Sciencemadness Discussion Board

Potassium carbonate from wood ashes

footpetaljones - 2-5-2015 at 12:42

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)?

blogfast25 - 2-5-2015 at 12:51

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]

footpetaljones - 3-5-2015 at 04:02

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.

Milan - 3-5-2015 at 04:59

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]

blogfast25 - 3-5-2015 at 07:01

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]

Texium - 3-5-2015 at 07:02

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]

Milan - 3-5-2015 at 11:54
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.

blogfast25 - 3-5-2015 at 12:53

Quote: Originally posted by Milan  

Quote:



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.

Milan - 3-5-2015 at 12:58

Quote: Originally posted by blogfast25  
Quote: Originally posted by Milan  

Quote:



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.

Hawkguy - 3-5-2015 at 14:31

You could try separating the KOH by adding Ethanol. Bisulfate, Carbonate, Bicarbonate, Sulfate, etc will precipitate, leaving a Hydroxide solution...

Texium - 3-5-2015 at 15:43

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]

blogfast25 - 3-5-2015 at 16:46

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...


Even if that precipitating worked, you'd end up with potassium ethanoate. Then what?

Texium - 3-5-2015 at 16:54

Huh? Since when were acetates/ethanoates formed from alkali hydroxide + ethanol?

blogfast25 - 4-5-2015 at 05:45

Quote: Originally posted by zts16  
Huh? Since when were acetates/ethanoates formed from alkali hydroxide + ethanol?


Sorry. Potassium ethoxide, of course. My bad.