In aqueous solution, what does pH measure?

• A. The total concentration of dissolved salts in the solution
• B. The hydroxide ion concentration only, defined as $\mathrm{pH}=-{\log }_{10}\left[{\mathrm{OH}}^{-}\right]$
• C. The temperature of the solution in degrees Celsius
• D. The hydrogen ion concentration, defined as $\mathrm{pH}=-{\log }_{10}\left[H^{+}\right]$

Answer: (D)