SPECTROPHOTOMETRIC DETERMINATION OF PHOSPHATE WITH ALIZARIN RED SULFONATE

The formation of complexes between orthophosphate and alizarin red sulphonate (ARS) has been ascertained. One or two kinds of complexes are formed in solution according to the pH of the medium with PO43-:ARS ratios of 1:3 and 1:2 at pH 6.6 and 8.3 with formation constants of 2.97 x 10(20) and 1.3 x 10(9) at lambda-max 478 and 520 nm, respectively. The development of the 1:3 complex was utilized for the determination of orthophosphate with a working concentration range of 0.03-6.00-mu-g ml-1 phosphorus (epsilon = 1.14 x 10(4) l mol-1 cm-1 at 478 nm). The interference of some anions and cations was investigated. Structures of the isolated solid complexes are suggested based on elemental analysis and infrared spectrophotometric measurements. A procedure for the determination of different forms of phosphorus has been elaborated and applied to differentiate between non-organic and organically bound phosphorus in re-used water effluents.