Extraction-spectrophotometric determination of niobium with 1,2,4,6-tetraphenylpyridinium perchlorate and thiocyanate

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Abstract
1,2,4,6-Tetraphenylpyridinium (TPP+) as the acetate or perchlorate was used as a counter ion in the spectrophotometric determination of Nb(V) by extraction into toluene of the anionic Nb(V)-thiocyanate complex from 4–6 M hydrochloric acid. The molar absorptivity of the ion-association complex, whose composition was shown to be NbOCI(SCN)3 –. TPP+, was 2.82 × 104 l mol–1 cm–1 at 395 nm. Beer's law was obeyed over the range 0.1–2.5 µg ml–1 of Nb(V). The method was applied to the determination of niobium in standard steels and ores (culombite type) with good precision and accuracy. 1-(4′-Nitrophenyl)-2,4,6-triphenylpyridinium (nitro-TPP+) perchlorate was also synthesised and used in the spectrophotometric determination of Nb(V), but did not show advantages over TPP+. The fluorescence of TPP+ and Nb(V)-SCN–-TPP+ solutions in toluene also disappeared when nitro-TPP+ was used, owing to the paramagnetic effect of the NO2 group.

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