Approach to Analytical Chemistry of Saponins : Application of Saponin as a Surfactant Having Complex-Forming Ability to Spectrophotometry of Iron Ion

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An attempt was made to apply saponin to analytical spectrophotomethy; color reactions between o-hydroxyhedroquinonephthalein (Qnph) and several metal ions in the presence of surfactants including saponin were studied in weakly basic media. A simple, sensitive and selective spectrophotometric determination of iron ion (iron<III>+iron<II>) was developed with Qnph and saponin. The apparent molar absorptivity for iron ion was 1.18×10⁵ 1 mol<-1>cm<-1> at 565 nm. It was suggested that saponin is a surfactant having complex-forming ability with metal ions, and iron ion combined with hydrophilic sugar groups in the saponin. The proposed method was applied to the assay of iron ion in rain water, tap water, human urine and calf serum, and the analytical results were in good agreement with those obtained by atomic absorption spectrometry, inductively coupled plasma atomic emission spectroscopy and spectrophotometry with bathophenanthroline.