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If I have a cation exchange membrane and need to remove 2 g/L of sodium, how much resin would I need.

I used the amount of substance of $\ce{Na}$ to calculate the amount of substance of $\ce{SO3H-}$ needed for complete sodium removal. But this will not give me the correct mass since the resin is a polymer with that functional group attached. However I haven't been able to find any kind of complete structures for a cation resin.

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    $\begingroup$ Resins typically include a capacity label in $\mathrm{mmol/g}$, don’t they? $\endgroup$ – Jan May 6 '16 at 11:24
  • $\begingroup$ I bought one for a project pretty cheap so it didn't have much technical information. $\endgroup$ – Jonathan May 6 '16 at 12:03
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First of all, you will need more sulfonic groups than sodium ions since sodium doesn't have a very good affinity for sulfonic groups.

Anyway, you can find some specs here — figure 2. This source is using sulfonated polystyrene with various crosslink percentages and expresses the capacity in $\mathrm{mmol}_\text{sulfonic groups}/\mathrm{g}_\text{resin}$ ($1\ \mathrm{meq} = 1\ \mathrm{mmol}$).

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