NEUTRALIZATION DIALYSIS - PROBLEMS AND OUTLOOK

Neutralization dialysis (ND) of model systems containing 0.1-0.5 M NaC l and various high-molecular-weight synthetic or natural substances ha ve been investigated. Desalination experiments were carried out in a t hree-compartment spiral module equipped with heterogeneous Ralex or ho mogeneous Neosepta membranes. Practically complete desalination procee ds due to the neutralization reaction in the desalination compartment. The time of desalination depends only on the membrane permeability an d on the initial salt concentration in solution. An important feature hindering larger application of this method is sharp pH changes occurr ing in the desalination solution. This phenomenon predicted theoretica lly is mainly caused by the difference in diffusivities of H+/Na+ and OH-/Cl- ion pairs. Independently of the type of membrane and desalinat ed solution, pH value jumps into acidic region at the beginning of the ND. In the course of the process the pH value increases and the ND pr oceeds mostly at pH < 7. On the basis of theoretical prediction, an ex perimental method consisting in adjustment of acid concentration is pr oposed, which makes it possible to maintain constant pH in the desalin ation compartment. The convenience of this method was demonstrated by the ND of protein solutions.