The Naked Scientists

The Naked Scientists Forum

Author Topic: Can potassium and sodium hydroxide (KOH and NaOH) react with glass?  (Read 32605 times)

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
In a chemistry book I have it's said water solutions of KOH reacts with glass less than those with NaOH. Why?
« Last Edit: 19/05/2007 11:28:13 by chris »


 

Offline Batroost

  • Sr. Member
  • ****
  • Posts: 276
  • There's no such thing as a dirty atom!
    • View Profile
Probably not very helpful but NaOh reacts with glass to form Sodium Silicate. There's loads on the web about Sodium Silicate but very little (that I could find) about Potassium Silicate except a suggestion that Potassium Silicate is made only a high temperatures. So, perhaps the energy balance between the hydroxides/glass/silicates is such that Sodium Silicate is readily produced (i.e. small energy output) but Potassium Silicate production by this route would require a significant energy input. This could explain why corrsosion of glass by KOH is not so readily seen.

Sorry - couldn't find the data I'd need to back this idea up, and it's a long time since I studied any Chemistry. But there are some Chemists' on the Forums....
 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8661
  • Thanked: 42 times
    • View Profile

The intersting thing is that the Na and the K both start and end the reaction as ions in solution so there shouldnt be any difference in the energy changes. In both cases the reaction is hydroxide ions  + glass gives silicate ions. The K and Na don't enter into the reaction.
One possible reason is that a 10% solution of NaOH has more OH in it than a 10% solution of KOH because the K weighs more than the Na.
If both the solutions were the same molarity then I'd expect very similar reaction rates.
 

Offline Batroost

  • Sr. Member
  • ****
  • Posts: 276
  • There's no such thing as a dirty atom!
    • View Profile
It's a good point and interesting - because I couldn't find any warnings about KOH attacking glass. Perhaps people don't use KOH as commonly as NaOH?
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
One possible reason is that a 10% solution of NaOH has more OH in it than a 10% solution of KOH because the K weighs more than the Na.
If both the solutions were the same molarity then I'd expect very similar reaction rates.
I don't think it's simply because of this, they wouldn't have written it in an Inorganic Chemistry book, I assume; furthermore, usually we talk of molar concentrations and not weigh conc.
Is it possible, it's just an idea, that OH- is more active in a NaOH solution because Na+ is more "shielded" from water molecules, being smaller than K+?
« Last Edit: 10/05/2007 23:35:50 by lightarrow »
 

Offline chris

  • Neilep Level Member
  • ******
  • Posts: 5336
  • Thanked: 65 times
  • The Naked Scientist
    • View Profile
    • The Naked Scientists
Does this reaction lead to "frosting" of glass, or does it gently erode glass? Is the silicate insoluble?
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
Does this reaction lead to "frosting" of glass, or does it gently erode glass? Is the silicate insoluble?

Is soluble:
2OH- + SiO2 --> SiO3- + H2O

Sodium silicate: Na2SiO3 is a soluble salt.
I had it. I enjoyed making "glass plants" with it: you add crystals of different salts, which silicates are insoluble, to a dilute solution of sodium silicate. After some days you have a little "forest" of coloured silicates, growing from bottom up. Very nice.
 

Offline eric l

  • Hero Member
  • *****
  • Posts: 514
    • View Profile

Sodium silicate: Na2SiO3 is a soluble salt.
I had it. I enjoyed making "glass plants" with it: you add crystals of different salts, which silicates are insoluble, to a dilute solution of sodium silicate. After some days you have a little "forest" of coloured silicates, growing from bottom up. Very nice.

Sodium silicate is known as "water glass".  It was used as a binder in a number of applications, and is still used as a fire retardant for fireproofing paper and textiles.  See also wikipedia article :  http://en.wikipedia.org/wiki/Waterglass
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
Sodium silicate is known as "water glass".  It was used as a binder in a number of applications, and is still used as a fire retardant for fireproofing paper and textiles.  See also wikipedia article :  http://en.wikipedia.org/wiki/Waterglass
Thank you eric.
 

Offline ChemEGuy

  • First timers
  • *
  • Posts: 1
    • View Profile
The strength of the alkaline solution is dependent on the dissociation constant of the NaOH or KOH. One mole of one or the other will not give exactly one mole of Na+ and OH- when put into aqueous solution, but NaOH dissociates (breaks into Na+, OH-) more easily than KOH. Descending this column of the periodic table ionic radius of the cation increases and so does the relative affinity of the cation for the OH- anion. So when KOH goes into solution there's more that remains solid as KOH than with NaOH.
 
Glasses are composed of silicates (silicon covalently bound to oxygen in a ratio of 1:4), quartz grains (silicon covalently bound to oxygen in a ratio of 1:2), and cations such as Na+, K+, Ca2+, Fe2+, etc.

The surface of the glass phase is, in essence, a salt interacting with H+ (H3O+), OH- and other ions in the surrounding solution. Picture Si-O-Si-O- with the O- sticking out and contacting (equilibrating with cations in the water phase. The exterior Si atoms which have partial positive charge are vulnerable to attack by excess OH- ions in solution. They separate from the rest of the glass forming anions that are more soluble than the rest of the glass.

In other words, the aqueous solution is trying to achieve equilibrium by balancing its charge. Excess OH- needs to go someplace and so it attacks the Si atoms and dissolves them as a gel containing the complex salts of Na+ -O-Si-OH with different ratios of each element.

The dessicant silica packets in sneakers are this kind of salt. Concrete used around salt water or de-icing salt is vulnerable to cracking because of this type of reaction.

Hopefully that helps/interests somebody.
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
Hello, ChemEGuy, welcome on this Forum!

The strength of the alkaline solution is dependent on the dissociation constant of the NaOH or KOH. One mole of one or the other will not give exactly one mole of Na+ and OH- when put into aqueous solution, but NaOH dissociates (breaks into Na+, OH-) more easily than KOH. Descending this column of the periodic table ionic radius of the cation increases and so does the relative affinity of the cation for the OH- anion. So when KOH goes into solution there's more that remains solid as KOH than with NaOH.
Maybe you intended "that remains as undissociated molecules"?
Quote
Concrete used around salt water or de-icing salt is vulnerable to cracking because of this type of reaction.
You mean Cl- ions react with Si atoms in a similar way as OH- do?
 

Offline DrDick

  • Sr. Member
  • ****
  • Posts: 162
    • View Profile
The strength of the alkaline solution is dependent on the dissociation constant of the NaOH or KOH. One mole of one or the other will not give exactly one mole of Na+ and OH- when put into aqueous solution, but NaOH dissociates (breaks into Na+, OH-) more easily than KOH. Descending this column of the periodic table ionic radius of the cation increases and so does the relative affinity of the cation for the OH- anion. So when KOH goes into solution there's more that remains solid as KOH than with NaOH.
 

This is actually tough to predict.  The larger K+ actually has a lesser affinity for the OH- because there is a larger distance between the center of charges.  However, in water, the alkali metals tend to act a bit funny, because the hydration enthalpy has a similar trend.  Thus, the K+ also has a lesser affinity for the water than Na+ does.  I personally don't know which one wins.

And KOH does dissolve glass quite well.  It is very commonly used in this way (as a KOH/ethanol bath) to clean glassware with stubborn material stuck in it.  Just dissolve the top few layers of atoms and the gunk comes right off with it.  Then you have some nice clean glassware to dirty up in your next reaction.  :)

Dick
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
The strength of the alkaline solution is dependent on the dissociation constant of the NaOH or KOH. One mole of one or the other will not give exactly one mole of Na+ and OH- when put into aqueous solution, but NaOH dissociates (breaks into Na+, OH-) more easily than KOH. Descending this column of the periodic table ionic radius of the cation increases and so does the relative affinity of the cation for the OH- anion. So when KOH goes into solution there's more that remains solid as KOH than with NaOH.
And KOH does dissolve glass quite well.  It is very commonly used in this way (as a KOH/ethanol bath) to clean glassware with stubborn material stuck in it.  Just dissolve the top few layers of atoms and the gunk comes right off with it.  Then you have some nice clean glassware to dirty up in your next reaction.  :)
Interesting. I wonder why KOH/ethanol and not, let's say, simply NaOH/water. Maybe for less hydrofilic materials to remove?
 

Offline DrDick

  • Sr. Member
  • ****
  • Posts: 162
    • View Profile
The strength of the alkaline solution is dependent on the dissociation constant of the NaOH or KOH. One mole of one or the other will not give exactly one mole of Na+ and OH- when put into aqueous solution, but NaOH dissociates (breaks into Na+, OH-) more easily than KOH. Descending this column of the periodic table ionic radius of the cation increases and so does the relative affinity of the cation for the OH- anion. So when KOH goes into solution there's more that remains solid as KOH than with NaOH.
And KOH does dissolve glass quite well.  It is very commonly used in this way (as a KOH/ethanol bath) to clean glassware with stubborn material stuck in it.  Just dissolve the top few layers of atoms and the gunk comes right off with it.  Then you have some nice clean glassware to dirty up in your next reaction.  :)
Interesting. I wonder why KOH/ethanol and not, let's say, simply NaOH/water. Maybe for less hydrofilic materials to remove?

I think that's it exactly.  Generally this is to remove caked on organic gunk (to use the technical term :)), which wouldn't dissolve well in water.  Generally, you follow this treatment with an acid dunk (which reprotonates the silanol groups) followed by a water rinse.

An interesting note about the KOH/ethanol baths that I should point out.  You should make sure NOT to get this on your skin.  KOH will turn any oils in your skin to soap.  I had this happen once when I was cleaning some glassware.  Unbeknownst to me, there was a pinhole leak between two fingers of the latex gloves.  Unfortunately, KOH also deadens the nerves, so I didn't realize it was happening until I took the glove off and found the skin on that hand all mushy.  After trying (and failing) to rinse off the base with water, I ended up having to neutralize the KOH by dunking my hand in the acid bath (~3M HCl), then rinsing with water.  There was no permanent damage fortunately, but it really played havoc with the moisture level of that hand, which required liberal use of moisturizer for several days after.

Dick
 

Offline DrDick

  • Sr. Member
  • ****
  • Posts: 162
    • View Profile
Does this reaction lead to "frosting" of glass, or does it gently erode glass? Is the silicate insoluble?

Chris,
  Bases don't tend to frost the glass.  It tends to be more even than that.  Frosting is usually done with gels containing fluoride - generally a mixture of HF (hydrofluoric acid) and NH4F (ammonium fluoride), which is pretty nasty stuff.

Dick
 

Offline maryuc

  • First timers
  • *
  • Posts: 1
    • View Profile
hi, regarding the mixing of naoh and silica powder ... i was wondering if there is a need (and if so how) to separate the sodium hydroxide from sodium silicate if the goal in the end is to have as pure sodium silicate as possible?
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
I think it's enough to have an excess SiO2 and to mix well for long time (maybe increasing T).

However remember that SiO3- hydrolizes in water:

SiO32- + 2H2O ↔ H2SiO3 + 2OH-

since H2SiO3 is a very weak acid.

So you will always have NaOH together with Na2SiO3, if you start from a water solution.

To have pure Na2SiO3 you should, e.g., melt Na2O with SiO2.
« Last Edit: 30/03/2011 12:35:53 by lightarrow »
 

The Naked Scientists Forum


 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums