Flow-injection spectrophotometric determination of trace iron in various salts. Elimination of blank peak effect and use of 2-(5-nitro-2-pyridylazo) -5-(N-propyl-N-sulfopropylamino) phenol as chromogenic agent

A highly sensitive flow-injection method is presented for the rapid and simple determination of iron in various salt samples. 2-(5-Nitro-2-pyridylazo)-5-(N-propyl-N-sulfopropylamino)phenol (Nitro-PAPS) was used as the chromogenic reagent for the spectrophotometric detection at 582 nm, yielding the iron(II) complex having a high molar absorptivity of 8.5 × 104 1 mol−1 cm−1. A large volume sample injection with 5-m sample loop was employed in a flow-injection analysis (FIA) system, which was very effective for eliminating the refractive index problem that is often found and should be solved in the FIA of samples containing an excess of salts. A linear calibration was obtained for 0–100 ng ml−1 iron in the presence of excess of sodium chloride and the relative standard deviation was 1.3% for 50 ng ml−1 iron (n = 5). The limit of detection is 1 ng ml−1 that corresponds to 0.05 μg ml−1 based on a solid sample when a 1 g sample is taken for analysis. Lower detection limits are attainable with larger samples. No significant interference was found by the ions commonly found in most salt samples. The proposed FIA system was successfully applied to “solar” salts and purified salts with satisfactory precision. The sample throughput is ca. 15 h−1.