[boca]

Background
  Honey bees have a serious parasite a mite called varroa. According to in field experiments, one of the effective chemical against them is the following solution applied on the body surface of the bees:
1000 ml water
1000 g   sucrose
    75 g   oxalic acid dihydrate.

This solution dries on the surface but not completely, since both Oxalic acid and sugar are hygroscopic.

The mechanism of action is unknown, probably the low pH damages the mite more than the bees. I am going to investigate mechanism of action and currently planning the materials and method.

My questions are:
  What happens with the solution while it dries? (pH, relative concentration)
  What crystallise first the acid or the sugar?
  What is the equilibrium concentration and pH if the temperature is 20 C and the relative humidity is 75%?

It would be a great help if you indicate how to model the process, and with which buffers I could modify the final pH.

Thanks
Gabor

[Arkcon]

Hmmm...tricky.  OK, you know sucrose is hygroscopic, logically it should crystallize first, given that its a huge excess (1000 >>75) but then, rehydrate into a syrupy coating, especially at 75 % humidity.  Maybe you can model this on glass slides in a sealed chamber, of some sort.  The pH will be hard to understand.  In a drying film, the solution will be practically saturated.  As I recall, buffers become very non-ideal under those circumstances.  If your hypothesis is that the effect is pH dependant, then just switching acids would be enough to show a difference.