[Babcock_Hall]

A question arose today about the best way to clean these frits for columns or glass filter funnels.  Sometimes there is discoloration, and sometimes there are clogs.   Piranha solution was something that a student read about, but it sounds as if this could go awry if not prepared carefully.  I was thinking nitric/sulfuric acid.  What are some standard go-to solutions and methods?

[hypervalent_iodine]

If I can’t get them clean by rinsing through with solvents or acid, then I generally soak in bleach overnight. Seems to do the trick.

[wildfyr]

Nitric acid is a good one that is a bit short of the power of pirahna.

Pirahana is indeed dangerous. However if whatever you have is only wet with water and not organic solvent it's not too tricky.

[OrganicDan96]

conc sulphuric? a lab i used to work in used aqua regia as their go to last resort cleaner

[pgk]

1). H2O2 solutions > 30% w/w (like not correctly prepared, Piranha one), are metastable and self-explosives in any moment.
2). The most effective is chromosufuric acid but it is associated with cancerogenic and environmental (waste) issues.
3). You can use conc. H2SO4, (attention that H2SO4 is completely removed), water, acetone and dichloromethane or alternatively, by the inverse sequence. Optionally you can also use H2O2 solution < 30% w/w but this can lead to an exothermic Bayer-Villiger reaction if H2O2 is not completely removed before washing with acetone (note that glass fits dramatically increases the contact surface area or the reagents).