Low‐pressure membrane separation processes are taking on importance for the purpose of permeate‐water reuse.

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Ion flotation studies have shown that a surface‐active agent is useful for qualitative analysis of complex ions in dilute aqueous solution, with the surfactant forming a particulate complex with the complex ion of concern. Experiments with a monovalent, cationic surfactant have established the prevalence of Cr2O72− (HCrO4−) and not CrO42−; of [Fe(CN)6]4− and [Fe(CN)5H2O]3−; and of [FeFe(CN)6]2− and not [FeFe(CN)6]− or [Fe(CN)6]3−. The results can be contrasted to those with ions that do not form particulate complexes with the surfactant, such as HPO42− and phenolate; with the latter, no qualitative analytical information can be gained. Ion flotation appears to be a promising technique in general for the determination of ionic species present in aqueous solution; the surfactant must react with the ion of significance to form a particulate complex and the initial surfactant concentration must be controlled carefully.

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