Determination of structural and electrical parameters for activated composite membranes containing di-(2-ethylhexyl)dithiophosphoric acid as carrier

The paper reports the results obtained on the physicochemical characterisat ion of activated composite membranes (ACMs). Membrane samples containing di fferent concentrations of di-2-ethyl-hexyldithiophosphoric acid (DTPA) as a carrier were prepared and characterised by different analytical techniques . The amount of the carrier incorporated in ACM appears to be directly prop ortional to the concentration of carrier in the casting solution. Respectiv e distribution dependencies can serve as calibration graphs applicable for pre-calculation of extractant concentration in the casting solution to manu facture an ACM with a given carrier content. The carrier distribution in th e membrane was evaluated by X-ray microanalysis (EDS), which allowed us to determine that only a limited concentration of the carrier can be incorpora ted into the top polyamide membrane layer, resulting in redistribution of t he excess of extractant along the polymeric structure of the membrane. Thes e results also correlate with those obtained by impedance spectroscopy meas urements. The equivalent circuits associated with the different membrane sa mples agree with a two-layer model for the ACM containing a high concentrat ion of carrier (C-DTPA >0.5 M), which allows for the location of a larger a mount of carrier into the polyamide layer. The results obtained by the indi cated techniques are in good agreement with each other, and provide for the structural characterisation of the membrane. (C) 2000 Elsevier Science B.V . All rights reserved.