A spectrophotometric method is developed for the determination of dissolved tri-n butyl phosphate (TBP) in aqueous streams of Purex process used in nuclear fuel reprocessing. The method is based on the formation
of phosphomolybdate with added ammonium molybdate followed by reduction with hydrazine sulphate in acid medium. Orthophosphate
and molybdate ions combine in acidic solution to give molybdophosphoric (phosphomolybdic) acid, which upon selective reduction
(with hydrazinium sulphate) produces a blue colour, due to molybdenum blue. The intensity of blue colour is proportional to
the amount of phosphate. If the acidity at the time of reduction is 0.5 M in sulphuric acid and hydrazinium sulphate is the
reductant, the resulting blue complex exhibits maximum absorption at 810–840 nm. The system obeys Lambert–Beer’s law at 830 nm
in the concentration range of 0.1–1.0 μg/ml of phosphate. Molar Absorptivity was determined to be 3.1 × 104 L mol−1 cm−1 at 830 nm. The results obtained are reproducible with standard deviation of 1 % and relative error less than 2 % and are
in good agreement with those obtained by ion chromatographic technique.