The Collaborative International Dictionary of English v.0.48:

Acidify \A*cid"i*fy\, v. t. [imp. & p. p. Acidified; p. pr. &
   vb. n. Acidifying.] [L. acidus sour, acid + -fy: cf. F.
   acidifier.]
   1. To make acid; to convert into an acid; as, to acidify
      sugar.
      [1913 Webster]

   2. To sour; to imbitter.
      [1913 Webster]

            His thin existence all acidified into rage.
                                                  --Carlyle.
      [1913 Webster]

WordNet (r) 3.0 (2006):

acidify
    v 1: make sour or more sour [syn: sour, acidify,
         acidulate, acetify] [ant: dulcify, dulcorate,
         edulcorate, sweeten]
    2: turn acidic; "the solution acetified" [syn: acidify,
       acetify] [ant: alkalify, alkalise, alkalize,
       basify]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "acidify":
   acetify, acidulate, alkalify, alkalize, borate, carbonate,
   catalyze, chemical, chlorinate, electrolyze, ferment,
   homopolymerize, hydrate, hydrogenate, hydroxylate, isomerize,
   nitrate, oxidize, pepsinate, peroxidize, phosphatize, polymerize,
   reduce, sour, sulfate, sulfatize, sulfonate, turn sour, work