TRANSPORT-PROPERTIES OF PBHPO4 PRECIPITATION ZONES GENERATED WITHIN CELLOPHANE SKINS

PbHPO4 precipitation zones formed by interdiffusion of equally concent rated aqueous solutions of Pb(NO3)(2) and H3PO4 across cellophane skin s act as membranes under certain experimental conditions. They exhibit different diffusional resistances to the ionic species present in the solutions. This is shown by measurements of the membrane potential ac ross PbHPO4 precipitation zones and of the zeta-potential of PbHPO4 cr ystallites suspended in aqueous solutions of Pb(NO3)(2) and H3PO4. Ess entially, one experimental condition has to be met to produce precipit ation zones with membrane properties: The supersaturation S = c/c(sat) (C-sat, saturation concentration of PbHPO4) within the skin for the g eneration of the precipitate has to be large enough so that a high con centration of small crystallites is generated in a short time. This co ndition is met for concentrations of the electrolytes which are equal to or larger than a characteristic concentration c approximate to 4 . 10(-3) c(+) (c(+) = 1 mol dm(-3)) corresponding to a supersaturation S = 3 . 10(2). The PbHPO4 precipitation zones are not completely imper meable to the ionic species forming the precipitate. In this respect t heir transport properties are different from those of precipitation zo nes of other sparingly soluble crystalline inorganic precipitates.