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I recently did a double displacement reaction in which I formed a solution of water with sodium hydroxide and sodium chloride dissolved in it. Now I want to separate both of them. How can I do this?

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    $\begingroup$ Full separation would be very difficult. So does partial separation $\ce{NaOH}$ from of $\ce{NaCl}$. Partial separation $\ce{NaCl}$ from of $\ce{NaOH}$ is duable, as @Hans says. $\endgroup$
    – Poutnik
    Oct 22, 2020 at 7:44
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    $\begingroup$ Is there some reason you need to recover the NaOH? I would consider neutralizing with HCl to get a solution that is just NaCl and then distill off the water. $\endgroup$
    – Andrew
    Nov 28, 2020 at 12:55

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Hans' idea (evaporation + filtration) is not easy to carry out in practice. Concentrated $\ce{NaOH}$ solutions are extremely viscous, and if they are mixed with $\ce{NaC}$l crystals, they cannot be filtered easily. Look ! If the filtration is done on paper, the paper is quickly transformed into a sort of parchment, that prevents any liquid from going through. And if it is done on a glass-sintered filter, the glass will be attacked by $\ce{NaOH}$. As a consequence, I am afraid there is no possibility of separating easily $\ce{NaCl}$ from $\ce{NaOH}$. The only really working process would be centrifugation, which requires an equipment that is not always available in all laboratories.

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One solution would be to reduce the amount of solvent (under vacuum for instance), then filter out the precipitated NaCl at high temperature (at higher temperatures NaOH has as much as >10x the water solubility of NaCl).

The common ion effect will play in your favour, as the high water solubility of NaOH at higher temperatures will help you reach NaOH concentrations that strongly disfavours NaCl solubility.

The way to go may be to first reduce the solvent mass under vacuum at acceptably safe temperatures, and once the desired mass is reached redissolving most of the precipitated NaOH by pushing the temperature up before filtration to a level at which most NaOH has redissolved.

Use appropriate protection equipment and contact material to handle very hot NaOH.

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    $\begingroup$ Sorry but concentrated NaOH solutions cannot be filtrated. . $\endgroup$
    – Maurice
    Nov 23, 2020 at 22:19
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    $\begingroup$ @Maurice: You bring up the point that concentrated NaOH solutions are not as easy to filter as any solution, which is a good remark. Two points on that (1) As for the case here we don't know the proportion of hydroxide vs chloride, so there's no indication how basic the solution would be once concentrated. (2) There are ways even for pure NaOH solutions: a) Cotton + Funnel works well, b) Crystalizing slowly and settling works also, and c) Even in the most extreme cases PTFE membranes with a water miscible solvent (to address the materials hydrophobicity), is another possible solution. $\endgroup$
    – Hans
    Nov 25, 2020 at 17:07
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    $\begingroup$ I'm writing this only to balance a remark that fully has its place on the topic. $\endgroup$
    – Hans
    Nov 25, 2020 at 17:08
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For amateur chemistry, maybe the best bet for separating NaOH from NaCl in solution, is to use low-current electrolysis across a cation exchange membrane, that allows Na+ ions to cross the membrane, while drawing Cl- to the anode. Nurdrage has several YouTubes on electrolytic production of NaOH that might be helpful.

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    $\begingroup$ Low current means low ion transport. Also how does getting all the Na+ on one side and Cl- on the other solve the problem? $\endgroup$
    – jimchmst
    Oct 10, 2023 at 23:37

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