Acid--and its 'counterpart'

Alkali is the word you've misplaced (I have trouble not spelling it "alkili"). Are you a Sherlock Holmes fan? Think of the "Alkali flats" in The Valley of Fear.

On the other hand it may be simpler to refer to the opposite of acid as basic, acid/base, both four letters, one a and the next b, whatever little trick you would like to devise.