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I got a drain cleaner in dust form, which consists of $\ce{Na2CO3}$ and $\ce{NaOH}$. I wanted to extract the $\ce{NaOH}$ so I tried to heat the mixture in order to just melt the sodium hydroxide, but it didn't really work as expected. Is there any easy way to do it?

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    $\begingroup$ @Mithoron Well, I managed to melt it. The melting point of NaOH according to Wikipedia is 323°C not a difficult temperature to achieve. The idea was that by heating the mixture to let's say to 400°C would melt the NaOH but not the Na2CO3 which melts at 851°C and separate them this way. What I didn't know is that Na2CO3 dissolves in liquid NaOH so separation in this way is not possible. $\endgroup$ – GeorgeTsak Mar 15 '18 at 18:32
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There is a forum which discusses the similar problem of separating sodium hydroxide and sodium carbonate from drain cleaner. The solution is taken from there(slightly edited):

This is relatively easy because sodium carbonate is practically insoluble in 50% sodium hydroxide solution so a glass frit type filter is needed to remover the insoluble sodium carbonate. Most commercial sodium hydroxide contains only a little sodium carbonate from absorption of atmospheric $\ce{CO2}$. So for carbonate free sodium hydroxide solution, make up one litre at a time of say 60% solution. To do this, add about 300g of sodium hydroxide pellets of flakes to 500ml of ice-cold water in a large beaker and stirr until practically dissolved, cover and leave to cool . Allow it to cool to about 20-25 C and the add another 300g of sodium hydroxide and stirr until most has dissolved (much less heat is generated on the second addition), dilute to one litre, pour into a 1 L thick-walled polythene bottle and leave it to stand for about 1 month in a cool place and then decant the clear sodium hydroxide solution.

If the material is say 50% sodium carbonate do the following:

Add 500g of mixed sodium salts to 500ml of ice-cold water, stirr the slurry periodically for a couple of hours and then filter through a glass frit funnel (a vacuum filtration set-up will be needed) and wash the cake with a little water, say 50-100ml, to remove most of the remaining caustic soda; some sodium carbonate will dissolve but this will precipitate on the next addition. The cake will be almost pure sodium carbonate but you can always treat it with saturated sodium bicarbonate solution to ensure that it is.

The clear filtrate should be placed in a calibrated vessel and another 500g of material added and again stirred periodically for a couple of hours then filter and wash as before. Hopefully there should be about 700-800ml of very strong sodium hydroxide solution. Place it in a polythene bottle and let it stand of a month so the sodium carbonate can crystallize out, decant the clear solution and dilute as required.

NOTE:

  • 60% caustic soda solution is rather viscous and will not filter without suction even on a fairly coarse frit, 50% is still pretty viscous and difficult to filter.

  • The sodium carbonate that crystallises out sinks very slowly hence the long standing time.

  • Sodium carbonate dissolves rather slowly in water form some reason so with vacuum filtation the residence time of the wash water will actually dissolve very little sodium carbonate, too much wash water will dilute the solution below 50% and make the carbonate more soluble.

  • Hot 50% caustic soda solution is extremely aggresive towards skin! When handling strong sodium hydroxide solutions, keep a bowl full of dilute citric around in case of emergency.

There are other method such as treating a more dilute solution with slaked lime, filtering (difficult because the precipitate is fine grained) and evaporating down.

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    $\begingroup$ SAFETY RE:keep a bowl full of dilute citric around in case of emergency. // You need a lot more than a bowl full of citric acid. You should have eye glasses, an apron, gloves, an eye wash, and a safety shower in case of an accident. You should also not be working alone with such chemicals. $\endgroup$ – MaxW Mar 15 '18 at 14:31
  • $\begingroup$ Thank you very much for your response. I didn't know that solubility of Na2CO3 decreases when NaOH is presented. That explains the white precipitate in an exerinment I did using the drain cleaner. $\endgroup$ – GeorgeTsak Mar 15 '18 at 18:34

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