Marcus Darkmoor
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Low Temp Decomposition of Sodium Sulfate ?
I am experimenting with producing sodium sulfate from magnesium sulfate using sodium carbonate.
The initial reaction:
5 MgSO4(aq) + 5 Na2CO3(aq) + 5H2O ---> Mg(OH)2.3MgCO3.3H20 + Mg(HCO3)2 + 5 Na2SO4
The Magnesium complex is soluble, but falls apart into insoluble MgOH and MgCO3 upon dehydrating / drying. The Mg(HCO3)2 also decomposes to MgCO3.
During boiling off the water, I have let the mixture cool and seen crystals of Na2S04 forming, and do have to filter out a white chalky precipitate
after redissolving the dried solid, so I am confident the reaction is proceeding as planned.
I am adding a slight excess of sodium carbonate to make sure any magnesium ions are removed from solution.
However, during the drying process (boiling off the water) I am noticing a slight yellow / green discoloration of the liquid, and upon redissolving
there is a noticeable tinge to the solution. If left on a low heat for hours when drying, I get a significant yellow / brown deposit where the
mixture has been in contact with the glass (pyrex). If heated gently for an excessive time (8-12 hours) this becomes more of a dark brown / black
residue.
It would seem that sodium carbonate does not decompose below 1500K and that sodium sulfate should not decompose below 880 C. I believe the pyrex
would melt before reaching either of these temps, so can anyone explain what is happening? I do not believe the temp is above about 400-500 C, this
is being done on a gas hob (reserved for chemistry only! no food!). Solder will melt in the bowl (so about 250 C), and the flame is as low as I can
get it without it blowing out.
Thanks in advance.
(I want to use the sodium sulfate in a later stage, and both ingredients are super cheap and easily available - much cheaper than buying the sodium
sulphate)
[Edited on 2-3-2018 by Marcus Darkmoor]
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Boffis
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The colour is due to impurities in your "super cheap" reagents, most likely iron. After filtering off the insoluble magnesium salts evaporate to
dryness as you have done and calcine the material gently until it is dry free-flowing granules and then redissolve it in the minimum of warm water
(about 45 C), filter off the insoluble iron oxide and chill the filtrate in the fridge. Filter of the sodium sulphate decahydrate crystals which
should now be colourless and glassy (they tend to loose water and become white on exposure to air). The filterate after recovering the crystals should
be colourless, if so you can evaporate it down and recover more sodium sulphate.
Alternatively you could recrystallize your starting materials first.
[Edited on 2-3-2018 by Boffis]
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Charlemagne
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Quote: Originally posted by Boffis  | | The colour is due to impurities in your "super cheap" reagents, most likely iron. After filtering off the insoluble magnesium salts evaporate to
dryness as you have done and calcine the material gently until it is a dry free flowing granules and then redissolve in the minimum of warm water
(about 45 C), filter off the insoluble iron oxide and chill the filtrate in the fridge. Filter of the sodium sulphate decahydrate crystals which
should now be colourless and glassy (they tend to loose water and become white on exposure to air). |
Would recrystalizing the reagents before use work?
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