[psm1212]

I learned something interesting a few weeks ago listening to Dr. Cameron Jack discuss oxalic acid applications. There has long been great confusion among beekeepers about whether we were actually vaporizing or sublimating oxalic into our hives.

Vaporization being taking a solid, converting it to liquid and then to gas. Sublimation being taking the solid and converting it straight to gas without ever liquefying it.

According to Jack, oxalic acid dihydrate, which is the approved compound in the US for treating varroa, is vaporized and not sublimated.

Only pure oxalic acid can be sublimated directly from solid to gas.

Virtually all of us (at least in the US) use oxalic acid dihydrate, therefore, we are vaporizing and not sublimating.

Many of you probably knew all of this, but that was not my previous understanding of the reaction. Thought I would share.

 

[johno]

Maybe we are just playing with words hers as there might be a difference in the reaction between heating up anhydrous Oxalic acid and oxalic acid dihydrate. But I think the importance here is that the water of crystallization provides the pressure which forces the vapor into the closed hive, but I am sure you would not get those tiny crystals if the OA did not sublimate. I think that the liquid we see from the heated OA is the water being released as the OA starts to become a gas.

 

[psm1212]

I honestly don't know johno. Before listening to Dr. Jack, I was under the impression that we were sublimating the oxalic acid dihydrate.

Here is the link to the podcast with Dr. Jack. I think it is a little over 20 minutes in.

https://podcasts.google.com/feed/aH...1NDBiZGQtMDc0OS00NmJlLWJkYzgtMzNmMTBiZDk5NGI4

 

[msl]

the device is a heat vaporizer... weather it goes liquid to gass (like the water in OAD) or from sold to gas doesn't mater, the tool still turns things to vapor.

Anhydrous sublimates after as the water is cooked out of it. while is may be a "liquid" supersaturated soulstion for a bit (the "splatter" you some times get), the OA is in a dissolved state and a sold not a liquid

How does oxalic vaporization work? In the CRC Handbook of Chemistry and Physics we find that upon heating oxalic acid, the water of hydration boils off first, then at 315°F the oxalic acid starts to sublime (go directly from solid to vapor), and finally at 372°F any oxalic acid which has not yet sublimed decomposes to initially to formic acid and carbon dioxide, and then to CO2, carbon monoxide, and water). One of the problems with some vaporizers is that they get too hot too quickly and decompose, rather than evaporate the acid.

http://scientificbeekeeping.com/oxalic-acid-heat-vaporization-and-other-methods-part-2-of-2-parts/

 

[psm1212]

msl:

Jack's point is that we are not using oxalic acid. We are using oxalic acid dihydrate. Pure oxalic acid would sublimate. Oxalic acid dihydrate does not. It must pass through the liquid stage. I posted a link to the podcast above where Jack explains it. Like I said, it was news to me.

 

[msl]

I will dig a bit deeper later
But on face value OAD melts at 101c, the water boils out of it and it becomes sold OAA, at 189-191c OAA melts, at 190c the OAA boils in to vapor, or sublimes depending on the melt point.. but +-1c doesn't mater in the practical sense like tossing a 1g ice chip on a white hot steel plate vs a 1g water drop the end is the same.

 

[msl]

Indeed that's what happens

Set the band heater vap to 115c, once to temp I loaded a charge
ended up with a pool of slightly bubbling liquid (very tiny bubbles).. a little bit of Vape comeing off.
Changed the setting to 150c.. by the time it hit 125c it was bubbling rapidity(very large bubbles), by 150c it was a dry sold, little bit of vape coming off
turned the heat back up to 230c and the sold started subliming hevey at 185 or so and was gone by 200

my observation is right in line with the CRC Handbook of Chemistry and Physics refrance and my sujested range in post 6


So it appears we do melt OAD , boil out the water to turn it to sold OAA then sublime the OAA
if we were vaporizing OAD as Dr jack suggests the vapor should have all boiled out of the liquid, not changed to a sold state

 

[psm1212]

From Dr. Cameron Jack's latest study:

"Often, the terms sublimation and vaporization are confused and incorrectly used. Pure OA heated to a temperature of 157 C will sublimate, going straight from a solid to a gas (International Labour Organization [ILO], 2009). Heating OA dihydrate, which is the legal treatment in the US (United States Environmental Protection Agency, 2015), to a temperature of 101 C will cause the crystals to melt to a liquid and then vaporize with continued heating (ILO, 2009). Thus, vaporization occurs when using OA dihydrate."

Cameron J. Jack , Edzard van Santen & James D. Ellis (2021): Determining the dose of oxalic acid applied via vaporization needed for the control of the honey bee (Apis mellifera) pest Varroa￿destructor , Journal of Apicultural Research, DOI: 10.1080/00218839.2021.1877447

https://doi.org/10.1080/00218839.2021.1877447

 

[crofter]

If you apply the heat slowly the water of hydration will be seen to boil off leaving a solid (crusty powder) which is the mono hydrate form. At this point it does not matter if you started with oxalic acid monohydrate or the oxalic acid didhydrate which for all practical purposes is the only form beekeepers will come in contact with.

The monohydrate we now have in the pan, if we raise the temperature slowly will slowly transition to a vapor without going through a liquid state. This meets the definition of sublimation; two changes of state without exhibiting the intermediate one. As the gaseous state material cools it immediately starts to revert to the solid state in micro particles. This is what we observe to be OA vapor which we are forcing into the hive. It is not really a vapor but a finely divided solid.

The rapid boiling off of the water of hydration overlaps with the sublimation process and for the most part obscures it. This water steam does have the propelling effect johno describes. The change of state of the oxalic acid vapor back to a solid fine powder absorbs the heat from the steam and the visible plume coming out of the applicator, as Johno has demonstrated with his bare but hairy arm, is not scalding hot! So much happens so quickly we dont observe or absorb what really happens.

I think some of Dr. Jack and Medhats explanations may be the dumbed down version.

Thus, vaporization occurs when using OA dihydrate." Yes, it does, but it very quickly resublimates to the solid (fine powder, which is the visible form we see going into the air within a very short distance of the nozzle. In reality we are powdering the bees, not vaporizing them with one of the phases of oxalic acid.
The vapor phase is very fleeting and would be invisible if it were not accompanied by the condensing water of hydration. The heat absorbed to cause the double change of state from solid to vapor is given back by the almost immediate reversion (back through 2 state changes) to the very finely air borne cloud of micro solid particles which most definitely are not vapor.

 

[johno]

Then the experts, even Medhat Nasse and Randy Oliver will tell you that when the temperature gets above 200C or 400F the OA will break down to all sorts of other things, however OA will never get to those temperatures as it will sublimate beforehand. You can boil water for ever at sea level and it will not matter how much heat you apply it will still only be 100C or 212F now if you measue the temperature of the bottom of the pot it may be a good deal higher than 100C or 212F but the temperature of the water is governed by the law of physics which does not seem to be understood by the entomology experts mostly cause engineers must be dumb. Oxalic acid is the same, there can be small variations due to purity but that will be negligeable. The other thing to bear in mind is that a wet substance will have a greater area in contact with the hot surface wheras a dry powder not as much and it does not convect heat as water does. You will find that the heat penetration of the dry powder will also be influenced by the amount of other materiels between the bottom and the powder which can create an insulating buffer so that the bottom can measure maybe 200C or 400F and still not be sublimating the OA powder mostly because that powder is not feeling that heat and this has been proven with dirty equipment that no longer sublimates OA. When I started using OAV about 8 to 9 years ago every expert would tell you that it does not work unless the colony is broodless, all that has happened over those years is that the time between treatmens has decreased to every 3 to 4 days and that the dosage has increased to a point where the colonies are well covered in crystals.

 

[psm1212]

Medhat Nasr contends that the ProVap, turned up to 230C, degrades the OA. He contends that we should drop the temp of the ProVap (if using ProVap) to 195C. Medhat's assertion is that applying 2g of OA at 230C is the equivalent of applying 1g at 195C because of the degradation of OA at higher temperatures.

If we are going to say that Medhat is incorrect, and OA will sublimate at 157C and never has the opportunity to get to 200C+, then I am assuming we have to acknowledge that OA dihydrate boils before that in our applicators (101C).

It seems like some are arguing that OA dihydrate boils off only the water molecules, leaving pure OA to then sublimate when it hits 157C. Jack seems to be arguing that this does not happen. That the compound does not separate into water vapor and pure OA. He says that at 101C, the entire crystalline structure of OA dihydrate melts and then vaporizes into the hive.

I am not a chemist, nor am I an engineer. I just find the topic interesting. I will follow along.

 

[crofter]

PSM1212;

I suggest that the two experts quoted are not engineers or chemists either! I think there is simplification and conjecture in their hypotheses. The set temperature and the overshoot can be reduced considerably but the extra mass and construction of the vaporizer pot would add a fair bit to the cost. If we were seeing bee mortality there might be motivation.

 

[johno]

If we were to sublimate anhydrous oxalic acid it would change from a solid directly into a vapor, however I am sure that this acid would have to be kept in a sealed container or it would absorb moisture then we would be back to square one.if you were to set your vaporizer temperature to just over the boiling point of water the oxalic acid will assume a liquid state but it is just the water bound to the oxalic acids molecules that is starting to boil off the mixture . If you hang around long enough you will find that a solid residue will be left behind and that will not do much until the temperature is lifted to above the sublimation temperature of oxalic acid. This experiment has been done and recorded on bee-l because of the same argument that Medhat claims to result from high temperatures.
Now you could just set your vaporizer temperature to below 200C or 400F and feel good about it but your sublimation times would be increase by a good amount, so in the interest of cutting down the sublimation time we allow the copper temperature to climb to around 230C or 450F to create a calorie bank as the water absorbs a large ammount of calories to change its state as does the OA so all you are doing is adding a kind of flywheel to your process to help the small amount of heat applied to get the job done as quicly as possible. Now if we wish to talk about the breakdown of the matteriels consider where the layers of ash on the bottom of the copper bowl, I suspect that it could be as a result of a little reaction between the copper and OA and if one was to heat a dirty bowl the could well be a small amount of CO2 released from it. But the proof of the pudding is always in the eating, years of multiple heavy dosing of OA with the band heater type of vaporizer has left me with a surplus of healthy bees even though I have not spent enough time manipulating my colonies for maximum gain.

 

[johno]

There is another thing to think about, if the vaporizer temperature is at 450F and the OA hits the bottom of the bowl the temperature immediately drops to around 300F or around 150C and then rises slowly and by the time it gets yo 400F or 200C on my vaporizers the OA is gone and the only vapor around comes from the OA condensed on the silicone caps. Now when the OA releases the water why does the temperature not drop closer to 212F or 100C? Have any of you who make fondant noticed that with sugar in the water the boiling point of water increases, I wonder if the OA in the water has some sort of effect on the temperature. Also remeber that the temperature that the thermocouple shows is not necessary the temperature that the OA sees, all that the thermocouple is showing is the temperature of the copper or aluminum bottom and it has been shown that a heavy layer of ash on the bottom will insulate the OA from thr bottom temperature, this also has been proved.