pharmacological
Harmless
Posts: 15
Registered: 22-5-2006
Member Is Offline
Mood: No Mood
|
|
Are NaOH and KOH Interchangeable for Basification?
I am trying to basify using 750ml of a 35% NaOH soln., but I only have 100g of NaOH left.
I do, however, have about a kilo of KOH.
I am skeptical as to whether I should simply substitute because of the different electronegativities of the two metals ( Na and K) and the pKa's (
NaOH=0.2, KOH=0.0)
So can i simply make a 35% ( or slightly weaker) KOH soln. and use that instead?
If more details are needed, i can fill in the gaps.
|
|
|
Sandmeyer
National Hazard
Posts: 784
Registered: 9-1-2005
Location: Internet
Member Is Offline
Mood: abbastanza bene
|
|
It's perfectly OK KOH is even smoother as it goes faster into soln... I prefer KOH, but it's not as OTC as NaOH which you can buy at the supermarket
around the corner.
[Edited on 7-7-2006 by Sandmeyer]
|
|
|
Magpie
lab constructor
Posts: 5939
Registered: 1-11-2003
Location: USA
Member Is Offline
Mood: Chemistry: the subtle science.
|
|
I agree that KOH should be an equivalent supplier of OH- compared to NaOH. But one should look at the whole context before substituting one for the
other.
As an example, liquid soap makers prefer KOH over NaOH. Why, I don't know.
The single most important condition for a successful synthesis is good mixing - Nicodem
|
|
|
ordenblitz
Hazard to Others
Posts: 259
Registered: 18-7-2004
Location: Northwest
Member Is Offline
Mood: Bohr'd
|
|
NaOH makes harder bar soaps while KOH is used for making more of a liquid or gel type soap with all else being equal.
|
|
|
neutrino
International Hazard
Posts: 1583
Registered: 20-8-2004
Location: USA
Member Is Offline
Mood: oscillating
|
|
Remember that a 35% solution of KOH will have a lower hydroxide concentration than a 35% solution of NaOH due to the differing molar masses. Depending
on what you're using this solution for, you may have to adjust the concentration.
|
|
|
guy
National Hazard
Posts: 982
Registered: 14-4-2004
Location: California, USA
Member Is Offline
Mood: Catalytic!
|
|
Why the heck do people still use mass percent? Molarity is so much better. Everyone one on earth should learn about moles along with mass.
|
|
|
The_Davster
A pnictogen
Posts: 2861
Registered: 18-11-2003
Member Is Offline
Mood: .
|
|
Yeah, I dislike mass percent as well, but it is necessary to read older works.
Now the worst is when solutions are measured in mass, such as '15g of 40% NaOH' I am always uncertain if they mean the volume of 40% NaOH that
contains 15g pure NaOH or 15 g of the solution. I have seen it both ways.
|
|
|
pharmacological
Harmless
Posts: 15
Registered: 22-5-2006
Member Is Offline
Mood: No Mood
|
|
I agree with both rogue and guy, i equally dislike % and am a strong proponent of molarity.
|
|
|
pantone159
National Hazard
Posts: 586
Registered: 27-6-2006
Location: Austin, TX, USA
Member Is Offline
Mood: desperate for shade
|
|
Personally, I prefer moles/kg of solution, rather than moles/liter (molarity).
Moles/kg doesn't seem to be very popular, it is called 'molinity' I think, and doesn't even have an abbreviation for the unit. Personally, I use 'm'
which I steal from molality (moles/kg of *solvent*), which I personally think is useless.
Some advantages of moles/kg vs moles/liter:
* Much more convenient to measure amounts with an electronic scale, rather than with a graduated cylinder (which needs cleaning every time, besides.)
* At least as accurate, and probably more. The best volume measurement precision I can get is 0.05 mL (one half of a gradation on a 25 mL graduated
cylinder), but my cheap electronic scale also measures to 0.05 g. I could get a more precise scale to improve the precision of my mol/kg
measurements, but I don't know how I could be more accurate with volumes.
* Density variations (with temperature, concentration of solutes, whatever) are of no consequence at all.
* It's easy to convert % w/w to molinity. E.g., 10% NaOH is 100 g NaOH/kg solution, 100g divided by FW of 40.0 gives 2.5 mol/kg. How the density of
the solution changes with NaOH concentration is completely irrelevant.
|
|
|
garage chemist
chemical wizard
Posts: 1803
Registered: 16-8-2004
Location: Germany
Member Is Offline
Mood: No Mood
|
|
Keep in mind that solid KOH is only 86% pure (rest being water).
A mole of KOH is therefore 66g.
|
|
|
unionised
International Hazard
Posts: 5102
Registered: 1-11-2003
Location: UK
Member Is Offline
Mood: No Mood
|
|
There could be problems using NaOH vs KOH with solubility for some materials (though it's not a big problem).
I await with amusement any attempt to get people to understand the alcohol content of wines and beers expressed as molarity. It's not as if the
conversion is a major exercise in mathematics. Frankly, if you are troubled by conversion from % to M them you should think carefully about doing
chemistry.
|
|
|
pharmacological
Harmless
Posts: 15
Registered: 22-5-2006
Member Is Offline
Mood: No Mood
|
|
| Quote: | Originally posted by pantone159
Moles/kg doesn't seem to be very popular, it is called 'molinity' I think |
It's called Molality
|
|
|
Darkblade48
Hazard to Others
Posts: 411
Registered: 27-3-2005
Location: Canada
Member Is Offline
Mood: No Mood
|
|
| Quote: | Originally posted by pharmacological
| Quote: | Originally posted by pantone159
Moles/kg doesn't seem to be very popular, it is called 'molinity' I think |
It's called Molality |
Moles/kg solvent is molality.
Moles/kg solution is molinity; though this measurement of concentration is rarely used (I've seen it maybe once or twice in my entire life).
|
|
|
pantone159
National Hazard
Posts: 586
Registered: 27-6-2006
Location: Austin, TX, USA
Member Is Offline
Mood: desperate for shade
|
|
| Quote: | Originally posted by Darkblade48
Moles/kg solution is molinity; though this measurement of concentration is rarely used
|
I think it should be used a whole lot more than it is. I find it very convenient to use, much much more convenient than using molarity.
In my home lab, it is MUCH easier to weigh something than to measure its volume, and at least as accurate if not more. I think this is very likely
true for many others.
OTOH, I can't imagine anybody finding molality (mol/kg solvent) to be other than useless.
|
|
|
unionised
International Hazard
Posts: 5102
Registered: 1-11-2003
Location: UK
Member Is Offline
Mood: No Mood
|
|
"OTOH, I can't imagine anybody finding molality (mol/kg solvent) to be other than useless. "
It's easy to convert to mole fraction which is useful in some instances- not least distillation. Anyway, for dilute solutions it's pretty near the
same as molinity and, if the solvent is water, pretty close to molarity too.
|
|
|
pharmacological
Harmless
Posts: 15
Registered: 22-5-2006
Member Is Offline
Mood: No Mood
|
|
| Quote: | Originally posted by pantone159
it is called 'molinity' I thinkquote]
Haha, i apologize, i am so unaccustomed to molinity that I simply assumed you meant malality. I've never heard of molinity except for here on this
board. |
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
