I would like to know if formic acid (concentrated to 85%) is stable (no chemical reaction) when frozen. Is it a good solution to store it for many years?
Formic acid is thermodynamically unstable to decomposition to water and carbon monoxide: HCOOH ==> CO + H2O. I assume this is what you are referring to.
A common use is acidification of silage to prevent smells. The fact that it decomposes makes it more environmentally friendly than alternatives. I was not aware that it was used in bee keeping to control varroa parasitic mites.
Lowering the temperature would reduce the decomposition rate from both a thermodynamic and kinetic point of view.
The MSDS I have looked at indicate that the bottle should be vented periodically to release the pressure built up from decomposition. However this one also says that formic acid should not be stored below its freezing point of 8C. http://ionics.ca/wp-content/uploads/MSDS-Formic-Acid.pdf Whether this is just for convenience of use or for reasons or safety I do not know.
To me, the build up of pressure seems far more of an issue than any loss of strength in the acid. In order to lose 50% of its strength, a 1L bottle of formic acid (1200g/46=26mol) would have to lose 13mol (about 312L) of carbon monoxide. That's a lot of venting! I would think a bottle should have a shelf life of 2 or 3 years.
I did my university chemical engineering project on a formic acid plant. It is made as follows:
1. CO + Methanol (20 bar, 80C, potassium methoxide catalyst) == Methyl Formate 2. Methyl Formate + H2O (acidic autocatylisis)== Formic acid + Methanol
From this you might conclude that formic acid is stable to decomposition at 20 bar, but that is not the case, because excess reactants and careful use of distillation are used to push both reactions in the desired direction.
That was a long time ago, but I will see if I can find any relevant data in the decomposition reaction. However you would probably be best off just following the manufacturer's recommendations.
Oh, and keep it in the original glass bottle. Many metals catalyse the decomposition.