Junk_Enginerd
Hazard to Others
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Registered: 26-5-2019
Location: Sweden
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Saponification of an oil -> adding an acid. What happens?
I've been doing some experimentation with linseed oil and it's soap for treating wood. I'm doing a little carpentry project.
I made my own linseed oil soap for the purpose of treating the wood. It came out a little darker than I had hoped though, same color as the linseed
oil I used to make it. I got a vague thought about organic contaminants and recrystallization and took a bit of the soap and added acid to it.
(Specifically sodium bisulfate, but I don't believe that's too relevant.) As I expected, the soap separated as the soap molecules converted to fatty
acids instead. I got a clear liquid and a coagulated slimey part which separated easily. I poured off the liquid and saved the coagulated bit. I then
proceeded to remake it into soap by adding potassium hydroxide.
Overall, I suppose I succeeded, because the soap I had now was a mellow pale yellow color instead of the rich amber it was before. That's what
commercial linseed oil soap looks like. Also, in contrast to the original soap, it now actually looked like liquid soap, you know, the
shimmering pearly look. The original soap is just a rich transparent amber color.
The idea was to see if whatever was making the soap dark was a contaminant that could be poured off with the liquid after acidification. Again, I
succeeded as far as the results go, the problem is I don't understand exactly what happened lol.
The liquid was clear, not dark, when I poured it off, and the coagulated stuff had already gone light. The color seemingly just... disappeared?
What might've happened with it?
Afaik, saponification takes a triglyceride, displaces the glycerin from it, and you end up with fatty acids hanging onto a hydroxide instead, or
something like that. At first I thought adding an acid basically just reversed the saponification, but thinking about it more I doubt the glycerin
goes back to hold the fatty acids. Does this mean that after acidification, I have a bunch of free fatty acids instead of the original triglycerides?
Do the above mentioned free fatty acids turn into the same type of soap when reacted with potassium hydroxide again? Or is something
irreversibly different?
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Tsjerk
International Hazard
Posts: 3032
Registered: 20-4-2005
Location: Netherlands
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The saponification broke the ester bonds between the fatty acids and glycerol, the fatty acids will now be in their deprotonated salt form. These
acids will dissolve in water, and when you added the acid they became water insoluble while your colored contamination remained in the water. The
ester bonds won't reform though.
When you add a base again you will reverse what you did with the acid, making the acids water soluble again.
[Edited on 7-8-2022 by Tsjerk]
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