Sciencemadness Discussion Board
Not logged in [Login ]
Go To Bottom

Printable Version  
Author: Subject: Weird stain on a test tube
Ashendale
Harmless
*




Posts: 47
Registered: 3-4-2005
Location: Estonia
Member Is Offline

Mood: Hydrated

[*] posted on 6-3-2006 at 08:38
Weird stain on a test tube


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! :o

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]
View user's profile View All Posts By User
Flip
Hazard to Others
***




Posts: 116
Registered: 7-12-2005
Member Is Offline

Mood: No Mood

[*] posted on 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]
View user's profile View All Posts By User
neutrino
International Hazard
*****




Posts: 1583
Registered: 20-8-2004
Location: USA
Member Is Offline

Mood: oscillating

[*] posted on 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?



View user's profile View All Posts By User
Ashendale
Harmless
*




Posts: 47
Registered: 3-4-2005
Location: Estonia
Member Is Offline

Mood: Hydrated

[*] posted on 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..
View user's profile View All Posts By User
Darkblade48
National Hazard
****




Posts: 411
Registered: 27-3-2005
Location: Canada
Member Is Offline

Mood: No Mood

[*] posted on 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?
View user's profile View All Posts By User
Ashendale
Harmless
*




Posts: 47
Registered: 3-4-2005
Location: Estonia
Member Is Offline

Mood: Hydrated

[*] posted on 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 :P

[Edited on 7-3-2006 by Ashendale]
View user's profile View All Posts By User
Dave Angel
Hazard to Others
***




Posts: 128
Registered: 22-3-2005
Location: UK
Member Is Offline

Mood: 0 K

[*] posted on 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.
View user's profile View All Posts By User
Ashendale
Harmless
*




Posts: 47
Registered: 3-4-2005
Location: Estonia
Member Is Offline

Mood: Hydrated

[*] posted on 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.
View user's profile View All Posts By User
darkflame89
National Hazard
****




Posts: 255
Registered: 1-3-2004
Location: With probability 1, "somewhere" in this
Member Is Offline

Mood: No Mood

[*] posted on 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.




Ignis ubique latet, naturam amplectitur omnem.
View user's profile View All Posts By User
scientistfromdarkness
Harmless
*




Posts: 25
Registered: 26-6-2006
Member Is Offline

Mood: schlenk

[*] posted on 4-7-2006 at 22:06


if you still cannot remove that stain with acids or alkalis, try ultrasonic bath. it must work...
View user's profile View All Posts By User

  Go To Top