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Spectrophotometric determination of trace anionic surfactants such as SDS and SDBS in water samples based on a simple and rapid preconcentration technique of collection on an organic solvent-soluble membrane filter, has been proposed. The anionic surfactants are collected on a nitrocellulose membrane filter as an ionassociate complex with the cationic complex of iron (II)-3- (2-pyridyl)-5, 6-diphenyl – 1, 2, 4 – triazine (pyridyl diphenyltriazine, PDT). The membrane filter with the analyte is dissolved in a small volume of organic solvent, and the absorbance of the resulting solution is measured against a reagent blank. The maximum absorption is at 555 nm and the apparent molar absorptivities are 8.93x103 1 mol−1 cm−1 for SDS and 8.51x103 1 mol−1 cm−1 for SDBS. Beer's law is obeyed over the range of 10-100 μg for SDS and SDBS and the detection limit is 0.0043 mg 1−1 for SDS and 0.0052 mg 1−1 for SDBS, respectively on a 3 [sgrave] basis. Interference from Cu2+ and Ni2+ can be eliminated by masking.

An organic solvent-soluble membrane filter (MF) is proposed for the simple and rapid reconcentration with subsequent spectrophotometric determination of trace levels of iron (II) in water. Iron (II) is collected on a nitrocellulose membrane filter as ion associate of an anionic complex, which is formed by iron (II) and Ferrozine and a cation-surfactant. The ion-pair compound and the MF can be dissolved in small volumes of 2-ethoxyethanol and the absorbance of the resulting solution is measured at 560 nm against a reagent blank with molar absorptivity of 4.01 × 104 L mol–1 cm–1. Beer’s law is obeyed over the concentration range 0–10 μg L–1 of iron (II) in water and the detection limit is 0.03 μg L–1 with a 50-fold enrichment factor. The proposed method can satisfactorily be applied to the determination of iron (II) in natural water and sea water.