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Author Topic: Thoughts related to the hydroxide/carbonate content of wood/plant ashes  (Read 380 times)

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MDG

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Composition of Wood Ash:

1)   The percentage of carbonate versus hydroxide depends on the temperature that the wood ash is fired at.  At 1000 oC, the initial composition is mostly hydroxide.  At 600 oC, the initial composition is mostly carbonate.  Intermediate temperatures produce intermediate results.  The temperature of wood charcoal is about 1100 oC.  Sodium carbonate is converted to sodium oxide at 851 oC.

2)   Finely powdered freshly prepared wood ash containing potassium and/or sodium hydroxide will quickly take up CO2 resulting in conversion of the hydroxides to carbonates.

3)   Making soap with carbonates is difficult (pKa of bicarbonate ≈ 10; pKa of water = 14), as hydroxide is 10,000 times as strong a base.  People have been making soap for quite some time (e.g., the 12th century Florentine soap making guild).  People in centuries past were clever, and soap makers likely determined that wood ash must be heated to high temperature followed by leaching with water and storage in a container that excludes contact with atmospheric CO2.  Alchemists in the middle ages were familiar with the construction of furnaces that could easily be maintained at 1000 oC.

4)   Several recent papers have reported analyses of the alkaline content of burned plant ashes.  These papers report hydroxide at between 1% - 20% of the total with the balance being carbonate.  These results are typical when the combustion is carried out “carelessly” and the fine ash produced is allowed to take up CO2.

5)   Let’s assume that medieval soap makers knew that a stronger base than carbonate could be produced and that this stronger base was necessary for efficient soap making.

6)   As an alternative to high T firing, sodium carbonate and/or potassium carbonate can be converted to the corresponding hydroxides by treatment with calcium hydroxide (aka, hydrated quicklime, slacked lime) in a process referred to as causticizing: Na2CO3(aq) + Ca(OH)2(aq)  :rarrow: CaCO3(s) + 2 NaOH(aq)

7)   Processes to produce quick lime (CaO) were known before 100 BC.  I am not sure when the first person realized that water could be added to quicklime to produce slacked lime, but I assume it was a long time ago.

8)   However, it seems that medieval alchemists (i.e., chemists) would use thermal methods in preference to the process of causticizing.

9)   High carbonate potash is fine for use as a pH adjusting agent, fertilizer, etc., but hydroxide is necessary for efficient soap making.

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Borek

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The percentage of carbonate versus hydroxide depends on the temperature that the wood ash is fired at.  At 1000 oC, the initial composition is mostly hydroxide.  At 600 oC, the initial composition is mostly carbonate.  Intermediate temperatures produce intermediate results.  The temperature of wood charcoal is about 1100 oC.  Sodium carbonate is converted to sodium oxide at 851 oC.

Doesn't make much sense. At 600°C you have mostly carbonate and for obvious reasons we can assume the sample is completely dry. You heat it further till carbonate decomposes leaving metal oxides. Where does the water come from to convert oxides to hydroxide?
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MDG

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The ashes are created/fired at normal wood fire coal temperatures of about 1000 oC.  The ashes are immediately cooled and taken up into water (the CO2 concentration in typical surface water varies depending on contact with carbonate containing minerals; unpolluted rain water with a pH of 5.6 contains about 350 ppm of CO2 ).  So a little hydroxide will be converted to carbonate by the CO2 in the water, but the majority will, assuming that unpolluted rainwater is used, be retained as hydroxide in the leachate.  The leachate is filtered and stored in a sealed container to avoid contact with the air.  The water is added by the person preparing the alkali.

It is acknowledged that several recent papers have shown typical procedures for burning plant material followed by collecting and leaching (with water) the ash produce alkali which is mostly carbonate (published results indicate the hydroxide content is 1% to 20% of total).  Also, medieval alchemy textbooks refer to the process of causticizing for the production of hydroxides: sodium/potassium carbonate (aq) + calcium hydroxide (aq)  :rarrow: sodium/potassium hydroxide (aq) + calcium carbonate (s)

My point is that causticizing seems to be extra work that could be avoided if the ash formation and alkali leaching steps are carried out carefully so as to insure the ash is fired at 1000 oC and then quickly leached, filtered and stored.
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MDG

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Note an error in my original post.  Calcium hydroxide is referred to as slaked lime and not slacked lime.  I assume the idea is that CaO (quicklime) produced by firing CaCO3 (limestone) at 1000 oC is thirsty and needs water to slake its thirst.
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MDG

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Additionally, note that the alkali calcium content of wood ash will be converted to CaO at 1000 oC.  Thus, the causticizing process can occur in the water leachate (i.e., the CaO will react with water to produce calcium hydroxide which will then go on to double ion exchange with any sodium/potassium carbonate present to give solid calcium carbonate and soluble sodium/potassium hydroxide),  After leaching the ash with water, the leachate is filtered and the insolubles discarded.  Again, it depends on the ash being heated to 1000 oC (temperature necessary to convert calcium carbonate to CaO).

While there are a number of good papers documenting the formation of predominantly carbonate from "typical" wood ash leachates, there are also reports from people who insist that hydroxide predominates.  Maybe the difference is in the temperatures maintained during firing of the ash and the follow-up techniques used to leach. filter and isolate the alkali.
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Borek

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Your original post never referred to the leachate, you were all the time writing about the ash composition.
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MDG

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Apologies, I should have been more clear.
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