On the pH scale, pH is a measurement of the concentration of what species in aqueous solution?

• A. Hydrogen ions, $H^{+}$ (equivalently, hydronium ions, $H_{3}O+$)
• B. Electrons, $e^{-}$
• C. Hydroxide ions, $O^H$
• D. Sodium ions, $N^a$

Answer: (A)