# What would happen if OH-ions were added to a very basic solution?

Say you have a solution of pH=12, that means there are ${10}^{-} 2$ moles of $O {H}^{-}$ already present. To raise this to pH=13, you'd have to add $9.9 \times {10}^{-} 2$ moles of $O {H}^{-}$, almost 10 times as much as already present.
Doubling the number of $O {H}^{-}$-ions would only raise the pH by 0.3 points.