# Potassium carbonate vs. Potassium hydroxide in Wood Ash

I'm trying to get more concrete information about the chemical process of creating soap from wood ash.

What I’m confused about: Many soap-making materials say that you create potassium hydroxide directly from filtering water through wood ash. As far as I can tell this is false? You are actually getting potassium carbonate (and some small amount of other residue.)

To create potassium hydroxide from this, historically you would add a solution of calcium hydroxide (calcium oxide/burnt limestone dissolved in water). Calcium carbonate would precipitate out of the solution, leaving behind potassium hydroxide. From my understanding, a lot of modern soap-makers are simply misunderstanding the chemistry, or not realizing that the lime has to be added before they actually have potassium hydroxide.

However, some sources say that if the wood is heated hotter and given a more complete burn, thus combusting more of the organic material, then more hydroxides will be dissolved instead of carbonates, thus negating the need for lime. Is this true?

I have also read that if you take the potassium carbonate, dehydrated, and heat it to an (uncertain) degree, it will burn off carbon dioxide and create potassium oxide, which added to water will become potassium hydroxide. Is this true? How hot do you need to burn it, if so?

However, other sources say heating potassium carbonate only purifies it, creating pearl ash, which is still potassium carbonate.

• According to this reference researchgate.net/publication/… Potassium carbonate has a melting point of 900C under a nitrogen atmosphere but some volatilisation occurs below that temperature so I guess it depends on how hot the wood fire got but you are probably correct that you mostly get potassium carbonate – Waylander Jun 21 '19 at 19:09

You can never get $$\ce{KOH}$$ directly out of wood ash. One can recall that potash $$\ce{=K2CO3}$$ is indeed pot-ash! So you are right that historically calcium hydroxide was needed to obtained $$\ce{KOH}$$ out of potash. The rest of the stories are all false. $$\ce{KOH}$$ is unstable in air with respect to carbonate formation. It is hygroscopic as well. Wood ash remains bone dry forever. I don't think there is even a trace of potassium hydroxide in wood ash.

Until and unless you don't have a furnace, which can generate temperatures above 1300 Celsius chances of getting $$\ce{KOH}$$ in useful amounts are almost negligible.

Secondly, a soap made with tons of calcium salts will be nothing but literally scum (=insoluble calcium soaps). Calcium should not be a part of soap.

Here is a nice abstract on the composition of wood ash as a function of temperature. [1]

The elemental and molecular composition of mineral matter in five wood types and two barks was investigated as a function of temperature using thermal gravimetric analysis, differential thermal analysis, inductively coupled plasma emission spectroscopy, and X-ray diffraction. Low temperature ash was prepared at 500°C, and samples were heated in a tube furnace at temperature increments to 1400°C. The dissociation of carbonates and the volatilization of potassium, sulfur, and trace amounts of copper and boron were investigated as a function of temperature. Overall mass loss of the mineral ash ranged from 23–48% depending on wood type. The mass of K, S, B, Na, and Cu decreased, whereas Mg, P, Mn, Al, Fe, and Si did not change with temperature relative to Ca which was assumed to be constant. Sintering of the ash occurred, but fusion of the ash did not occur. In the 600°C ash CaCO3 and K2Ca(CO3)2 were identified, whereas in 1300°C ash CaO and MgO were the main compounds. The implications for ash deposition in furnaces is discussed.

### References:

1. Misra, M. K.; Ragland, K. W.; Baker, A. J. Wood ash composition as a function of furnace temperature. Biomass Bioenergy 1993, 4 (2), 103–116 DOI: 10.1016/0961-9534(93)90032-Y.
• Great information, thank you. About the comment "Calcium should not be part of soaps," I believe you mean the act of adding CaCO3? If you use that method to get from potassium carbonate to hydroxide, after precipitating, I was under the impression you would have 2 KOH + CaCO3, and all the CaCO3 would settle out of the solution. Is this incorrect? Where does the insoluble soap scum come from? – Azalea Ellis Jun 24 '19 at 15:38
• The scum would originate from remaining calcium hydroxide. Calcium hydroxide needs to be added in excess to convert K2CO3 to KOH. Calcium hydroxide has some solubility in water. I assume the scum during soap making can be removed. I have tried this experiment, but at one time I was also very curious about wood ashes for feeding to plants. – M. Farooq Jun 24 '19 at 15:41
• Thanks for the clarification. Looking at the solubility of calcium hydroxide (0.066 g/mL) vs. potassium hydroxide (162.9 g/100 mL) --This is for a solution at boiling 100Celcius -- I imagine it would be possible to simply boil off the water from the solution. Basically, using recrystalization as a purification technique. The Calcium hydroxide would be forced to precipitate, and then the solution containing the potassium hydroxide could be poured off. Does that sound reasonable? – Azalea Ellis Jun 24 '19 at 20:57
• Sounds reasonable on paper. KOH cannot be crystallized as far as I know. I have never tried this experiment (sorry for the mistake in the previous comment). As I said my only interest in wood ashes was its use as a plant food but never for soap making. Are you making an organic soap for some reasons? – M. Farooq Jun 24 '19 at 21:44
• No worries, I understood your previous comment. It's fine that you haven't tried it, but your understanding of chemistry is probably better than my own. I will have to try it myself, when my accommodations allow. I am a science fiction author and am writing a new series where authenticity is important. The least I can do is take some time to research the material I'm writing about so it's not blatantly wrong. – Azalea Ellis Jun 25 '19 at 14:47

$$\ce{CaCO3}$$ is easily decarboxylated to $$\ce{CaO}$$ in the heat of the fire. By dumping the ash into water you get a solution of $$\ce{Ca(OH)2}$$ and $$\ce{K2CO3}$$, and since $$\ce{CaCO3}$$ is less soluble than $$\ce{Ca(OH)2}$$ and $$\ce{K2CO3}$$ you end up with $$\ce{KOH + Ca(OH)2}$$ solution in one step and $$\ce{CO2}$$ dissolving into this solution will deplete calcium ions before it will start depleting $$\ce{KOH}$$