Q.Which of the following impurity can be removed by electrodialysis method ?

Freshly prepared colloidal solutions commonly carry dissolved electrolyte and other soluble impurities that, left unremoved, destabilise the sol and trigger premature coagulation. Purification exploits one physical fact throughout: colloidal particles are too large to cross a semipermeable membrane, while dissolved ions and small molecules pass through freely.

Dialysis (T. Graham, 1861) places the colloidal solution inside a bag of semipermeable membrane suspended in a trough of flowing water; small electrolyte ions diffuse out through the membrane and are carried away by the water flow, while the larger colloidal particles are retained. Medically, kidney dialysis recycles a patient's blood through semipermeable tubing in isotonic saline to remove excess electrolyte in exactly this way, when the kidneys themselves cannot.

Electrodialysis speeds this up by applying an electric field: the impure sol sits in a central compartment flanked by two dialysing (ion-selective) membranes, beyond which are two water-filled compartments with electrodes; current drives the impurity's cations and anions through their respective membranes into the two water compartments, from which they are periodically flushed — considerably faster than passive dialysis alone, since the field actively pulls the ions out rather than relying on diffusion. …