Longevity of concentrated STAB

[pukalo]

I have a bottle of concentrated formaldehyde based Tetenal STAB (stabilizer, 1:100 mixture ratio for creating stabilizer solutiion for E6/C41) that I have been using over the last year for slides. There is no printed expiration on it, but I believe it may have been made in 2010. Does anyone (PE?) know if formaldehyde based stabilizer lasts for a very long time? Or is it prone to oxidize or break down over time once opened?
Thanks in advance.

 

[Gerald C Koch]

Formaldehyde solutions can become cloudy or throw down a white powdery precipitate. This is caused by polymerization of the formaldehyde to paraformaldehyde. However this can be prevented by the addition of a small amount of methanol to the solution. I would assume that Tetenal does this but you can check the MSDS for the stabilizer. As long as the concentrate is clear it should still be OK.

 

[Rudeofus]

Tetenal states in their instructions that opened concentrate bottles last for 24 weeks, the same as used working solution, yet I have never seen a precipitate forming in STAB well over a year old. This makes me wonder whether there is another mechanism at work which decays the STAB. Would Formaldehyde eventually diffuse through the bottles/lids and render the liquid ineffective? What about the biocide (1,2-Benzisothiazolin-3-one), is it prone to aerial oxidation?