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)

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 lambdasub(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. (author)