Melgar
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Potassium hydroxide vs. glass
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:
Making potassium cyanate, by heating KOH and urea/cyanuric acid to about 400C, then turning off the heat when the molten salt loses its brown
color. I've done this one myself with no visible damage to the glassware.
Potassium metal, by electrolyzing the molten cyanate, which melts at 315C.
Making potassium hypochlorite solution by bubbling chlorine into KOH solution. This could be used to clean charred glassware, and wouldn't
damage it as much as its sodium counterpart.
So, does anyone out there either want to burst my bubble, or have other suggestions for reactions?
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nitric
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KOH will in fact corrode glass, and from what i have read, even more so than NaOH.
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Sedit
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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.
Knowledge is useless to useless people...
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nitric
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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.
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blogfast25
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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.
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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.
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Melgar
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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.
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Nitro-esteban
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Interesting, this may be useful to identify sodium or potassium hydroxide. I wonder if barium hydroxide can corrode glass.
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blogfast25
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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.
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plante1999
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| 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*
I never asked for this.
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blogfast25
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Explain?
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plante1999
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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.
I never asked for this.
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watson.fawkes
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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.
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blogfast25
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| 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...
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plante1999
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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.
I never asked for this.
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ScienceSquirrel
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| 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...
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markanon
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naoh vs koh
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?
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blogfast25
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What experience specifically?
In my experience neither NaOH or KOH solutions etch glass significantly, except at high concentrations/prolonged exposure/boiling.
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chemrox
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or you can get glassblowers goggles
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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metalresearcher
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Really ? I thought KOH is more aggressive towards glass than NaOH.
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