Reaction of thorium(IV) with tris[2,4,6-(2-hydroxy-4-sulpho-1-naphthylazo)]-s-triazine trisodium salt as a spectrophotometric method for the determination of phosphate and thorium(IV)

Abstract

Thorium(IV) reacts with tris[2,4,6-(2-hydroxy-4-sulpho-1-naphthylazo)]-s-triazine trisodium salt (THT) to give a 1 : 3 dark red complex between pH 4.8 and 7.0. Sandell's sensitivity of the colour reaction is 0.0063 µg cm^–2 for thorium at a λ_max. of 535 nm. Phosphate is the only anion which decomposes the thorium-THT complex, quantitatively replacing the THT from the complex. Phosphate can therefore be determined indirectly in the range 5.5–65 µg per 25 ml.