Hugo wrote: So I wonder for how long can a beekeeper keep his formic acide
before it looses its qualities (given that it is kept in a tight container)
?

Go to Univar Canada.  The shelf life is one year.  I have had acid for 3
years and it is still okay.  Even when open it will evaporate and lose
concentration until it reaches 72% then both water and acid molecules
evaporate equally.  It does survive freezing.  The buzzing of bees is caused
by overdose.  You did not cause it.  Read the handbook on formic acid
available from www.mitegone.com .

Bill
Bill's Honey Farm - Home of the MiteGone Formic Acid Treatment
Kelowna, British Columbia, CANADA, V1V 2B6

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