a-Benzoinoxime is used most frequently for the determination of copper(II),
but it gives highly stable complexes with molybdenum(VI), tungsten(VI) and
vanadium(V), which are insoluble in water and can be extracted with chloroform.
Copper can be quantitatively precipitated from hot solutions containing
ammonium hydroxide or sodium tartrate. The excess reagent ensures the quantitative
precipitation of the copper but it requires careful work to avoid contamination
of the precipitate by the excess of the reagent. Hence combined solvent
extraction-spectrophotometric measurements have proved more suitable than
the gravimetric method. The absorption maximum of the copper chelate in chloroform
solution is at 440 nm.
