user10302
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Concentration of KOH solution
Hi everyone,
I'm currently involved in a project to make soap using avocado oil and KOH solution extracted from banana skin ashes. However, the problem I'm having
currently is with concentration of the KOH solution. To prepare the solution, I ran soft water through the ashes of burnt banana peels, resulting in a
basic KOH solution with a pH of 10.7. For the saponification of the avocado oil, the target pH of the solution is around 14 (so the solution requires
alot of concentrating i know!). However, when i concentrated 120 mL of the pH 10.7 KOH solution down to 8 mL of solution, the pH decreased to 10.4,
and I have no clue why. I'm fairly certain the problem doesn't lie in the pH meter as we are using an expensive meter from the chemistry department of
my university. We also repeated the experiment with KOH solution extracted from wood ash, and experienced the same problem (the pH decreased upon
concentration). Any ideas as to what is going on would be really appreciated, thanks!
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Zyklon-A
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If you're in a chemistry university, why not use lab grade KOH? Are you trying to make it completely natural?
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macckone
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You probably have more K2CO3 than KOH.
Also any KOH may absorb CO2 fairly quickly in solution.
And K2CO3 may also absorb CO2
resulting in KHCO3 which doesn't break down until
100C.
*added*
The usual method for making KOH from K2CO3 is to add Ca(OH)2 solution.
[Edited on 3-2-2014 by macckone]
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user10302
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Thanks for the fast replies. Yes we have access to lab grade KOH and have successfully performed the saponification with it (we did this to ensure
that avocado oil can form solid soap, as many vegetable oils are only suitable to prepare liquid soap), however the aim of the project is to prepare
the soap from completely natural resources. Ok macckone thanks for the answer, I just find it puzzling how this isn't working as the extraction and
subsequent concentration of KOH solution from various ashes appears to be a old tried and tested procedure (from what I have read). I will try adding
some calcium hydroxide to the solution and retest the pH . Thanks again
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blogfast25
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At the homelab level only.
@User:
A pH of 11, even assumed only KOH (no carbonate), is still only about 0.001 M (mol/L) or 56 g/mol x 0.001 mol/L= 0.056 g KOH / L or 0.0056 % KOH,
ridiculously low. You're not gonna saponify diddlysquat with that, nothing 'puzzling' there. But ashes contain carbonates, not hydroxides. Ask
yourself why potassium is named after 'potash' and what potash really is: the leachate from wood (or plant) ashes is rich in K2CO3, not KOH.
To go 'all natural', you'd have to leach serious amounts of banana peel ash, using the same leaching solution over and over again, until some
reasonable concentration of potash is reached. Boiling in then yields impure but concentrated K2CO3.
I doubt if as an alkali it is strong enough to effectuate saponifications.
[Edited on 3-2-2014 by blogfast25]
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phlogiston
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Quote: Originally posted by user10302  | | However, when i concentrated 120 mL of the pH 10.7 KOH solution down to 8 mL of solution, the pH decreased to 10.4, and I have no clue why. I'm fairly
certain the problem doesn't lie in the pH meter as we are using an expensive meter from the chemistry department of my university.
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Even very expensive pH meters don't always cope very well with highly concentrated solutions. I have often needed to dilute highly concentrated
well-buffered solutions to get the correct reading.
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"If a rocket goes up, who cares where it comes down, that's not my concern said Wernher von Braun" - Tom Lehrer |
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Zyklon-A
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Well, his solution is not highly concentrated.
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