Ashendale - 6-3-2006 at 08:38
I need help removing a weird stain on my test tube. It's a long white stain and it runs from top to bottom. I have really no idea how it got there. I
left my test tubes to soak in a weak sulfuric acid solution to get dirt off. In the morning I poured away the solution, washed them and dried them.
And there it was! 
I don't really know what was in there, but the stain is outside...I have no idea how to get it off. And I don't like my test tubes dirty 
(If I get my digicam working, I'll make a pic)
[Edited on 6-3-2006 by Ashendale]
Flip - 6-3-2006 at 11:31
Try a strong sodium/ potassium hydroxide solution, and if that doesn't work, something non-polar should be your best bet. Steel wool will work but it
will score the glass on the tube.
[Edited on 3/6/06 by Flip]
neutrino - 6-3-2006 at 14:14
What kind of stain is it? Organic or inorganic? Do you have any information about what it might be made of?
Ashendale - 7-3-2006 at 07:11
Inorganic. The test tube MIGHT have had KOH and I2 in it (not only that tho), but I'm not really sure. The thing is, I don't remember what I put in
there. I mean, I knew at the time, but not anymore.
I also tried to use ethanol and at first I thought "Hey, it's gone". But after drying it became visible again..
Darkblade48 - 7-3-2006 at 08:50
It sounds like it's a calcium carbonate stain (those "hard water" stains you sometimes get around tea kettles, bath tubs, etc)
Maybe a dip in HCl rather than H2SO4 might do the trick? What concentration sulfuric acid did you use?
Ashendale - 7-3-2006 at 09:33
I put about 5-6 drops of 96% sulfuric acid to a full test tube. (15*2,5, err, ~37 ml?)
I'll try HCl then, altough I do not like working with it. Smells sooo bad 
[Edited on 7-3-2006 by Ashendale]
Dave Angel - 7-3-2006 at 10:16
My guess is that it is etching of the glass by the KOH you had in there. Can't be sure without pics but I had a similar thing happen to some bottles
that got splashed by some conc. NaOH soln. a while back: I cleaned them up but when they dried the 'stain' came back.
Ashendale - 7-3-2006 at 10:49
Ok, here's a pic, sorry for the bad quality.
http://www.unicorn.pri.ee/crap/stain.jpg
As you can see, the stain runs from top to bottom...And it feels kinda dry when touching. Like if you would touch chalk.
darkflame89 - 8-3-2006 at 01:17
Instead of sulphuric acid, try using dilute nitric acid. That stain looks like a ppt kind of stain. I used to encounter those kind of stains while
doing qualitative analysis in school. My teacher had a very good tip, instead of using mechanical ways to try rub the ppt stain off, just add dilute
nitric acid to ppt. The acid will dissolve most usual ppt(if you were dealing with basic oxides/hydroxides) and the resulting salt dissolves in water
too(all nitrates are soluble). Sulphuric acid and hydrochloric acid isnt such a good choice because there are some salts that are insoluble.
For example, if that stain was calcium carbonate, then adding sulphuric acid is pretty much useless since the resulting salt calcium sulphate is also
insoluble.=__= And you will end up with the same stain. If you add nitric acid instead, the resulting salt calcium nitrate is soluble, and you have
cleaned the stain up.
Of course, this depends on whether you have nitric acid available to you.
scientistfromdarkness - 4-7-2006 at 22:06
if you still cannot remove that stain with acids or alkalis, try ultrasonic bath. it must work...