OPTIMAL CONDITIONS FOR PHOSPHORUS ANALYSIS BY THE ASCORBIC-ACID MOLYBDENUM BLUE METHOD

Phospho-molybdo blue color intensity, its relation to P concentration in solution, and color stability are greatly influenced by the solution acidity and matrix. Our objectives were to: find the acid-stability plateau for the ascorbic acid-Mo blue complex and its application to colorimetric P analysis; identify the stability of the calibration graphs for Soltanpour-Schwab, Watanabe-Olsen, and Bray-Kurtz P-1; and study the stability of the mixed and color-developing reagents. Standards containing 0.0, 0.2, 0.4, 0.6, 0.8, 0.9, 1.0 and 1.1 mg P L-1 were prepared across a range of acidity from 0.0 to 0.6 M H2SO4. The acid-stability plateaus were between 0.17 and 0.28 M and 0.17 and 0.46 M H2SO4 when absorbances were obtained after 10 min and 24 h, respectively, after adding the color-developing reagent. The blue color was stable with time for P concentrations up to 0.30 mg L-1 for AB-DTPA and 0.80 mg L-1 for other extracts. The mixed reagent was stable for 16 wk and the color-developing reagent was stable 2 and 14 d for the open and sealed-cold conditions, respectively.