Membrane filters for concentration of trace elements in water: distribution of 2-(2-pyridylazo)-5-diethylaminophenol between the membrane filter and the aqueous phases

Abstract

The distribution of 2-(2-pyridylazo)-5-diethylaminophenol (PADAP) between the membrane filter (MF) and the aqueous phases was studied over a wide pH range in the presence of various electrolytes by treating the distribution as a form of solvent extraction, i.e., by taking the MF as one of the solvents. In the presence of large anions (X^–) or large cations (Q⁺), the protonated and dissociated species of PADAP distribute as the ion-associates, H₃PADAP²⁺·2X^–, H₂PADAP⁺·X^– or PADAP4^–·Q⁺. The uncharged form of PADAP distributes in the MF as such. Equilibrium constants relevant to the distribution of PADAP as the ion-associates and as the uncharged species were determined for many combinations of electrolytes and MFs made of different materials; cellulosedinitrate (CN), cellulosediacetate, polyethersulfone, regenerated cellulose and poly(tetrafluoroethylene). The distribution ratio calculated from the equilibrium constants agreed fairly well with the observed one over a wide pH range. Small differences observed between the calculated and observed distribution ratio were interpreted as being due to the ion-exchange adsorption of a small fraction of the ionic species H₃PADAP²⁺, H₂PADAP⁺ and PADAP^–, on anionic and cationic sites on the MF. The extraction power of CN MF was compared with that of chlorobenzene. The results have shown that as the extraction media for the ion-associates, CN MF is stronger than chlorobenzene, while the latter is more effective for the extraction of the electrically neutral form of PADAP.