Direct Determination of Phosphite in Fertilizers by Flow‐Injection Amperometry

A sensor based on graphite electrode modified with palladium‐platinum‐palladium film is proposed for phosphite determination by flow‐injection amperometry. The modified electrode was prepared by a sequential cathodic deposition of Pd, Pt and Pd on a graphite electrode from 0.5% m/v PdCl 2 +28% m/v NH 4 OH and 2% m/v H 2 PtCl 6 +10% v/v H 2 SO 4 solutions. After suitable conditioning, the electrode showed catalytic activity for phosphite oxidation when 0.15 V was applied. The proposed system handles approximately 50 samples per hour (0.01–0.05 mol L −1 Na 2 HPO 3 ; R 2 =0.9997), consuming ca. 70 μL of sample per determination. The limit of detection and amperometric sensibility were 5×10 −4 mol L −1 and 1.5 mA L mol −1 , respectively. The proposed method was applied to analysis of fertilizer samples without pre‐treatment. Results are in agreement with those obtained by spectrophotometry and titrimetry at 95% confidence level and good recoveries (96–109%) of spiked samples were found. Relative standard deviation ( n=  12) of a 0.01 mol L −1 Na 2 HPO 3 sample was 2%. The useful lifetime of modified electrode was around 220 determinations. For routine purposes it means that this electrode can be continuously used for 5 hours.