The definition of sublimation is simple.  It is just the transformation of solid directly to a gas without ever being a liquid.  Dry ice is a nice example.  Dry ice sublimes at about -80C at atmospheric pressure.  Carbon dioxide can not exist as a liquid at pressures below 5.1 atmospheres or about 75 pounds per square inch.  Below that pressure it only exists as either a solid or a gas. For phase diagram data see:  https://webbook.nist.gov/cgi/inchi?ID=C124389&Mask=4

You can watch the whole sublimation process in any pan evaporator.  When you heat hydrated oxalic acid the first thing that happens is it melts.  After the hydrate melts the water boils off.  This causes the melt to foam.  As the water boils off the anhydrous oxalic acid goes back to a solid.  Then with further heating it hits a temperature where it rapidly goes from a solid to a gas.  The gas goes back to a solid with slight cooling just above the pan forming the familiar cloud or fog. The above is personal observation.  The particle size of the fog is going to largely be a function of the oxalic acid concentration in the gas above the pan.  If you had an apparatus like a band heater sublimer and blew air thru the chamber during sublimation I would expect the fog formed to have smaller particles versus no air blown.  A fast sublimation should make a courser particle fog than a slower sublimation.  I know of no evidence that any of the sublimers are either better or worse at killiing mites because of differences in particle size.  I would not expect particle size, with in reason, to be particularly important in mite kill unless it was so course the fog dispersed poorly.

The big difference between a pan sublimer and a band heater sublimer is the later has a big heat sink and is hot right at the start.  So, the oxalic acid hydrate is not going to be at a fairly uniform temperature in a band heater sublimer like it is in a pan sublimer.  Some of it is already going to be dehydrated and subliming before the last of the hydrate has had time to heat to melting temperature.

Many solids have a vapor pressure while at temperatures where they are solid.  Even iron at room temperature is very slowly subliming.  The last data I saw for iron said it has a vapor pressure of about one iron atom per m^3 at room temp.  As the temperature increases the vapor pressure of the solid increases and at some point the vapor pressure is high enough that sublimation is fast enough to see it happening.  All the pan or band heater evaporators do is heat the oxalic acid to a high enough temperature that sublimation is fast.  But, you can be sure that oxalic acid is subliming at temps far below those we use to treat bees.

Decomposition also takes place over a very wide temperature range.  Oxalic acid is decomposing even in a bottle at room temperature.  However at that temperature the decomposition rate is so slow that bottle of oxalic acid could sit there a million years and you might not notice much change.  As temperatures rise the rate of decomposition will also change.  Oxalic acid has more than one decomposition route.  One route can give carbon monoxide, carbon dioxide and water.  Another route can give formic acid and carbon dioxide.  As the temperature changes the partition between these two different decomposition routes will also change because those different routes have different activation energies and different entropies of reaction.  For some details on both of these decomposition routes see:  https://pubs.acs.org/doi/10.1021/jp9638191

How much decomposition you get will depend on the time at temperature.  An apparatus that is slow could decompose more than an apparatus that is fast if both are run at the same temperature.  I have even run oxalic acid thru a propane heated fogger where the heating coils are red hot and shown that at least some of the oxalic acid survived.  All the sublimers run at temps hundreds of degrees below that in the fogger.

At the end of the day I really do not care at all if part of the oxalic acid I sublime decomposes.  Nor do I care at all what it decomposes to.  What I care about is killing mites.  I have lots and lots of data that tells me my band heater vaporizer is very good at killing mites and I know I can treat a hive in under one minute with that apparatus.  The treatment is fast, easy, cheap, effective and safe to both me, my customers and my bees.  That seems like a real nice combination to me.  If I was not killing a lot of mites my winter survivals would not be in the 90% range most years.  I also know others who use nothing  but oxalic acid sublimed who also have very low winter losses.  Losses below 10%.  I have production hives that have lived ten years without me requeening them or restarting the hive because it died simply because I keep mite counts low.  It is really simple.  Keep the mite counts low at all times and your hives that die become very few.  If you want to restart hives every year or two because they died out just do not control mites.  If you think a 2% wash number is good the first of August in NE Ohio get used to hives dying.  If you want maximum production requeen every year.  If you are ok with making 2/3s the honey crop it is possible to make why bother requeening when the hive will requeen itself often enough to make a decent honey crop on average.  If half the oxalic acid ends up as formic acid or even carbon monoxide why should I care?  I am interested in killing mites not reducing a minor loss of cheap oxalic acid due to decomposition.  Other acids will also kill mites.  Acids like formic acid are widely used.  I do not like using something as dangerous to me as formic acid if I can avoid it.  Formic acid is way more dangerous to handle than oxalic acid.  I could easy argue formic acid is the most dangerous chemical bee keepers use unless they are doing some off label, really stupid thing. I will admit I have heard of a few bee keepers doing really stupid things. Formic acid is way more dangerous than any of the "synthetic" chemicals.  Of course formic acid is also a synthetic chemical.  It is made in big metal vats at high temperatures and pressures in chemical factories.

The anatomy of a mite is very different from that of an insect.  In a mite what we would call the head is actually just a feeding apparatus.  The normal parts associated with a head like eyes and brain are pushed back into the section behind the mites feeding apparatus.  Due to lack of space parts of the central nervous system are even pushed out into the leg parts.  
Ref:  https://link.springer.com/article/10.1007/s10493-006-9039-9
This might tell us why a mite is sensitive to acids while insects are far less sensitive.  Absorption of acid into foot parts could very well wreck the functioning of the brain parts that are in the legs.  In an insect the legs are attached to the thorax while the brain is in the head.  So, in an insect that acid would have to move a long way to get to the brain giving lots of chance to neutralize the acid before it gets there.  In a mite part of the brain is directly exposed when acid is taken up by the feet so little to no acid transport is needed.

Dick

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