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 H 2 SO 4 . The acid‐stability plateaus were between 0.17 and 0.28 M and 0.17 and 0.46 M H 2 SO 4 when absorbances were obtained after 10 min and 24h, 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.