Novel indirect spectrophotometric methods for determination of phosphate and arsenate using polyoxometalates and micellar medium

Five various methods based on the use of polyoxometalates (POMs) have been developed for indirect spectrophotometric determination of phosphate or arsenate. Amplification procedure involving formation of the 12-molybdophosphate, its extraction with butyl acetate, and stripping with ammonia differs from known method by applying sensitive reaction of molybdenum(VI) with phenylfluorone in presence of the nonionic surfactant neonole at the last step of analysis. The calibration curve for phosphate is linear over the range of 2×10−8 to 2×10−7 M. The molar absorptivity was 1.68×106 l mol−1 cm−1 at 524 nm. Ion-exchange separation of metal ion excess from mixed POMs PMeW11O395− [Me-Co(II), Zn(II)] allows to realize indirect determination of phosphorus on metal atoms by means of spectrophotometry or atomic absorption spectrometry. Stability of POMs with Keggin structure is greatly increased by solubilization in micelles of nonionic surfactant. An explanation for this effect was proposed, and two analytical procedures based on this phenomenon were created. They include determination of phosphate or arsenate by means of measurement of absorbance in near UV and visible region of the spectrum as well as another amplification procedure which is based on destroying the molybdenum(VI) complexes with alizarin S or brompyrogallol red due to formation of POMs in the presence of phosphate. Finally, it was found that reaction of Keggin POMs with cyanine dyes leads to significant changes in colour of the ion associate by contrast with other dyes. The value of molar absorptivity is equal to 9.0×104 l mol−1 cm−1 at 580 nm.