Salt bridges are used in electrochemical cells with an aim to connect electrodes, minimizing the risk of contamination between the electrolytes of the two electrodes. While choosing a salt bridge, care must be taken to avoid precipitation of insoluble salts at salt bridge interfaces.
In the case of an electrochemical cell formed between half cells, Pb+2 /Pb and Cu+2 /Cu, Pb /Pb2+ serves as the anode and Cu2+/Cu as the cathode. Consequently, Pb is oxidized to Pb2+ ions in the spontaneous anode reaction. If aqueous NaCl solution is used in the salt bridge, the Cl- ions will form a precipitate of PbCl2 at the anode-salt bridge interface, which will interfere with the passage of electrons adversely, eventually stopping the cell reactions altogether after some time.
Therefore, it is not advisable to use aqueous sodium chloride solution as the salt bridge here. Use of an alternative salt bride (ammonium nitrate, for example) is indicated.
sodium chloride will react with pb+2 to form insoluble pbcl2.This prevents free movement of ions.