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I'm trying to understand the chemical formula of soap using potash ($\ce{K2CO3}$) as they did in antiquity instead of using sodium hydroxide ($\ce{NaOH}$) or potassium hydroxide ($\ce{KOH}$). I've found the formula for making soap with sodium hydroxide on the Khan Academy, but I didn't make much progress on my own.

soap

Can somebody help me understand how the soap and the glycerin molecule would look like?

Or is the saponification not possible and ancient soap makers first needed to create potassium hydroxide from potassium carbonate?

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    $\begingroup$ Closely related, would be a duplicate if there was an answer. But it has useful comments. $\endgroup$
    – Poutnik
    Apr 23, 2023 at 10:21
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    $\begingroup$ I closed the related answer as duplicate now that we have multiple answers here. $\endgroup$
    – Karsten
    Apr 23, 2023 at 20:11
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    $\begingroup$ Unrelated to the potassium carbonate question, but where does the oxidation (carbonyl) on the central carbon come from? Saponification should produce glycerol (and/or related alkoxides), not dihydroxyacetone. $\endgroup$
    – R.M.
    Apr 24, 2023 at 14:58

3 Answers 3

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Ashes have been thoroughly studied more than $100$ years ago. See the Dictionnaire de chimie (in French) by A. Wurtz, Hachette ed., Paris, 1872, p. 787 - 789. Potassium hydroxyde $\ce{KOH}$ was never found in all ashes studied (Wood, fruits, vegetables) : in the presence of air, potassium hydroxide $\ce{KOH} $ is quickly transformed into potassium carbonate $\ce{K2CO3}$. The composition of wood ashes is $\ce{CaCO3}$ $5 - 50$%, $\ce{K2CO3}$ $12 - 32$%, $\ce{SiO2}$ $5 - 42$%, $\ce{Ca3(PO4)_2}$ $5 - 15$%, $\ce{MgCO3}$ $2 - 6$%, and $\ce{FePO4}$ $0 - 2$%. So, wood ashes do not contain any $\ce{KOH}$, as the $\ce{OH-}$ ion is entirely transformed into carbonate in the presence of air. But, as mentioned by Oscar Lanzi, this $\ce{CO3^{2-}}$ ion is hydrolyzed in water and produced about $1$% $\ce{OH^-}$ ion, which is able to saponify the vegetable oils. But the yield is poor.

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Can somebody help me understand how the soap and the glycerin molecule would look like?

Once you add water, you have potassium ions and hydroxide ions in solution, and the products look the same as with aqueous potassium hydroxide.

In contrast to the reaction with sodium hydroxide, which gives the sodium salts of the fatty acids, you would obtain the potassium salts of the fatty acids with potassium hydroxide or with potassium carbonate.

If you use potassium carbonate, there would be some form of carbonate as a side product. Depending on the final pH, it could be carbonate, hydrogen carbonate or carbon dioxide or some combination. According to this source, traditional recipes used an excess of wood ash, so the pH would be fairly alkaline.

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    $\begingroup$ I am afraid that with comparable ratio bicarbonate/carbonate, the saponification would be already effectively stopped due too low pH (pKa2 10.32) , not even speaking about CO2. $\endgroup$
    – Poutnik
    Apr 23, 2023 at 20:02
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    $\begingroup$ @Poutnik I added that you would usually work with an excess of wood ash. I am wondering whether it is possible to change the pH after the fact (this is soft soap, so semi-liquid). $\endgroup$
    – Karsten
    Apr 23, 2023 at 20:04
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    $\begingroup$ Yeah, I have noticed that after posting my comment. From CO2 it would be just a small step to precipitation of free fatty acids. I think there would be needed much more potash than caustic potash, as with the former pH would quickly drop. $\endgroup$
    – Poutnik
    Apr 23, 2023 at 20:07
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Once you introduce water, potassium carbonate can generate potassium hydroxide through the hydrolysis reaction

$\ce{CO3^{2-} + H2O <=> HCO3^- +OH^-},$

and this reaction is fairly efficient because carbonate ion is a rather strong base ($\pu{pK_b} = 3.75$ from the conjugate acid $\pu{pK_a}$ reported by Wikipedia). That basicity combined with the good solubility of potassium carbonate in water allows a $\pu{pH}$ between $12$ and $13$ to be obtained.

In addition, the moderately strong-base carbonate ion can promote nucleophilic attack by the water itself, by acting as a base to remove a proton from the nucleophilic substitution product:

$\ce{RCOOR' + H2O -> RCOOH + R'OH}$

$\ce{RCOOH + CO3^{2-} -> RCO2^- + HCO3^-}$

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    $\begingroup$ With spending about 9% of carbonate, pH will drop to 11.3. With 50% to 10.3. $\endgroup$
    – Poutnik
    Apr 23, 2023 at 20:13
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    $\begingroup$ So I propose a mechanism that directly uses carbonate ion as the reactant. $\endgroup$ Apr 23, 2023 at 20:13
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    $\begingroup$ Hmm, I am not sure about that, but I am not an organic chemist. $\endgroup$
    – Poutnik
    Apr 23, 2023 at 20:15
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    $\begingroup$ In response, I switch roles to carbonate acting as a proton remover, which seems better documented in the litersture. $\endgroup$ Apr 23, 2023 at 20:35
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    $\begingroup$ We don't need to boil off CO2 or specifically create more KOH. But we may need hotter water to make the soap more effective. $\endgroup$ Apr 24, 2023 at 13:16

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