Sciencemadness Discussion Board

Potassium hydroxide vs. glass

Melgar - 25-2-2011 at 11:05

I know a lot of reactions that use sodium hydroxide are hard to do in regular glassware because of how sodium hydroxide will corrode glass. However, experimentation seems to reveal that potassium hydroxide is far less corrosive to glass than its sodium counterpart. Although I'm sure it can damage glassware over time, this seems to allow a number of experiments that wouldn't be possible with NaOH, provided some damage to glassware can be tolerated, or some potassium silicate contamination isn't problematic. For example:



So, does anyone out there either want to burst my bubble, or have other suggestions for reactions?

nitric - 25-2-2011 at 18:55

KOH will in fact corrode glass, and from what i have read, even more so than NaOH.

Sedit - 25-2-2011 at 21:46

A shot in the dark here but perhaps water content, KOH being more hygroscopic means a more "dilute" concentrated solution.

IE: if you melt the two KOH and NaOH in simular glassware which will destroy the glass more? If we base this experiment on weight then given KOH greater love for water I would think that there would be less KOH avalible then appears meaning it seems to attack the glass less even if its kinetics are higher per molar ratio.

Im more then likely very wrong or at best half right since I dont have enough personal experience with KOH to give anything in greater detail.

nitric - 26-2-2011 at 04:38

KOH is the stronger base, and will attack glass faster, as even solutions of it will attack glass, as it is used to clean glassware and has been known to etch glass joints and flasks if left too long.

blogfast25 - 26-2-2011 at 11:18

Quote: Originally posted by Sedit  
A shot in the dark here but perhaps water content, KOH being more hygroscopic means a more "dilute" concentrated solution.

IE: if you melt the two KOH and NaOH in simular glassware which will destroy the glass more? If we base this experiment on weight then given KOH greater love for water I would think that there would be less KOH avalible then appears meaning it seems to attack the glass less even if its kinetics are higher per molar ratio.

Im more then likely very wrong or at best half right since I dont have enough personal experience with KOH to give anything in greater detail.


No, even at exactly the same concentrations, KOH is more caustic than NaOH but only just. But using hot 50 w% NaOH on kitchenware Pyrex (actual borosilicate but not lab grade) I noticed attack but only very mildly. Glass is seriously corrosion resistant.

Melgar - 26-2-2011 at 12:21

Quote: Originally posted by nitric  
KOH will in fact corrode glass, and from what i have read, even more so than NaOH.

http://www.springerlink.com/content/n5523871x634l523/

"Sodium hydroxide corrodes silicate glasses of all compositions to a greater extent than all the other hydroxides. It is proposed to call this phenomenon the sodium anomaly. The sodium anomaly probably arises from peculiarities in the peptization of silicon acids by hydroxyl ions in presence of sodium ions."

Yes, KOH is a stronger base, but NaOH has a stronger corrosive effect on glass. Molten KOH MAY be more corrosive than molten NaOH, but this would probably be more due to KOH's higher melting point.

Nitro-esteban - 20-6-2013 at 13:54

Interesting, this may be useful to identify sodium or potassium hydroxide. I wonder if barium hydroxide can corrode glass.

blogfast25 - 21-6-2013 at 04:13

Quote: Originally posted by Nitro-esteban  
Interesting, this may be useful to identify sodium or potassium hydroxide. I wonder if barium hydroxide can corrode glass.


Ba(OH)2 is only sparingly soluble at RT and such a solution would not be very corrosive to glass. At BP a solution of about 50 % Ba(OH)2 can exist and that starts to get corrosive towards glass.

To use NaOH or KOH corrosiveness to glass as a way to identify either, is folly IMHO. A simple flame test should do: while the KOH will in all likelihood also test positive for Na, the strength of colour should be much higher with NaOH.

plante1999 - 21-6-2013 at 04:51

Quote: Originally posted by blogfast25  

To use NaOH or KOH corrosiveness to glass as a way to identify either, is folly IMHO. A simple flame test should do: while the KOH will in all likelihood also test positive for Na, the strength of colour should be much higher with NaOH.


*cobalt glass*

blogfast25 - 21-6-2013 at 08:21

Quote: Originally posted by plante1999  

*cobalt glass*


Explain?

plante1999 - 21-6-2013 at 08:33

It is a standard procedure to use cobalt glass to mask the sodium flame colors, as such it much easier to see the difference between element color even with sodium contamination.

watson.fawkes - 21-6-2013 at 08:47

There's a picture of a cobalt glass filter on this page. It was the first one I found; there's better data out there.

blogfast25 - 21-6-2013 at 09:19

Quote: Originally posted by plante1999  
It is a standard procedure to use cobalt glass to mask the sodium flame colors, as such it much easier to see the difference between element color even with sodium contamination.


Well, live and learn. I doubt if anyone here has cobalt glass though...

plante1999 - 21-6-2013 at 09:40

I do... a 2 by 2 inch square, it does the trick wonderfully. Time to time I think I'm the only one on the board who read about mineralogical chemistry...

I guess one could try to make a temporary borax cobalt glass. Melt 10g of borax on a steel sheet with a tiny pinch of cobalt carbonate, then melt back, it does not take much to over-color the glass.

ScienceSquirrel - 21-6-2013 at 09:40

Quote: Originally posted by plante1999  
It is a standard procedure to use cobalt glass to mask the sodium flame colors, as such it much easier to see the difference between element color even with sodium contamination.


That takes me back. That used to be standard school lab equipment in the seventies.
They were glass rectangles about 10 cm by five cm and about 5mm thick.
I suspect that you could use something like this;
http://www.ebay.co.uk/itm/Small-STUNNING-Cobalt-BLUE-Bowl-DI...

naoh vs koh

markanon - 31-12-2013 at 03:27

I has been my experience in the cosmetic industry that NaOH will etch glass more easily than KOH (at least in aqueous solutions. Could this be because Na has a higher electronegativity than K?

blogfast25 - 31-12-2013 at 05:58

What experience specifically?

In my experience neither NaOH or KOH solutions etch glass significantly, except at high concentrations/prolonged exposure/boiling.

chemrox - 31-12-2013 at 17:22

or you can get glassblowers goggles

metalresearcher - 1-1-2014 at 13:01

Really ? I thought KOH is more aggressive towards glass than NaOH.