Behaviour of hydrophobic pH indicators in aqueous–organic two-phase systems

Abstract

Very hydrophobic indicators can remain completely in an organic phase, even in ionized form, but nevertheless respond to the pH of an equilibrated aqueous solution. They are useful to monitor the inaccessible aqueous phase of low water media, such as may be employed in enzymic processes. We present a theoretical model for the indicator response, based on the presence of both free ions and ion pairs in the organic phase. This adequately describes the behaviour observed with C₁₅ alkyl esters of fluorescein at concentrations around 10^–5 mol dm^–3, as a function of aqueous pH and cation activity or concentration. Analysis of data for toluene suggests complete ion-pairing in the organic phase, with either tetraalkylammonium ions or dibenzol-8-crown-6 and Na⁺. In more polar solvents Na⁺ counter-ions can be extracted alone: fitting of the titration curves suggests complete dissociation in pentan-3-one and butan-1-ol, but partial ion-pairing in ethyl acetate. Fitted parameters obtained from the model may be related theoretically to other literature measurements, notably for picrate salt extraction, and are generally consistent.