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 solutio n acidity and matrix. Our objectives were to: find the acid-stability plateau for the ascorbic acid-Mo blue complex and its application to c olorimetric P analysis; identify the stability of the calibration grap hs for Soltanpour-Schwab, Watanabe-Olsen, and Bray-Kurtz P-1; and stud y 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 pre pared across a range of acidity from 0.0 to 0.6 M H2SO4. The acid-stab ility 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, af ter adding the color-developing reagent. The blue color was stable wit h 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 seale d-cold conditions, respectively.