Spectrophotometric Determination of Phosphate in Natural Water after Flotation as an Ion Pair of Molybdophosphate with Rhodamine B

Abstract

A sensitive and accurate spectrophotometric method is described for the determination of phosphate in natural water using Rhodamine B. The ion pair of molybdophosphate with Rhodamine B is floated at the phase boundary between an ethyl ether phase and an aqueous phase: the aqueous phase comprises 1.0 mol dm^-3 nitric acid, 33 mmol dm^-3 sodium molybdate and 20μmol dm^-3 Rhodamine B. The small amount of excess Rhodamine B extracted into ethyl ether is removed by washing the organic phase with 25 cm³ water. The floated ion pair is dissolved by adding methanol into the ethyl ether phase after discarding the aqueous phase. The calibration curve for phosphate is linear up to 1.0μg of phosphorus with the correlation coefficient of 0.999. The apparent molar absorptivity of phosphate was 4.1×10^-5 dm³mol^-1cm^-1 at 555 nm; the relative standard deviation was 2.0% for 1.0μg of phosphorus (10 determinations). Arsenate causes positive error, while no ions commonly existing in natural water, except large amounts of silicate ions, interfere with the determination of phosphate. The proposed method was applied to the determination of phosphate-phosphorus in some samples of natural water; satisfactory results were obtained.