EXTRACTIVE SPECTROPHOTOMETRIC METHOD FOR THE DETERMINATION OF NIOBIUMIN STEELS

A strongly coloured complex of niobium with PAR is formed in oxalate s olutions and at pH 7.4 can be extracted into nitrobenzene as an ion-as sociation complex with xylometazolonium ion (XMH), with a molar absorp tivity of 46300 1 mol-1 cm-1 at 545 nm. In contrast to cationic extrac tants such as tetraphenylphosphonium or tetraphenylarsonium chloride, XMH is an effective extractant over a wide concentration range without any difficulties in either the separation of the phases or the absorb ance of the reagent blank. Job's method of continuous variations revea led that the composition of the extracting species as 1 : 1 : 1 : 1 fo r niobium : oxalate: PAR: XMH. Based on this extraction, a highly sens itive and selective spectrophotometric method for the determination of niobium in various steels, after prior separation of iron, is describ ed. 4-(2-Pyridylazo)resorcinol (PAR) reacts with the niobium tartrate complex in nearly neutral medium to form a red, water-soluble 1 : 1 an ionic complex [1]. PAR is an unselective reagent and one way of improv ing selectivity is by ternary and ion-association complex formation. T he extraction of some highly coloured metal-PAR complexes with tetraph enylarsonium or tetraphenylphosphonium ion has been described [2, 3]. The PAR complex of niobium, which is formed in oxalate solutions has b een shown to be extractable by chloroform using tetraphenylarsonium an d phosphonium salts [4, 5]. However, in many of these extraction spect rophotometric procedures difficulties arise due to the delay in the se paration of the two phases and significant absorbance of the reagent b lank solution, particularly when concentrated solutions of reagents ar e used. The extraction of anionic complexes of Pd, V, and Cr with xylo metazolonium (XMH) cation has been reported elsewhere [6-9]. In the pr esent study we have tried to extract Nb(V) and it is observed that 1 : 1 oxalatoniobium-PAR complex can be extracted with XMH. A method is p roposed for the determination of niobium in standard steels.