I am trying to avoid any ion in the solution as it may produce other compounds with my cation of interest. Fluoride is also problem for me, otherwise I would be able to use HF easily.
I need to reduce the pH without adding any anion to the solution!
Except injection of a ion beam of H+ or H3O+ etc. into the solution (hardly possible) I cannot imagine a solid or liquid addition (a neutral system) giving only positive ions without the anion couterpart.
it depends also from the final ph that you would like to reach.
Generally for the preparation of buffers for ionic exchange which cannot contains anoins as CL- or S04 -- i'm adding MES which is able to drop down the pH up to 5.5-6