IceDahl
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Registered: 28-2-2016
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Seperate Sodium Hydroxide and Sodium Carbonate
Today i bought some Sodium Hydroxide drain cleaner and thought it was only sodium hydroxide, but as it turns out it was a mix between Sodium Carbonate
and Hydroxide. I now need to separate them or just turn it all to hydroxide since that is my desired product.
My first thought is to just add Calcium hydroxide to get Sodium hydroxide and insoluble calcium carbonate, the problem is that i don't have nearly
enough Calcium hydroxide (I made it myself).
Is there any other good way to separate or at least get sodium hydroxide from them?
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Boffis
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This is relatively easy because sodium carbonate is practically insoluble in 50% and stronger sodium hydroxide solution but you will need a glass frit
type filter to remover the insoluble sodium carbonate. Do you know roughly how much sodium carbonate is present? Most commercial sodium hydroxide
contains only a little sodium carbonate from absorption of atmospheric CO2. So when I want carbonate free sodium hydroxide solution I will usually
make up one litre at a time of say 60% solution. To do this I 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 (the beaker will be too hot to touch by now!). I 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 1L 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 your material is say 50% sodium carbonate thats going to be tough but you could probably do it like this:
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 odf 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 you should have about 700-800ml of very strong sodium hydroxide solution. I wouldn't dilute to one litre yet
but 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.
You should be able to extrapolate between these two limits depending on your carbonate content.
A few note to take into account. 1) 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. 2) the sodium carbonate that crystallises out sinks very slowly hence the long standing time. 3)
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. 4) hot 50% or so
caustic soda solution is extremely aggresive towards skin! When handling strong sodium hydroxide solutions I usually keep a bowl full of dilute citric
or similar acid around to wash and neutralise it quickly.
Let us know how you get on, I have never tried this on sodium hydroxide containing large amounts of deliberate contamination. I am usually working
with commercial caustic soda which is usually <2% sodium carbonate unless it is very old.
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. I have never tried evaporating strong sodium hydroxide solution back to a solid but it is feasible I guess, in stainless steel
possibly. I use the solution directly.
Good luck.
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