kaviaari
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Very stubborn stain in a flask
I recieved some old equipment from the nearby university. Most of them were quite dirty but sulfuric acid managed to dissolve most of the stains.
Except this one.
I don't have a clue what the stuff is but it does not dissolve in any acids or acid mixtures (aqua fortis, piranha solution..). Nor does it dissolve
in mixture of strong oxidizers (K2Cr2O7+H2SO4, CrO3+H2SO4). Acetone, toluene, xylene, ethanol and methanol didn't work either. I have tried boiling
acids and cold acids and nothing seems to work. I have scraped it with all sorts of things from pens to screwdrivers.
Of course it won't hurt the reactions carried out in it because it just won't dissolve in quite about anything. But it would be nice to get it clean.
A picture of it can be found from http://koti.mbnet.fi/kaviaari/stainedflask.jpg
[Edited on 18-7-2007 by kaviaari]
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Waffles
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heh. i think it deserves to be left in peace for all you put it through. or, if you just can't let it go, molten alkali should do the trick...
\"…\'tis man\'s perdition to be safe, when for the truth he ought to die.\"
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YT2095
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it`s quite possible that the glass itself has become pitted at a micro scale and "frosted", in that case you`ll never really get rid of it.
if as you say Nothing is attacking it, then just use it anyway as if it wasn`t there.
\"In a world full of wonders mankind has managed to invent boredom\" - Death
Twinkies don\'t have a shelf life. They have a half-life! -Caine (a friend of mine)
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The_Davster
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Steel wool on the end of a spatula with soap and water?
Sometimes the simple way works.
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sparkgap
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"I have scraped it with all sorts of things from pens to screwdrivers."
I have the feeling doing any more scraping will only net you a flask with a nice hole around where your stain used to be, given what you've put the
flask through. 
Leave it alone; I don't think the gunk'll react with anything you'd be slapping into the flask.
sparky (~_~)
"What's UTFSE? I keep hearing about it, but I can't be arsed to search for the answer..."
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Bromide
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This is no time for play, this is no time for fun. This is no time for games, there is work to be done.
Why, everyone knows--for cleaning stained flasks, you see--only one thing will work: little cats A through Z. (As a last resort, it may be necessary
to apply a catalytic amount of Voom.*)
References:
* Geisel, T.S. "The Cat in the Hat Comes Back." Random House, 1958.
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hinz
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If you should have hydroflouric acid around (I don't suppose so), you could try to etch the surface of the stained glass away. HF gives you a more or
less clear etch if it's applied as liquid.
The stains at the photo look as if someone cooked two inmiscible solvents with some kind of hydroxide or other basic stuff dissolve in the upper
layer.
I once also dissolved the surface on an flask with NaOH, not the whole glass is etched, only fine structures on the glass which looked like small
cracks in the glass.
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chemkid
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isn't hydrofluroic acid in 'wink' rust remover?
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not_important
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Wash it wekk with plain water to remove anything you may have gotten stuck in it, then rinse with acetone and dry. Add enough kerosene to cover the
gunk, and reflux for 20 or 30 minutes. Let cool just a little, and pour the kerosene off hot into a tin can or such. If anything falls out of the
kerosene when it's cooled to room temperature, repeat.
If nothing comes over in the kerosene, rinse with a little toluene, then a couple of times with acetone, let dry. Make up a solution of KOH in hot
water, 40 to 50 gr of KOH per 100 ml hot water; add the KOH slowly with stirring to get it to dissolve. Then add the hot solution to isopropyl
alcohol, 1 L alcohol per 100 cc of water used; stir, let sit for about a day (in a HDPE or poly-olefin container, stoppered not too tightly to keep
CO2 out but still allow pressure venting if needed. Pour enough of this solution into the flask through a funnel, don't get it on the ground glass
joints. Let sit for 20 to 30 minutes, pour off the solution, rinse one with water, then dilute HCl, then well with water.
At that point if there goop left it is likely that it won't bother anything you do in it, except for the possible effects of high surface area of the
goop.
The hot kerosene will dissolve most plastics including silicones, and high molecular weight organics that aren't heavily cross-linked, to some extent.
The alcohol+KOH also attacks silicones and silica gel, and many ester/amide polymers.
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Sauron
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If this stain was not removed with chromic acid (hot) then I fear it is permeated into the glass and nothing will get it out.
All that scratching you are doing has probably rendered the flask unsafe for use at temperatures above ambient, and for any vacuum. My advice would be
to scrap the flask. It happens.
I have bought glassware off eBay that I had to scrap.
Certain fluorine compounds, very strong caustics and hpt concentrated phosporic acid can attack glass even borosilicate glass, so there you are.
That's why the university gave it away.
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bio2
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Your photo of the flask looks like a severe case of etched/
frosted glass such as is caused by prolonged boiling of a
saturated/ concentrated Na or K Acetate solution.
There is no cure for this and I ruined a 2L heavy wall round
bottom flask this way.
As was mentioned it's advisable to not use this flask above
room temp. A fancy wash bottle substitute perhaps.
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chemrox
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No cure? If it is indeed etched, couldn't it be annealed in a glassblower's oven? I'd go through the outline from not_important and then take it to
a glassblower. Little bastards are expensive!
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YT2095
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even if you did that, it would still be significantly weaker than a new one, and maybe even weaker than it is now.
if it`s etched it`s purely cosmetic if it`s only light surface etched, and totally ignorable for the most part, it will make cleaning it a little more
time consuming though.
\"In a world full of wonders mankind has managed to invent boredom\" - Death
Twinkies don\'t have a shelf life. They have a half-life! -Caine (a friend of mine)
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chemrox
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in general: cleaning glass..
Is Alconox everyone's first choice for cleaning glass? I don't like dishwashing soap but will use the 'hypoallergenic' kind if I don't have other
options. What's your favorite? Why?
For hard stains do you go to strong base or chromic acid first (after the detergent that is)?
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Fleaker
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After detergent, scrubbing, and acetone salt I usually go for the sulfuric acid, heat it slightly, then if that does not work I will add some
potassium dichromate and then heat it. If that still does not remove it, I'll give it a shot with KOH and ethanol. This is only if it's a big or
expensive piece of glass. If it's a cheap erlenmeyer or a beaker, I'll toss it if it doesn't come clean.
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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Ozone
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Looks etched to me. With everything you have washed it with so far, you are probably OK, from the contamination, etc. point-of-view. It's probably OK
to use, just don't strongly heat it, over pressure it, or use strong, hot alkali (or, of course HF--the increased surface area will greatly expedite
erosion). Unless... lifespan is not really important, then I'd use it only for the stuff that I know will kill it (sacrifice to save my other glass).
Best of luck,
O3
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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