A reaction based on the formation of a purple-coloured complex between niobium and 4-(2-pyridylazo)resorcinol (PAR) in a tartrate-ethylenediaminetetra-acetate (EDTA) solution has been successfully applied to the direct spectrophotometric determination of niobium, in the range 50 p.p.m. to about 1 per cent., in zirconium, zirconium alloys, hafnium, tungsten and molybdenum. The absorbance is measured at 550 mµ.

By modification, this direct procedure can be used for the determination of down to 200 p.p.m. of niobium in titanium and its alloys.

Following an extractive concentration, amounts of niobium down to about 10 p.p.m. can be determined in titanium, zirconium, and their alloys, and in hafnium, molybdenum and iron. For tungsten (and molybdenum), a preliminary collection of niobium on zirconium hydroxide is recommended.

The effects of common alloying elements, and likely impurities, have been investigated.

The absorption of the tantalum complex is only about one eighth of that of the niobium-PAR complex.