1. The degree of ionization of the reactants and products in ATP hydrolysis vary markedly with pH. Because pH in systems may vary from 6.5 to 8.0 and of hydrolysis is defined at pH 7, the ionization of ADP in the reaction:

ADP^2- <=> ADP^3- + H⁺

will be favored more as the pH is increased because the phosphates are repulsed more in the 3- state than in the 2- state. In turn, the G for hydrolysis of ATP to ADP is even more negative in alkaline pHs.

2. Divalent ions, like Mg²⁺ affects the G of ATP hydrolysis in complicated ways.

3. The actual concentrations of ATP, ADP, and Pi are very different from the 1M values of the standard state. In a bacterial cell, these concentrations may make the G for hydrolysis of ATP as high as -50kJ/mol versus the -31 kJ/mole of the standard state.

See also: ATP as Free Energy Currency, The Importance of Metastability, The Importance of Differences Between G' and