I am using magnetic stir bars to agitate solutions for 4 weeks at a time. I have noticed at the bottom of the glass bottles that white marks have accumulated where the stir bar spins. Even with soaking, thorough washing and scrubbing, acid washing, and sonicating, the marks remain. I suspect these marks are white PTFE that has eroded from the stir bars then stuck to the glass.

I have 2 questions:

  1. Are the marks removable? I know PTFE is very resistant to most solvents, which makes removal seem difficult.

  2. Is it necessary to remove these marks? Perhaps because PTFE is so resistant to solvents, it will not affect my analysis.

I am extracting ammonium from soil extracts. The soil extract is an acidified water solution, which I immediately add MgO to, in order to increase the pH to above 9. This pushes the NH4+ into the headspace of the bottle. Floating on the solution is a glass filter in a sealed Teflon packet. The ammonium ions diffuse through the Teflon, and react with citric acid on the filter to make an NH4Cl salt. This is then analyzed on an EA-MS for d15N.

  • 4
    $\begingroup$ Are you absolutely sure it's traces of PTFE and not just scratched/etched glass? Such matte coating is likely formed due to mechanical friction between the glass surface and some particles or stirring bar itself in alkaline medium. This would also explain why you cannot remove those marks. $\endgroup$
    – andselisk
    May 3 '19 at 15:37
  • 2
    $\begingroup$ Agreed with andselisk. Glass, and expecially borosilicate glass, is attacked by bases. For sure the long exposure time and the alkaline pH are the causes. No worries: it's the normal destiny of glassware in such reaction conditions. Just be aware, in the future, because eroded glass could trap particles, so it might not be suitable for every use, and it might require cleaning with "aggressive" agents in order to remove them. Also, it's a good habit not to use eroded glassware under vacuum or strong pressure, because it might lose mechanical resistance and fail. $\endgroup$
    – The_Vinz
    May 3 '19 at 19:41
  • $\begingroup$ That makes sense! Thanks! $\endgroup$
    – patawatt
    May 8 '19 at 15:41

Stir bar has a very rough grinding action. It is quite well known among silica manufacturers that if you stir micron sized silica with a stir bar for a long time in a reaction mixture, the spherical particles are damaged to some extent. In such cases, overhead stirring is preferred. This is not applicable to your case , I guess. If you had soil as well, the scenario is no different. It is a grinding action of the stir bar on glass. The "frosting" of glass is like rubbing a sand paper on glass for a long time. Nothing can heal this, except a glass-blower's flame treatment. Of course this is not worth it, as it is not going to affect the analysis of ammonia. I don't think that this is PTFE on glass. It will not stick to glass and secondly it is so hydrophobic that it would be floating on your solution rather than firmly sticking to glass.


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