Field method for determination of available phosphorus in soil by selective formation of protonated crystal violet dimer:12-molybdophosphoric acid [(R)22+ H + -12MPA] complex

Abstract

Abstract Phosphorus (P) is an important nutrient for agricultural soil and it accelerates eutrophication in environmental water. In aqueous medium, sensitive method for P determination by ion-pair (IP) of triphenylmethane (TPM) dyes with 12-molybdophosphoric acid (12-MPA) is hampered due to multiple IP complexes, overlapping of IP spectrum with unassociated dye and potential interference from silicate. 12-MPA (H3PMo12O40) ion associate with monovalent and protonated crystal violet ions [R+, R+H+, R+(H)22+] forming R+-H2PMo12O40 (violet), R+H+-HPMo12O40 (bluish green) and R+(H)22+-PMo12O40 (yellow) complexes with λmax at ~ 590, 610 and 450 nm respectively. In this developed method, R+H+ of R+H+-HPMo12O40, aggregate to dimerization induced by complexing ion (HPMo12O402−) forms (R)22+ H+.PMo12O40 (blue, 560 nm). This ion-pair complex is selectively utilized for visual detection of phosphorus. No dimer formation of unassociated R+H+ and disappearance of shoulder of characteristic CV absorption spectrum remove spectral overlap at 560 nm. R+H+ at 610 nm, double protonated R+(H)22+ and its IP complex [R+(H)22+.PMo12O40] at ~ 450 nm don’t interfere in the analysis. Rapid formation of (R)22+H+- PMo12O40 and double protonation of R+H+ into R+(H)22+ on interaction with sample solution containing molybdate ion removes silicate interference. This method is successfully applied for agricultural soil for determination of available P in Olsen, Morgan, Mehlich, Bray and Acetic acid extraction procedures. The results obtained by the developed method are validated by ion-chromatography (IC). Five distinct colour formation for 0, 0.25, 0.5, 1.0 and 2.0 mg/L of P offers infield screening of agricultural soil as low, medium, high and very high available P for fertilizer management.