Ion chromatographic determination of phosphorus soluble in different extracting media in fertilizers

A new method based on ion chromatography (IC) was developed for the determination of phosphorus in fertilizers. Fertilizers were extracted with water, mineral acids, and 2% formic acid, 2% citric acid, and neutral ammonium citrate solutions according to European Regulation No. 2003/2003 of the European Parliament and the Council of October 13, 2003, or the Decree of the Italian Agriculture Minister of June 17, 2002; the extracts were analyzed by direct injection, after simple filtration, by IC on an IonPac AS19 (250 x 4 mm id) column, using a KOH (21-50-21 mM) gradient and suppressed conductivity detection. The calibration plot was linear over the range of 5-50 mg/L (r(2) of >0.999). The method was evaluated by comparison with a gravimetric method according to established norms. Associated uncertainty at the 95% confidence level was established as 0.47% for the determination of 3-46% P2O5 by IC. A good chromatographic separation of phosphorus forms such as phosphates and phosphites, and some other important anions like nitrates, chlorides, and sulfates present in many commercial fertilizers was also possible, with a linear response over the range of 5-50 mg/L. After a more complete validation, this IC determination of phosphorus could replace more tedious methods such as those using gravimetric determinations.