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My measured cell voltages for a Cu/Zn cell don't seem consistent. Why?
The measured voltage for the cell is the total voltage across all of the solution/metal boundaries in the cell. For example, in a cell with a copper and a zinc electrode, the cell voltage is the total of the voltages across the Cu(s)/Cu2+(aq) and Zn(s)/Zn+2(aq) boundaries. If the concentration of electrolyte solution increases these voltages change.
If electrolyte and metal ion concentrations aren't held constant from one experiment to the next, the cell voltage will change.
If the cell contains a salt bridge, that also contributes to the total cell voltage. Since ions will diffuse (unequally) from the solution into the surfaces on either side of the salt bridge, the solution interface on each end of the bridge will have a different voltage associated with it. It's difficult to estimate the size of the net 'liquid junction potential' contributed by the bridge.
It can be positive or negative, and it happens to be very sensitive to pH (since hydrogen ions are better than most ions at diffusing into the bridge).
Author: Fred Senese firstname.lastname@example.org