Determination of soluble phosphorus in the presence of organic ligands or fluoride

In the ascorbic acid-molybdenum blue method for measurement of soluble P, t he relationship between color intensity and P concentration in solution and color stability is greatly affected by organic and inorganic ligands such as oxalate, citrate, tartrate, or F. The objective of this study was to eva luate the effect of these ligands and to optimize the conditions for determ ination of inorganic P in the presence of the interfering Ligand. The criti cal concentrations of the ligand at which P recovery was significantly decr eased were 1.5 mM for oxalate, 3.0 mM for citrate or tartrate, and 10 mM fo r fluoride. The interference of the ligand with P determination was overcom e by excess amounts of ammonium molybdate (AM) added before the color devel oping reagent (CDR), The critical molybdate/ligand molar ratios (excluding the amount of molybdate added in the CDR) required to completely eliminate the interference of the ligand were 1.12 for oxalate, 0.84 for citrate, 0.5 6 for fluoride, and 0.34 for tartrate. The presence of ligand and excess am ount of AM did not affect the linearity of color intensity against P concen tration but did alter the slope, suggesting that the modified method is rel iable for P determination in the presence of an interfering ligand provided that the standard curve is prepared in the same matrix as the sample.