Spectrophotometric determination of trace amounts of phosphate in water and soil

Abstract

A simple spectrophotometric method has been developed for the determination of phosphate dissolved in soil and water. The method is based on the formation of phosphomolybdate with added ammonium molybdate followed by reduction with hydrazine in acidic medium. Orthophosphate and molybdate ions condense in acidic solution to give molybdophosphoric (phosphomolybdic) acid, which upon selective reduction (perhaps with hydrazinium sulphate) produces a blue colour, due to molybdenum blue of uncertain composition. 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 830 nm. The system obeys Lambert-Beer's law at 830 nm in the concentration range of 0.5-5 μg/mL of phosphate with a relative standard deviation (RSD) of 0.1% and correlation coefficient of 0.99. Molar absorptivity was determined to be 2.9 × 10⁴ L mol⁻¹ cm⁻¹ at 830 nm. The method is also applicable for the determination of phosphate in nuclear reprocessing plants, medical science, clinical science, agriculture, metallurgy and environmental science.

The molybdenum blue reaction for the determination of orthophosphate revisited: Opening the black box

“…For example, it is frequently noted in studies which make use of organic reductants without heating that the absorbance of the product continually increases for periods up to several hours [49,63,70,74]; given the slow reduction kinetics of these species, it is now clear that the reported 'instability' of the colour arises from the slow, ongoing disproportionation of PMB(2e À ) to 12-MPA and PMB(4e À ), which exhibits a different absorption spectrum. However, once formation of PMB(4e À ) using organic reductants is complete, the product appears to be stable for at least several hours with respect to oxidation by dissolved O 2 [49,60,72,73,124]. Sb(III) methods behave in the same manner; once the reduction finishes after several minutes, the Sb 2 PMB(4e À ) adduct appears stable for many hours with respect to oxidation [51,85].…”

Microwave Parboiling: Reduction in Process Time, Browning of Rice and Residual Phosphorus Content in the Waste Water

“…Residual phosphorus content. The total phosphorus content in soaking water was determined by spectrophotometry by the molybdate assay, as described by Ganesh et al (2012). The results were obtained through a calibration curve, and the results were expressed in mg/L.…”

Indigo Carmine-Cu complex probe exhibiting dual colorimetric/fluorimetric sensing for selective determination of mono hydrogen phosphate ion and its logic behavior

“…1 mL ammonium molybdate 0.5 M, 1.0 mL hydrazine hydrate 0.5 M, 1 mL sulfuric acid 2.0 N and desired amounts of disodium hydrogen phosphate stock solution or fertilizer solution was added to each flask and the solutions were made up to 10.0 mL with distilled water and kept in 60º C for 30 min. after that the solution were cooled and measured at wavelength 830 nm [41].…”

“…The accuracy of the proposed method was checked by the spectroscopic molybdenum blue method [41]. The results obtained using present method and molybdenum blue method were compared using t-test.…”