On the pH scale, pH is a measure of the concentration of which species in an aqueous solution?

• A. Hydroxide ions, $O^H$
• B. Undissociated water molecules, $H_{2}O$
• C. Electrons, $e^{-}$
• D. Hydrogen ions, $H^{+}$ (or hydronium, $H_3{O}^{+}$)

Answer: (D)