Q.Collodion is a 4% solution of which one of the following compounds in an alcohol–ether mixture?

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. …