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

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 (H 3 PMo 12 O 40 ) ion associate with monovalent and protonated crystal violet ions [R + , R + H + , R + (H) 2 2+ ] forming R + -H 2 PMo 12 O 40 (violet), R + H + -HPMo 12 O 40 (bluish green) and R + (H) 2 2+ -PMo 12 O 40 (yellow) complexes with λ max at ~ 590, 610 and 450 nm respectively. In this developed method, R + H + of R + H + -HPMo 12 O 40 , aggregate to dimerization induced by complexing ion (HPMo 12 O 40 2− ) forms (R) 2 2+ H + .PMo 12 O 40 (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) 2 2+ and its IP complex [R + (H) 2 2+ .PMo 12 O 40 ] at ~ 450 nm don’t interfere in the analysis. Rapid formation of (R) 2 2+ H + - PMo 12 O 40 and double protonation of R + H + into R + (H) 2 2+ 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.