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I'm building a galvanic cell, Zn and Cu as anode and cathode, and ZnSO4 and CuSO4 as the solution respectively. I want to replace the salt bridge with porous membrane. Me question is which porous membrane to use? will filter-paper of 2 micrometer work? thanks

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Bridges and membranes are supposed to keep solutions separated while keeping ion conductivity. It is usually trade-off between both conditions.

Does filter paper keep solutions separated? I do not think so. Difference of pressures or levels would cause flow to the other side.

Somewhat better would be soaked thick paper for drawing/water colour painting. Or even better cardboards But consider long term desintegrating effects.

Try some ceramics, like porous flower pots(?), long term soaked in some suitable inert salt solution.

Or (not too porous) fritted glass as @Ed V has mentioned.

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    $\begingroup$ In the old days, people sometimes used soaked strips of asbestos cloth. A former colleague likes to used folded paper towels. Your answer is much better. Personally, I liked a little glass cell, made by our university glassblower, that had a fritted glass disk between the solution reservoirs. Wish I still had that. $\endgroup$
    – Ed V
    Nov 1, 2021 at 12:53
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    $\begingroup$ Glassblowers vanished so rapidly from universities. $\endgroup$
    – AChem
    Nov 1, 2021 at 14:08

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