use orthophosphoric acid, glacial acetic acid or formic acid to make acidic and triethylamine to make it basic. pH could be adjusted between 3-6.5 for c18 column otherwise column will get damaged. You can adjust your pH from 1-11, if you have a HPLC column, which is pH stable.
Phosphoric acid is commonly encountered in chemical laboratories as an 85% aqueous solution, which is a colourless, odourless, and non-volatile syrupy liquid.
ORTHOPHOSPHORIC ACID FOR HPLC