Question

Where do the OH- ions go as additional acid is added?

Answer 1

When an acid is added it usually donates protons or H+ ions that react with the OH- ions to form H₂O

Hope that helps :)

Answer 2

An acid such as HCl ionizes in water to release H+ ion

HCl → H⁺ + Cl⁻

A base such as NaOH ionizes in water to release OH- ion.

NaOH → Na⁺ + OH⁻

The H+ and OH- ions combine to form one molecule of water.  

H⁺ + OH⁻ →  H₂O