nora - 27-4-2018 at 16:15
Hello scientists,
I'm just starting to pursue my interest in chemistry and wanted to do something relatively fun and easy. I saw this video by NileRed called "Extracting the Spicy Chemical in Black Pepper". Nile uses Potassium Hydroxide (KOH) in their extraction, but it is
proving quite hard to come by. I saw a comment somewhere that said KOH could be substituted for Sodium Hydroxide (NaOH), is this true? If so, why is
that? Nile says that the KOH "reacts with acids as well as some fats" to "convert them into something that's soluble in water"; what in the KOH
molecule is reacting, and why would that make the result water soluble? I'm assuming it's the -OH part, since NaOH would allegedly react similarly?
Sorry about all the newbie questions; I learn a lot better by dialog and hands on experience than from a book (which I am also reading, don't worry).
tl;dr: Can I substitute NaOH for KOH in this synthesis? Why?
DraconicAcid - 27-4-2018 at 16:21
Sodium hydroxide is usually good enough for any application that requires potassium hydroxide.
nora - 28-4-2018 at 00:55
Thanks, DraconicAcid! Do you know why it works, or where I could learn why?
j_sum1 - 28-4-2018 at 01:54
Both NaOH and KOH are strong bases which is what is required for the extraction Nile is doing.
If you are studying this stuff (and I recommend you do) look up acid base chemistry in an intro textbook or online in something like Khan Academy.
At an elementary level, a base is something that can accept H+ ions. In the case of KOH, when it dissolves in water you get K+ and OH-. It is the OH-
that gives the base property. If you think about it, it is easy to see that OH- combines with H+ to give water. The K+ is just a spectator and doesn't
do much. In many situations K+ and Na+ behave exactly the same way and are interchangable. There are some times when you might prefer one over the
other.
Acid base theory does get more involved than this. But to get you in the door, if you see a metal ion with OH, you have a base. If you see a non-metal
ion with an H you have an acid. If you need more than that (including numerous exceptions) then it is time to begin some study.
LearnedAmateur - 28-4-2018 at 02:01
Alkali metal hydroxides react with fats in a saponification reaction, which is how you can make soaps. Fats consist of a glycerol backbone which is
connected to three fatty acids (long chain carboxylic acids, R-COOH) via ester linkages (R’-COO-R). In the presence of bases such as NaOH sand KOH,
these esters are cleaved to create glycerol which is miscible with water, and the carboxylic acid portion combines with the cation to form a water
soluble salt.
R’-COO-R + OH(-) -> R-OH + R’-COO(-)
The reason I didn’t add in the cation is because it is a spectator ion - it doesn’t actually take part in the reaction but is always there,
identical on both sides of the equation, so it doesn’t need to be included.
[Edited on 28-4-2018 by LearnedAmateur]
S.C. Wack - 28-4-2018 at 06:05
If the spicy chemical is piperine and the solvent is alcoholic one should consider some difficulty in dissolving NaOH without water, the availability
to most of 85% KOH, and the best (some needless drying) directions in existence for the isolation going either way: http://www.sciencemadness.org/talk/viewthread.php?tid=4698&a...
Piperine crystallizes nicely with the alkali even if the alcohol is mostly foul crapness. I could see some alcohol-soluble salt other than those
having the same effect.
Melgar - 28-4-2018 at 21:45
Two main differences: If you're trying to separate layers, K+ ions are more likely to "salt out" the top layer, since the main differences between
sodium and potassium compounds usually come down to either solubility or melting point.
Also, potassium hydroxide attacks glass less than sodium hydroxide, despite being a stronger base. Potassium hydroxide is just slightly better for
almost all applications, and is quite cheap from Duda Diesel if you're in the US.
nora - 29-4-2018 at 12:19
Thanks for the answers, they really helped a lot!