Sciencemadness Discussion Board

Sodium and Potassium Hydroxide

Chill - 7-10-2013 at 21:32

I know that NaOH and KOH are sometimes interchangeable in reactions, but are there any reasons why I would need one or the other (or both)?

I ask because I'm about to purchase some, and I'm wondering if I should just get NaOH because it's a tad bit cheaper, or shell out extra to get both. I figure, if I can use NaOH in place of KOH for many situations, I should be fine with just NaOH.

Anyways, do you guys know of any reactions in which you would want KOH exclusively, or is NaOH so interchangeable that I will be fine for most (let's say 9 out of 10) reactions. Any other opinions regarding the subject are also welcome.

Thank you

elementcollector1 - 7-10-2013 at 21:41

The only reactions in which you would specifically want KOH are reactions where you need a K+ ion - perhaps, like in the case of KNO3, it is much less soluble than its Na+ equivalent.

phlogiston - 7-10-2013 at 23:53

9 out of 10 times they are interchangeable. The sitatuations in which it makes a difference that I quickly think of:

1) Flame color (sodium imparts a very intense yellow color to flames that tends to overwhelm other colors)
2) Solubility, like elementcollector1 says, some sodium- and potassium salts have very different solubilities. For example potassium perchlorate is only sparingly soluble (16.8 g/l at 20 deg C) in water whereas sodium perchlorate is very soluble (2000 g/l)
3) Hygroscopicity. Some sodium salts are very hygroscopic whereas the equivalent potassium salt is not (potassium and sodium perchlorate are again a good example). I don't know of any examples of the reverse situation, but perhaps it occurs as well. It is not only a matter of convenience for storage. It makes a difference, for example, when you try to accurately weigh an amount or in pyrotechnic mixtures, where moisture absorbtion is undesirable.
4) In biological experiments (living cells and certain enzymes handle sodium and potassium very differently)

[Edited on 8-10-2013 by phlogiston]

woelen - 8-10-2013 at 00:17

Another place where there is a difference is in situations where hydroxide is dissolved in methanol or ethanol instead of water. KOH can more easily be dissolved than NaOH in these solvents. For certain organic experiments, hence KOH is preferred above NaOH.

If you confine yourself to aqueous chemistry, then in nearly all cases the two can be used equally well. If you are on a tight budget, then I would say, first buy some NaOH and if at a later time you really need KOH for a specific experiment, then buy that at that time.