normal acidity in solution/volatile compounds/fixed mineral residue
meu companheiro is right, for normal acidity in solution, you could probably just say "acidity," which implies all those other words, but if you want to be safe, it doesn't hurt to use them, and it's even common to see redundant phrases like this in chemical literature.
volatile substances or compounds or chemicals, etc. all say about the same thing: something that evaporates easily into the air
fixed or set mineral residue
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