KOH vs. NaOH

I was looking at the chemical compatibility of some things and noticed some were fine against sodium hydroxide but not against potassium hydroxide. This doesn’t make sense enough for me.

  1. is there really significant difference in reactivity between the two?
  2. what would cause KOH to be more aggressive in damaging materials then NaOH
  3. Is it likely the sources I’m looking at are wrong (Cole Parmer for one)

You are NOT a Chemist!

YES! Isn’t that what you just noted?

It’s chemical properties. Also see the Periodic Table of elements.

Naturally you would say that not having a smattering of chemistry in your background. Why would you criticize or doubt Cole Parmer with a long history in the Chemical and Lab Supplies business?

P.S. I am not a chemist and 60 years since being in a chemistry class but the above answers were obvious from a visit to the Periodic Table of the Elements and to the Cole Parmer website and a little exploration

That’s one of the least polite responses I’ve seen on this board.

And now for a nicer answer from a chemist.

  1. Yes, there is a significant difference between the two. While KOH and NaOH are both strong bases, potassium is further down on the periodic table than sodium. That gives you valuable information regarding the nature of these elements. A general rule of thumb is that atoms are more reactive going down a column, and are more reactive as you go toward the left of a row.

  2. The reactivity of potassium (or more specifically K+ (how does one superscript around here anyway?)) accounts for the added agressiveness. That is the layperson’s answer. If you want a more detailed answer it’s available, but not from me today.

  3. Nope. You’ve learned something valuable!


Please lose the snarky tone, mkay?

General Questions Moderator

10 years experience
6 years teaching chemistry
6 years in biotechnology
1 year in chemical spectroscopy

Ok. Yes I recognize that potassium is more reactive than sodium. But I’m dealing with potassium ions and sodium ions…not the reactive elements.

Both KOH and NaOH almost completely dissassociate when dissolved into water, and liberate equal numbers of hydroxide ions.

So if they both liberate equal numbers of OH ions, there must be something about the potassium IONS that are different then the sodium IONS that make the potassium IONS more likely to attack the material.

What confuses me is that I don’t recognize that K+ is terribly more reactive than Na+ otherwise other materials like potassium salts would be significantly more detrimental than similar sodium salts.

Solfy…so you are saying the K+ is significantly more reactive than Na+…I’m talking ions here not elemental atoms. Can you show me something backs that up?

How much more reactive? and why?
And yes, I’m pissed at the response a@@hole

I understand why, but I’ve already told **springears **to knock it off. Can we all just get along? Seriously, no more name calling please.

General Questions Moderator

Why couldn’t you have been a layperson?
You’re making an organic-er stretch her brain.

I see what you are saying. What I am questioning is why *wouldn’t *a potassium ion be more reactive than a sodium ion? Just because they are ionized, it doesn’t change the fact that they are different on the atomic level. Why do you say that potassium salts are no more “detrimental” than sodium salts? Would you say that potassium carbonate is the same as sodium carbonate? (maybe you would, I would not, but I’m having a bugger of a time coming up with concrete numbers to illustrate the difference)

The pkb value of potassium hydroxide is 0.5, while the value for sodium hydroxide is 0.2. Their first ionization energies are different as well: 496kJ/mol for sodium, 419kJ/mol for potassium.

If there weren’t a difference in the reactivity of potassium ions and sodium ions, you wouldn’t be able to separate them via ion exchange chromatorgraphy.

So how much more reactive? What units would you like that in? Define “reactive.” Why? Because the ions are ions of their respective atoms.

Is there a pchemist in the house? I know there are people around here with a better grasp of this at the atomic level than I.

With a fairly fundamental knowledge base in chemistry I have to admit this question has vexed me for some time, too - particularly since products like drain cleaner contain both compounds.

Is the difference in pKb values really what’s at work here? The relatively small excess of hydroxide- released by KOH compared with NaOH doesn’t seem like it would impart that much added reactivity.

I also agree that the cationic forms of the alkali metals would seem to be extremely nonreactive - seems like they’d be perfectly happy to float around in solvation shells and have little affinity for attacking much of anything. To me it seems like a perfectly valid question and, so far, nobody’s really answered it - unless the contention is that the (relatively insignificant, to me) difference in reactivity between the two placid cations is the culprit here.

It would be helpful if you provided precisely what compounds have this differing recommended compatibility. I suspect that the answer may not be strictly in terms of the reactivity of the ions but possibly something more along the lines of:

  1. Ionic radius effects. If you’re dealing with some chelating compound like EDTA or a crown ether, the size of the ion is important. 18-crown-6 ether has a particular fondness for potassium ions purely due to the fact that the ion fits well into the ether hollow. Similarly, the standard glass pH electrode has an alkali error associated with it–at high concentrations of sodium ions, the pH is measured erroneously. This is due to the fact that the standard glass pH electrode uses Na+ as a charge carrier because it is relatively mobile in the hydrated gel in the electrode’s construction. Could something along these lines be the factor?

  2. Is this in a biological application? The concentrations of Na+ and K+ are highly regulated in the body and are what generate the resting voltage across the membrane of a neuron. We eat NaCl as table salt, but Kevorkian’s death machine used high doses of KCl to stop the heart.

  3. Something related to the overall ionic atmosphere or ionic strength. These ions have different mobilities and activities in solution based on their different sizes. More information on precisely what system you’re talking about would be helpful :slight_smile:

Nitpick from a generalist and non-chemist (not past high school level anyway): the first part of that rule of thumb is good for metals, but exactly wrong for non-metals; consider the halogens for a start. (It is glaringly right for alkali metals, as you know; lithium is placid stuff compared to rubidium, f’rinstance.)

I agree that the two ions play different roles biologically (which is more evidence to me of their fundamental difference), but KCl is also a salt substitute for people on low sodium diets. The dose makes the poison. (as well as the administration mechanism)

I wholeheartedly agree. What materials are being aggressively damaged?

I don’t know if this will help, but one of the reasons why potassium ions are more reactive than sodium ions (and part of the explanation for the “going down and to the left” of the periodic table for increased reactivity in metals) is that the electrons that will be interacting in a chemical reaction are physically further away from the nucleus (because they have more electrons, in more orbitals, which increase the atomic radius). By being further from the nucleus, they are less attracted to the protons in that nucleus, and therefore are more available for reacting with atoms.

The atoms are less reactive towards the right (or more reactive towards the left) because they have more electrons that need to react: sodium only needs to lose one electron but its immediate neighbour, magnesium, needs to lose two. There is a greater energy required to strip two electrons from an atom than just one.

Of course, the other reagents in the mix also have a huge impact on reactivity, or simply usefulness. I’m trying to remember the exact reagents, but we discovered during an HPLC method development that one mixture using a sodium salt was fine but caused a precipitate with the potassium counterpart, all else being the same.
I have no idea if that helps answer the question(s) at all.

My qualifications:

B.S. in Chemical Engineering
M.S. in Environmental Engineering
Taught college-level chemistry for 5 years

I also would not be particularly cognizant of any particularly important distinction between NaOH and KOH. Both compounds dissociate in water completely (for all intents and purposes). While there is a marked difference in the reactivity of elemental sodium and potassium (potassium being the more reactive of the two alkali metals under discussion), I am not aware of any significance difference in reactivity for the ionic forms.

I would be curious to discover what spingears finds so obvious from a perusal of the periodic table. Perhaps he will return and be so kind as to elucidate on what we are missing?

This explanation is valid for explaining why elemental potassium is more reactive than elemental sodium. However, in the case of potassium ions and sodium ions, the outermost electron has already been lost.

I think mnemosyne gave a good explanation.

Not really; the sodium or potassium and hydroxide haven’t been harmlessly solvated yet. If you doubt me, drop water on either.

No, wait, don’t.

Regardless of whether the electron has been lost or not, the fact remains that potassium is a larger atom than sodium. And a larger ion as well.
Are you willing to claim then than Li+ should be chemically equivalent to Na+? Just because they’re all group 1A and all have a spare electron to lose does not make them all the same in terms of reactivity. Ionizing an atom does not take away all of its inherent properties.

If NaOH and KOH are so chemically similar, why does the result of saponification reactions with potassium result in a mostly liquid product and sodium give a solid product?

for the materials I was interested in I noticed KOH had a worse effect than NaOH. Now when I went to Cole Parmer I see some things are harmed more by NaOH and some things are more harmed by KOH. It just so happened the specific items of my current interest tended to be harmed by KOH more than NaOH.

I don’t inherently trust things that don’t make sense to me. So I’m trying to make sense of them or simply recognize that the Cole Parmer site is not necessarily authoritative.

Below are some examples of items and the more detrimental metal hydroxide

316 stainless–NaOH
Nitrile – KOH
cast iron–NaOH
ceramic(Al2O3) --KOH
Natural rubber --KOH
Some of them are listed as substantially more reactive to the particular hydroxide, not just slightly different. It’s perplexing.

Well, how about organic bases? Say LiHMDS vs. NaHMDS vs. KHMDS. Or another inorganic set of bases, like NaH vs. KH. Or K2CO3 vs Cs2CO3 (which I’ve found makes a real difference in some of my reactions, presumably due to the greater organic solubility of Cs2CO3)?

But honestly, I’m a synthetic organic chemist. I tend not to care as much how something works as long as it does.

Correct me if I’m wrong, but on the site you linked the choices for NaOH are all aqueous solutions, whereas the selections for KOH are for a solid product. That’s apples to oranges in terms of compatibility. For instance, solid pellets of KOH in a cast iron container shouldn’t be a problem as long as you kept all moisture away (rated on the site as B-good). Add water, and I warrant that KOH is every bit as corrosive as NaOH solutions (D-servere effect).

The difference in material damage of a solid pellet (assuming an anhydrous environment) versus an aqueous solution is not at all perplexing.

Seconded! But I will admit to having devoted way too much time today to musing on the nature of Na and K ions. (as evidenced by the fact that this is how I’m spending my Friday evening)