Final answer:
To neutralize hydroxide ions left in the hair after a relaxer, an acidic shampoo should be used.
Step-by-step explanation:
To neutralize the hydroxide ions left in the hair after a relaxer, you would use an acidic shampoo. Acidic shampoos contain acidic compounds that can react with the hydroxide ions, forming water molecules and restoring the pH balance of the hair. These shampoos are designed to specifically target and neutralize any residual alkalinity in the hair.