Colorimetric determination of available iron in soils by flow injection analysis

This work presents a flow injection system with colorimetric detection for the automatic determination of available iron in soils. The system is based on the formation of the coloured complex produced from the reaction of iron(II) with 1,10-phenanthroline. Problems with refraction index effects (Schlieren signal) were overcome by using a large injection volume yielding the formation of a plateau (analytical signal) between two refractive index peaks in each interface of the sample plug. The developed system is very simple and easy to implement, allowing a sampling rate of 60 determinations per hour. Results obtained for 20 soil samples are in good agreement with those obtained by the reference method with relative deviations lower than 4%. Relative standard deviations were never higher than 1.7% for the entire concentration range studied (1-10 mg L^-1).