Cathodic charge imbalance

The salt bridge is necessary to complete the electrical circuit. Without it, the solution in the anode compartment would become positively charged as Zn2+ ions appeared in it, and the solution in the cathode compartment would become negatively charged as Cu2+ ions were removed from it. Because of the charge imbalance, the electrode reactions would quickly come to a halt, and electron flow through the wire would cease. 

Fig. 3-1. An example of a voltaic cell spontaneously generating current in a wire. The impetus for electron movement in the wire comes from the difference in oxidation potential between Zn,s) and Cu. The reducing agent, Zn(s), gives up electrons at the anode to become Zn ". The oxidising agent, Cu acquires electrons at the cathode and plates-out on the copper electrode as Cu(s). The semi-permeable membrane allows ions to move between the two solutions preventing charge imbalances and completing the electrical circuit (from Hamilton, 1998).

In the Grignard reaction, the medium is poorly conducting. The requirement of effective conduction to prevent the build-up of excessive charge imbalance implies that the cathodic and anodic sites cannot be very far apart. 

The net imbalance in injected charge is equal to the net difference in unrecoverable charge between the cathodic phase and the combined anodic phase... 

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