Panache
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KOH vs NaOH
i find in most aspects of chemistry where one is interested in the anion, that if the counter-ion potassium for a particular reagent this is usually
interchangeable with the sodium form of the regent.
However not for the hydroxide.
Could someone more enlightened on this matter please succinctly summarise what the main differences between KOH and NaOH with respect to their use as
a reagant.
Also i have about 10L of 23% KOH solution, if i wanted to properly dehydrate and pelletise this what temperature would i be aiming for and what is the
most convenient solvent to drop the molten pellets into.
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chemrox
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have you looked up dissociation constants and ion radii to see if answer is suggested therein?
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Panache
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i am lazy and have looked up nothing, however please do not let my laziness deter you non-lazy people from spending time answering my question.
However that is a good idea regarding the dissociation constant except my crc is soooooo heavy and my feet are up on the desk (it's friday afternoon
5.30 i have a beer and well you know).
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pantone159
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Both NaOH and KOH essentially completely disassociate in water.
Comparing Na and K salts, they sometimes have significant differences in solubility or hygroscopic-ness (sic?) which makes one or the other preferable
in some cases.
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Nerro
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Often it comes down to the choice between a softer or a a hardar cation. K is a softer cation so it's more stable with soft anions. This might help to
precipitate salts or to facilitate reactions.
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Nicodem
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In water solutions KOH and NaOH behave quite similarly, but in most other solvents their basicity differs. See this post where this solvation effect is explained on the examples of alkali fluorides.
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Ephoton
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I know for organic isomerisations of alkenes KOH is far superiour also KOH is a pain to messure
with out titration as it collects water very easily and never comes completely dry well never did
from any supplier I purchased it from.
last time I made some KOH from calcium hydroxide and potassium sulfate it ate right through
my glass when trying to evap the water. this was fixed by doing the evaporation under vacuum
still I would only use it for things that absolutly needed it for instance the isomerisation of alkenes.
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woelen
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There are also some synthesis reactions for inorganic chemicals, where it does matter, whether KOH or NaOH is used, even from aqueous solution. E.g.
the produced potassium salt can be well crystallizing, while the produced sodium salt is difficult to get in a pure solid state.
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MagicJigPipe
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Ephoton, I have found that stainless steel works great for boiling K/NaOH solutions to dryness. Glass is definitely not the way to go as they both
dissolve glass at elevated temperatures.
I also have a similar question. I have some (what appears to be) semi-pure NaOH/KOH (35% Drain Cleaner; 1-3.5% KOH) solution. For reactions where
extreme purity is not essential the small amount of KOH shouldn't be that bad, right?
BTW, the solution appears perfectly clear with no obvious suspensions or other dissolved solids (when boiled to dryness the crystals are perfectly
white and clean).
"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any
question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and
that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think,
free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
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Magpie
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Woelen says:
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There are also some synthesis reactions for inorganic chemicals, where it does matter, whether KOH or NaOH is used, even from aqueous solution. E.g.
the produced potassium salt can be well crystallizing, while the produced sodium salt is difficult to get in a pure solid state.
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NaOH is used for making solid handsoaps whereas KOH is used fo making liquid handsoaps. I've always wondered what was the theoretical basis for this
difference. Does anyone know? Does it go back to the differences in ionic radii, solvation, and basicity given earlier by Nicodem?
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Ephoton
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personally I would not think so its not so much the purity thing thats important but weather
the reaction actually calls for one or the other as a specific reagent. thinking on what woelen said
I can think of two inorganic uses where KOH is a must or at least better. on being ionic swaps of
some types were KOH is to take a anion off some salts. the other as a drying agent in a desicator.
if your useing them in aqua solution see what nicodem had to say as that is the truth of it.
personaly I would just measure the ph and titrate how many moles in solution and use it
as is with some adjustments to fit the reaction. I think water has a kind of base line of how
basic or acid it can get were as solvents like DMSO or even alcohol are not as tied to the
same restrictions of ph. KOH is quite a bit more basic than NaOH and I think this including its
greater need to attract water is why it works so well for isomerisations. water ends up working
as a buffer to an extent as it will not let the ph get as high as KOH will clime in other solutions.
I think it might be atomic weight and shell diamiter that makes a soap
or detergent. if you freeze a detergent you get the same thing as a soap so I think its just like any other organic in that the size of the molecule
gives the melting points of the compound. the reason I would
also add shell diamiter to this thought though is due to lithium being
one of those weird elements that always seems to do what is not
expected if following the properties of the alkali metals up the chart.
this is due to its tightly packed electron shells making it quite dense
I belive.
[Edited on 10-3-2008 by Ephoton]
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jwarr
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I hate to bring up an old topic, but it's probably better than starting a new one. No one really explained here why KOH or NaOH may be preferred over
the other in organic reactions (outside of solvating affects). Is this really the only thing that is considered in, for example, alkene
isomerization?
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zed
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The one that works better is preferred.
In O-Chem, experiments often come first, and theories are later formulated to explain the results.
There are many variables effecting final results. Temp, Pressure, Rate of addition, Method of heating, Solubilities, Solvents, Competing reactions,
Electronegativity, Steric Hindrance, Etc..
Fortunately, based upon the outcomes of thousands of experiments, chemists have gradually devised best methods for certain preparations.
We are the inheritors of that knowledge. But, it is case by case, stuff.
A routine that works well in the seduction of one woman, may prove completely useless when you try to approach her cousin.
Practical Chem is a subtle craft. An art actually.
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