>> Have you calculated how much acid it would take to change the pH of a
>> typical hive significantly

> Here in Europe, it is claimed that the pH of oxalic acid must be equal to one to
> be effective. A higher pH would not be effective, a lower would even be
> harmful to the bees.

There are a number of acid treatments in use, and all use varying amounts of different 
acids, and are applied differently, so one would expect the impact on the hive as a 
whole, the internal atmosphere, each cell, and individual bees to differ greatly between
them.  Thus I was wondering how valid a generalization might be.

In the case of drizzling 50 mL of a 3.5% solution of the dihydrate, very roughly speaking,
less than 1.5 g of the actual acid is applied directly to the bees, but is assumed to be 
ingested.

In the case of evaporation, an amount of about 2g of the dihydrate is disseminated
through out the hive.  How much gets onto the bees is variable and debatable.

In the case of a "Mitewipe", 40 mL of dilute formic acid is released into the atmosphere
inside the hive over hours.  During that time there is air exchange with the outside. 
How much actually stays in the hive is a question.  I believe that traces can be detected 
in the brood honey, but is there enough to alter the pH detectably?  I have doubted that.

1 or 2g of acid applied to 3 or 4kg of bees in a hive massing 20kg or more would seem 
to be insufficient to cause a significant pH change other than locally and transiently.

Not only are there acids, but there are buffering agents in the hives that may confuse 
the issue as well.

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