footpetaljones
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Potassium carbonate from wood ashes
So I have a slight fascination with making things from scratch and historical chemistry, so I have been looking into the use of wood ashes and none of
the sources that I have seen explain how or if they separate the sodium carbonate from potassium carbonate. I know that getting pure sodium carbonate
from the solution is possible due to the differing solubility ratios, but I can't figure out if it is possible to isolate the potassium carbonate from
the solution. Since the usual next step is to go straight to sodium/potassium hydroxide, would isolation be possible at that step (or would there be
conflicting side reactions)?
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blogfast25
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Welcome to ScienceMadness!
Please use the search facility, this subject has been covered over and over again.
Potassium carbonate (aka 'potash', for that reason) from wood ash was once a major route to this 'precious' commodity. Leach plant ashes with hot
water, filter and boil filtrate down to dryness. The solid obtained is crude K2CO3. Only a few percent of the ashes is potash, though.
KOH can be obtained by reacting K2CO3 with slaked lime (Ca(OH)2) and water. A solution of KOH and insoluble CaCO3 is obtained.
[Edited on 2-5-2015 by blogfast25]
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footpetaljones
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Thanks for the reply blogfast. The terms I used in my search before posting were less helpful than the ones I used after. Recrystallization seems to
be the key, so I will have to read up on that.
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Milan
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Quote: Originally posted by blogfast25  | Welcome to ScienceMadness!
Please use the search facility, this subject has been covered over and over again.
Potassium carbonate (aka 'potash', for that reason) from wood ash was once a major route to this 'precious' commodity. Leach plant ashes with hot
water, filter and boil filtrate down to dryness. The solid obtained is crude K2CO3. Only a few percent of the ashes is potash, though.
KOH can be obtained by reacting K2CO3 with slaked lime (Ca(OH)2) and water. A solution of KOH and insoluble CaCO3 is obtained.
[Edited on 2-5-2015 by blogfast25] |
Interesting idea, though may I ask what would you do with soluble CaCl2 contamination.
Here's a list of what wood ash usually contains:
-A large amount of Calcium Carbonate (25-45%)
-Both K2CO3 (less than 10%) and Na2CO3
-Potassium chloride and sodium chloride
-A sulfate compound
-A phosphate compound (going by Wikipedia it makes 1% of ash)
-Also some Iron oxides (very small amounts)
-And lastly some miscellaneous organic compounds (which give a brownish color in solution)
Since you are going for KOH it would be better to get soft-wood ashes because they contain more potash (K2CO3) rather than lye (Na2CO3), unlike the
hard-wood ashes where the situation is reversed.
Also, after you dissolve the soluble contents of the ash, you should heat them till only one third of the volume is leave it to cool to get rid of the
sulfates. After this boil it down till dryness while stiring with a wooden or iron rod to prevent a hard cake from forming, then proceed to calcine it
to destroy the organic contaminates.
After that you can follow the guide at caveman chemistry to help with re-crystallization.
Here's a link: http://cavemanchemistry.com/oldcave/projects/potash/
Or if you would like to go the route blogfast suggested then you might just calcine the ash itself, this would destroy the organic contaminates and
provide you with CaO which give you Ca(OH)2 when it comes in contact with water.
Also, i would suggest looking at this book on HathiTrust: http://catalog.hathitrust.org/Record/005858638
It's a soap makers handbook, contents about potash (K2CO3) start from page 155, lye is a little bit before that.
Edit: You should be able to do all the calcining with a regular propane torch. Also I think aga has some experience with this matter so you could ask
him if he has free time to help you.
[Edited on 3-5-2015 by Milan]
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blogfast25
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| Quote: Originally posted by Milan |
Interesting idea, though may I ask what would you do with soluble CaCl2 contamination.
Also, after you dissolve the soluble contents of the ash, you should heat them till only one third of the volume is leave it to cool to get rid of the
sulfates. |
Where is this CaCl2 contamination supposed to come from?
How is that going to get rid of sulphates? Because they crystallise as K2SO4? You'd really have to know the level of sulphate content very well to so
precise about it. The whole point of recrystallizing is that minority constituents stay in the liquor because their solubility limit hasn't been
exceeded yet.
[Edited on 3-5-2015 by blogfast25]
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Texium
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Where is this CaCl2 you speak of? If the composition of the ash is as you say it is, then leaching with water should dissolve the K/Na
carbonates and chlorides and trace amounts of a few other things. Any calcium present in the solution would precipitate as calcium carbonate, due to
the alkalinity of the solution. Also, the sulfate content would probably be in the form of nearly insoluble calcium sulfate, and thus there would
likely be very little in solution.
Edit: Looks like blogfast beat me to it while I was typing up my post, sorry about that
[Edited on 5-3-2015 by zts16]
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Milan
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| Quote: |
Where is this CaCl2 contamination supposed to come from?
How is that going to get rid of sulphates? Because they crystallise as K2SO4? You'd really have to know the level of sulphate content very well to so
precise about it. The whole point of recrystallizing is that minority constituents stay in the liquor because their solubility limit hasn't been
exceeded yet.
|
Ah, sorry about that I forgot about the reactivity series, but still instead of CaCl2 there would be chlorides of Sodium and Potassium.
About the sulfate, it is mentioned in the one of the other threads about the same topic and also in the book I listed. Here's an extract from the book
(page 157): "Potash obtained from wood-ashes is, as previously mentioned, chiefly a mixture of potassium carbonate, sulphate and chloride."
Also that method will crystallize out the potassium sulfate because it has the highest difference of solubility in cold (or room temperature) and hot
water.
At 20 degrees Celsius it's 111 g/L while at 100 degrees it's 240 g/L.
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blogfast25
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| Quote: Originally posted by Milan |
Ah, sorry about that I forgot about the reactivity series, but still instead of CaCl2 there would be chlorides of Sodium and Potassium.
About the sulfate, it is mentioned in the one of the other threads about the same topic and also in the book I listed. Here's an extract from the book
(page 157): "Potash obtained from wood-ashes is, as previously mentioned, chiefly a mixture of potassium carbonate, sulphate and chloride."
Also that method will crystallize out the potassium sulfate because it has the highest difference of solubility in cold (or room temperature) and hot
water.
At 20 degrees Celsius it's 111 g/L while at 100 degrees it's 240 g/L. |
The 'reactivity series', whatever you mean by that, has nothing to do with this.
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Milan
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| Quote: Originally posted by blogfast25 | | Quote: Originally posted by Milan |
Ah, sorry about that I forgot about the reactivity series, but still instead of CaCl2 there would be chlorides of Sodium and Potassium.
About the sulfate, it is mentioned in the one of the other threads about the same topic and also in the book I listed. Here's an extract from the book
(page 157): "Potash obtained from wood-ashes is, as previously mentioned, chiefly a mixture of potassium carbonate, sulphate and chloride."
Also that method will crystallize out the potassium sulfate because it has the highest difference of solubility in cold (or room temperature) and hot
water.
At 20 degrees Celsius it's 111 g/L while at 100 degrees it's 240 g/L. |
The 'reactivity series', whatever you mean by that, has nothing to do with this. |
I think that's how it's called in English (don't judge me if I made a mistake, I'm a non-native English speaker). Basically what I meant by that is
that Ca(OH)2 won't react with say NaCl because Na is more reactive than Ca.
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Hawkguy
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You could try separating the KOH by adding Ethanol. Bisulfate, Carbonate, Bicarbonate, Sulfate, etc will precipitate, leaving a Hydroxide solution...
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Texium
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| Quote: Originally posted by Milan | | Quote: Originally posted by blogfast25 | | Quote: Originally posted by Milan | Ah, sorry about that I forgot about the reactivity series, but still instead of CaCl2 there would be chlorides of Sodium and Potassium.
About the sulfate, it is mentioned in the one of the other threads about the same topic and also in the book I listed. Here's an extract from the book
(page 157): "Potash obtained from wood-ashes is, as previously mentioned, chiefly a mixture of potassium carbonate, sulphate and chloride."
Also that method will crystallize out the potassium sulfate because it has the highest difference of solubility in cold (or room temperature) and hot
water.
At 20 degrees Celsius it's 111 g/L while at 100 degrees it's 240 g/L. |
The 'reactivity series', whatever you
mean by that, has nothing to do with this. |
I think that's how it's called in English (don't judge me if I
made a mistake, I'm a non-native English speaker). Basically what I meant by that is that Ca(OH)2 won't react with say NaCl because Na is more
reactive than Ca. |
No, that does not apply in this scenario. Ca(OH)2 doesn't react to completion
with NaCl because both NaOH and Ca(OH)2 are soluble, thus existing in an equilibrium of aqueous Na<sup>+</sup>,
Ca<sup>2+</sup>, Cl<sup>-</sup>, and OH<sup>-</sup>. More likely than not, there will also be some solid
Ca(OH)2 laying around as its solubility is quite limited.
[Edited on 5-3-2015 by zts16]
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blogfast25
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| Quote: Originally posted by Hawkguy | | You could try separating the KOH by adding Ethanol. Bisulfate, Carbonate, Bicarbonate, Sulfate, etc will precipitate, leaving a Hydroxide solution...
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Even if that precipitating worked, you'd end up with potassium ethanoate. Then what?
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Texium
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Huh? Since when were acetates/ethanoates formed from alkali hydroxide + ethanol?
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blogfast25
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Sorry. Potassium ethoxide, of course. My bad.
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