sulphuric acid between boric acid and a series of hydroxyanthraquinones (1,4-dihydroxy-,
1,5-dihydroxy-, 1,8-dihydroxy-, 1,2,3-trihydroxy-, 1,2,4-trihydroxy-, and 1,2,3,5,6,7-
hexahydroxyanthraquinone). He found that quinizarin (1,4-dihydroxyanthraquinone) was
the only reagent to produce a fluorescent complex and he described the determination of
boron with this reagent.
Alizarin Red S changes its yellow fluorescence to pinkish red in the presence of boron
[66,67]. A chelate ester with a six-membered ring is formed in the reaction. Boron can be
determined with this reagent in the presence of a 1000-fold excess of most cations and
anions; only iodides, chlorates, antimony and iron interfere. The interference of iodides is
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