Removing Silca Gel?

[THX-1138]

I recently made up some silica gel, and I inadvertently allowed the beaker to dry without cleaning it thoroughly. So now there are traces of it on the glass, and they're proving a right bugger to get out. I don't think anything short of hydrofluoric acid is going to touch it chemically, and that wouldn't be such a good idea in a glass beaker anyway.

 

Any suggestions for getting the last traces out?

 

TIA..

[Fuzzwood]

Actually, you just added a layer of unpolished glass to your beaker. Glass is called sodiumboronsilicate

[YT2095]

you may find it`s the NaOH that`s attacked the glass, in which case you wont remove it as it`s Physicaly etched (be happy you didn`t use LiOH!).

 

the addition of HCl may get rid of the Gel forming sodium chloride and silicic acid, but I doubt that`s what what the problem is.

[John Cuthber]

"Any suggestions for getting the last traces out?"

Toothpaste on a bit of rag.

[THX-1138]

NaOH? What reaction are you postulating? From a quick glance at mixing Na₂O-2SiO₂ + HCl, I was anticipating salicic acid (not sure which one) and NaCl... And why would NaOH etch borosilicate glass?

 

By the way, I'm unfamiliar with that hyphenated notation. Is it equivalent to Na₂O(SiO₂)₂ ?

 

Also, since silica gel is silicon dioxide, I wouldn't expect HCl to touch it under pretty much any circumstances..

[YT2095]

silica gel is Not silicon dioxide, it`s Sodium Silicate, Na2SiO3.

[THX-1138]

I believe you're mistaken. From the Encyclopaedia Britannica:

silica

, also called SILICON DIOXIDE...

silica gel

, a highly porous, noncrystalline form of silica

 

See also http://en.wikipedia.org/wiki/Silica_gel, http://en.wikipedia.org/wiki/Silicic_acid, and http://en.wikipedia.org/wiki/Silicon_dioxide

 

(Yes, I know Wikipedia isn't always a reliable source, but in this case it's supported by the Britannica, which I do regard as reliable and authoritative.)

 

An aqueous solution of sodium silicate treated with a strong acid (like HCl) yields silicic acid.. and some other byproduct(s). When you heat a silicic acid, it decomposes into silica gel and H₂O. I'm not sure which silicic acid was formed, though, and even if I assume the simplest I'm not familiar with the hyphenated notation to be sure I evolve the reaction correctly.

 

What I have in my beaker is the residue of that reaction after being allowed to mostly dry, a substantial amount of water re-added (to dissolve any of the soluble byproducts, which I was expecting to be NaCl), the precipitate filtered out, and the beaker inadvertently left to dry completely.

 

Upon review, I did in fact fail to heat the precipitate to drive out the water, so what I've got (in another container) is probably silicic acid that hasn't been decomposed. And what's in the beaker is probably the same. Both potentially contaminated by other byproducts of the first reaction.

[YT2095]

Hmm...

 

in that case I stand corrected on that count, however the reaction with NaOH and SiO2 will indeed make sodium silicate, and the addition of HCl will do as I said, silicic acid need not form solid products (as you yourself pointed out) unless you heat it.

heating it was never suggested by either John or Myself.

 

there is always the Bright side though, unless the vessel is actually Damaged, the stuff is fairly inert for most purposes, and shouldn`t be too much of a problem

[THX-1138]

I'm not following you. I didn't add NaOH to anything. An aqueous solution of Na₂O-2SiO₂ (sodium silicate) was one of the primary reactants. An aqueous solution of HCl was the other.

 

Unless you're suggesting that I add NaOH to the residue in the beaker in order to reverse it to sodium silicate and then wash it out..?

 

Which would seem to imply that NaOH would attack quartz, as well as laboratory glassware, which is certainly foreign to my experience..

[YT2095]

HF, LiOH, KOH, NaOH even hot H3PO4 will attack glass (and SiO2).

 

you could try heating NaOH to reverse it (it may get attacked 1`st as it`s porus.

[THX-1138]

A little empirical test here.. a little aqueous NaOH in the beaker, some agitation, a thorough rinsing, and the residue seems to be pretty much gone. So I would guess that it was, in fact, undecomposed silicic acid, and not silica gel as I mistakenly thought and originally stated.

 

Thanks again for leading me to take a closer look.

 

(I'm still trying to find an explanation of the hyphenated notation, though, so I can try to trace the actual reactions.)

 

Very interesting! So hot NaOH might be the ticket for removing some tiny mica flakes I have in another situation. Brilliant! Thanks!

[THX-1138]

Well, I tried boiling a bit of glass tubing in a concentrated solution of NaOH for twenty minutes, and there was no discernible effect. I examined it at about 100X.

 

Mmph. I guess I'll try to get some LiOH and try that.

[DrDNA]

Either Piranha bath or Aqua Regia bath should work. Chromic acid bath may help, but then again may not because it mainly oxidizes organics.

 

I'm not going to list the details on making and using Piranha or Aqua Regia because they are both potentially VERY dangerous and you should read up on these before attempting. For example, the process for making up Piranha involves an exothermic reaction and if not performed properly, it can boil over onto you. It will eat just about anything, including the flesh right off of your bones (hence the name).

The information to make and use these are readlily available, so do a lot of research first (I've listed some links below to get you started, but don't stop there!).

No matter what, PLEASE remember to make up the solutions in a hood, wear proper eye protection, lab coat, acid apron, thick rubber gloves, etc and add the components SLOWLY, CAREFULLY, and IN THE CORRECT ORDER.

Here are some links to get you started, but please do not stop with these...keep reading and ask for assistance from someone that has worked with these before.

http://engineering.tufts.edu/microfab/index_files/SOP/PiranhaClean_SOP.pdf

http://www.engineering.ucsb.edu/~memsucsb/Facilities/mems%20fab/PIRANHA%20CLEAN.pdf

http://www.ampel.ubc.ca/nanofab/sop/glass-cleaning.pdf

 

Be safe!!!

[THX-1138]

The NaOH YT suggested cleaned the beaker adequately, and the discussion moved on a bit from there. I haven't the least desire to get with twenty metres of either aqua regia or piranha. I'll start a new thread on the 'getting rid of silicates' bit.

 

Thanks!