Different acids have different ionization constants.  Oxalic acid for instance is actually two acids hooked end to end.  The first acid has an ionization constant of something like pKa = 2 and the second acid has an ionization constant of something like pKa = 5.  The smaller the ionization constant the stronger the acid.  So, if you dissolve oxalic acid in water what you see is a pH dominated by the stronger acid function and a pH of more or less 2.  At such a pH the second acid on oxalic acid is for practical purposes not important as it does not ionize.

The exact same situation exists when you mix two different acids together.  The weaker (higher pKa) acid contributes little or nothing to the solution pH.  If you added sulfuric acid (a very strong acid) to vinegar (a dilute solution of the fairly weak acid acetic acid) the pH observed would be entirely the result of the sulfuric acid.  In a mix of formic acid and oxalic acid the pH is going to be dominantly due to oxalic acid as formic is considerably weaker.

What does this mean in terms of killing mites?  That is an entirely different question and the acid strength is probably not the only factor involved in killing mites.  So, the formic acid could be real important even thou it is the weaker acid.

Note:  Do not take any of the actual numbers above as gospel.  I did not look up the ionization constants and am doing them from memory and am off by some margin.  Perhaps as much as a full 1 pKa unit.
--------------------------------------------
On Fri, 5/19/17, Bill Hesbach <[log in to unmask]> wrote:

 Subject: Re: [BEE-L] Varromed.
 To: [log in to unmask]
 Date: Friday, May 19, 2017, 7:50 AM
 
 Thanks Ghislain
 
 
 Apparently, the formic and oxalic acids
 are blended together in a liquid form so the Varromed can be
 dribbled into the seams. It seems like the two acids would
 change when they're mixed into a single solution. Does
 anyone know the chemistry of what happens when two acids of
 different ph levels are mixed?  
 
 
 
 Bill Hesbach
 Cheshire CT
 
          
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