The unsteady-state flux behavior for silica and dextran in a stirred ultrafiltration cell was studied. The 2 compds. show clearly different flux behaviors. During the filtration of dextran, only a polarization layer is built up. For silica, a gel layer formation also occurs. As a result, the time to reach steady-state flux is 1 h. The osmotic pressure model provides a good description of the flux for the expts. with dextran. If mass-transfer coeffs. are used that are higher than those electrochem. measured, the transient flux for silica can be rather well predicted by the gel-polarization model. The use of flux measurements under unsteady-state conditions as an indication for the presence of a gel layer is discussed. [on SciFinder (R)
