Uses
Due to the steric hindrance of the methyl groups attached to the carbon atoms
adjacent to the nitrogen donor atoms of phenanthroline, the reagent does not
give the low spin vivid red complex characteristic of phenanthroline derivatives
with iron(II). However, in the presence of reducing agents it reacts with copper to
give a copper(I) complex of composition MA2 and of tetrahedral symmetry. This
chelate is insoluble in water and can be extracted by chloroform in which the absorption
maximum of the complex appears at 457 nm. In this way the concentration
of the complex can be measured. The method is suitable for the determination
of copper in iron, manganese and vanadium ores even in the presence of
aluminium, germanium, titanium and silicon.
Neocuproine is today considered one of the most selective reagents. Unfortunately,
it is rather expensive. 2,3-bis-(2-pyridyl)quinoxaline, prepared by Belcher
et al. by condensation of o-diketone, 2,2'-dipyridyl and 0-phenylenediamine(44)
contains the functional grouping characteristic of cuproine; it is quite suitable for
the determination of copper and it is cheap. Starting with various substituted
o-phenylene-diamines Belcher synthesized 25 different quinoxaline derivatives,
of which 2,3-bis-[2-(-methyl)-pyridyl)]quinoxaline proved to be identical with
neocuproine as regards analytical selectivity. Besides copper, titanium(III) is the
only metal ion which gives a colour reaction with the reagent. However, the
coloured titanium(III) complex is formed only at lower pH and hence it does not
interfere with the determination of copper. The copper complex of the reagent can
be extracted quantitatively with isopentyl alcohol usually as a perchlorate ion pair.
