To counteract acidity you add alkaline. To counteract alkalinity you add acid. It depends on what type of item you are dealing with. If you are around people, you would use a weak alkaline like...
pH level, the lower the number, the more acidic (down to 1), the higher, the more base (up to 14), water is at 7. Acidity and base level is what makes food sour or bitter, respectively.