2,9-Dimethyl-1,10-phenanthrolin

VerwendenDue 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.