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A salt bridge is when you have two different solutions that are in different beakers while a porous disc is used in a concentration cell in which you have two solutions that are the same but are at different molarities. Also, a porous disc is used when there is only one beaker while a salt bridge is for two beakers. The porous disk would allow for movement of ions in the solution (6ed Figure 14.3 for a Daniel cell). The porous disk method is not as accurate when measuring cell potential, since there is an intermixing between the two redox species.
The porous disk will allow ions from both the anode and cathode to diffuse into the opposite subcell, utilizing the charges of the aqueous species involved in the reaction rather than some proxy (as with a salt bridge). This website does a good job of explaining the intricacies of a salt bridge and porous disk: https://chemistry.stackexchange.com/questions/93883/salt-bridges-porous-disks
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